Heater and heat pump system

Abstract

The invention relates to a heater for connection to a liquid circuit and configured to heat liquid, in particular as a back-up heater in addition to a heat pump unit, the heater comprising a tank for receiving liquid and having a tank wall extending in a longitudinal direction from a tank bottom and defining a liquid tank volume, the longitudinal direction running opposite to gravity, when the heater is installed on site, an inlet port and an outlet port for passage of liquid of the liquid circuit into and out of the tank, respectively, a heating means at least partially inside the tank for heating liquid inside the tank, the heating means extending in the longitudinal direction, and at least one hollow element provided inside the tank and extending in the longitudinal direction, a hollow element defining a liquid sub-volume and for separating the liquid sub-volume from the remaining liquid tank volume along the longitudinal direction, wherein a hollow element is in fluid communication with the remaining liquid tank volume at an opening of a hollow element, wherein the heater is configured such that the location of the opening of a hollow element, seen in the longitudinal direction, changes the passage of liquid from the inlet port or to the outlet port in the longitudinal direction, compared to the absence of a hollow element and the heater further comprising a thermostat inside the tank, in particular configured to measure the temperature of the liquid inside the tank or of steam generated by the heating means.

Description

[0001] Heater and heat pump system

[0002] Technical field

[0003] The invention concerns a heater and a heat pump system comprising such heater as well as a method of operating such a heater and / or heat pump system, a data processing device, a computer program product a computer readable data carrier and a data carrier signal, as well as the use of such a heater.

[0004] Background

[0005] Heating of water in buildings, such as heating water of a central heating system or heating of water for domestic use, may be accomplished by means of a heat pump system. Often, an electrical back-up heater for additional heating of the water is needed. For instance, when the outside air temperature is too low, a heat pump system without an electrical back-up heater may not (be able to) extract enough heat from the air to heat the water to a target temperature. Thus, an electrical back-up heater is required to assist the heat exchanger circuit within a heat pump system to achieve the target temperature. In case the heat pump fails or during extremely cold weather, the backup heater can act as the primary heating source, ensuring that the indoor environment remains warm. Using a backup heater allows the heat pump to be sized for average conditions rather than extreme cold. This optimizes the system's efficiency and reduces the need for an oversized heat pump. The backup heater is typically controlled operationally by a thermostat of the heat pump system and activates when the heat pump alone cannot meet the heating demand. As an example of such a heat pump system, the system can comprise one or more outdoor units, one or more indoor units. The outdoor and indoor units are connected by way of refrigerant lines which allow the refrigerant to circulate and transfer heat or heat transfer fluid lines such as water pipes. The electrical backup heater is typically located in or in proximity of the indoor unit. Common placements include in series after the indoor unit, inside or parallel to a storage tank or even after the storage tank in series. The storage tank is usually placed close to the indoor unit to minimize heat loss and ensure efficient transfer of heated water or air. This setup helps maintain the desired temperature with minimal energy waste.

[0006] Back-up tanks / heaters (in the following heater or (back-up) heater) are conventionally made of steel which are durable and non-flammable. The (back-up) heater is typically housed in a cylindrical casing, in particular a metal casing, in particular a steel casing. The cylindrical shape allows for efficient heat transfer and easy integration into the heat pump system. Inside the cylindrical casing an electric heating element is located. The electric heating element or heating means (in the following heating means) is often made of materials like steel or copper. The heating means heats the water as it flows through the (back-up) heater. The heater comprises a fluid, in particular water, inlet and a fluid outlet connection which are designed to be easily connected to the existing water pipes of the heat pump system. These connections are typically threaded or use quick-connect fittings for secure attachment. The casing can be insulated to minimize heat loss and improve energy efficiency.

[0007] However, a back-up heater with a (cylindrical) steel casing is expensive, adds additional weight and is not desirable in terms of their carbon footprint and sustainability profile. Commonly known back-up-heaters / tanks made of thermoplastic materials for cost effectiveness compared to steel and for a reduced carbon footprint and improved sustainability profile compared to steel have the additional advantage of more flexibility in design compared to steel. However, such known back-up heaters / tanks have inherent risks due to the thermoplastic material commonly used. For example, in the event of leakage and system malfunction, known back-up tanks made of thermoplastic material, in particular comprising a tank wall / housing made of a thermoplastic material, have the risk of melting, electrical hazards, and possible fires. Existing safety systems, such as pressure sensors and thermal sensors are not sufficiently reliable in mitigating said risk. In case of a fire generated by the back-up heater, the severity of the fire may be increased further by the proximity of flammable refrigerant. In some heaters of the prior art a fuse is used, which is also called a thermal cut out (TCO): In order to enhance security in an immersion heater a thermal fuse is used wherein the wired TCO is installed in a thermowell close to a heating element so that the TCO is triggered if the heater is used when not submerged. This solution allows the change of the fuse during a maintenance operation. However, these fuses cannot be reset in case they are triggered and thus need to be replaced which means opening the heater and replacing the fuse in case the same was triggered which means that until the replacement, the heating system comprising the backup heater would be not functional or would have at least limited functionality. This solution is thus requiring time consuming repairs and numerous spare parts every time a fuse is triggered.

[0008] EP2312224B1 is directed to providing a heat pump with a hydraulic module and to equip the hydraulic module in such a way that the hydraulic module can be flexibly installed as a compact component independently of the existing combined service water and heating system. In order to achieve this, EP2312224B1 discloses a backup heater as part of a hydraulic module. The hydraulic module has a flow connection where water enters and two return connections where water exits. The first return connection has a stop and a specific contour to fit securely. The hydraulic module further comprises a 3 / 2- way valve. The 3 / 2-way valve directs the water flow either to the first return connection or the second return connection, depending on the system's needs. The module is built with two main walls and several ribs that strengthen and stiffen the structure. These ribs converge in a rib ring for added stability. Inside the module is a cavity where the water flows. The inlet and outlet lines are angled to direct the water properly into and out of this cavity. The module includes several safety features, including a safety valve to prevent overpressure, and a volume flow sensor monitoring the water flow. The hydraulic module also comprises an automatic bleeder for removing air from the system to prevent airlocks. The hydraulic module also comprises a protective conductor lug which ensures electrical safety. The heating flange, in other words, the actual heating element, is securely fastened to the module and includes safety features like the protective conductor lug for electrical safety. The overall design is complex, with multiple components working together to ensure efficient water flow and heating while incorporating several safety mechanisms to prevent overheating and other issues. However, the hydraulic module of EP2312224B1 would have to rely on the volume flow sensor to prevent overheating due to the low volume of water in the system. The heating flange is introduced into the cavity and is fastened securely. If the water level drops too low, the heating element could overheat, potentially causing damage to the casing or creating a fire hazard. The protective conductor lug while providing electrical safety, does not directly prevent overheating due to low water levels. The disclosed hydraulic module is not suitable for reversing the flow direction. The hydraulic module is designed with specific flow paths to optimize performance. The flow connection (220), return connections (210 and 320), and the cavity (160) are all configured to ensure efficient flow and heat transfer. Reversing the flow would disrupt this optimization. Components like the 3 / 2-way valve, volume flow sensor (341), and servomotor (332) are designed to operate with the flow in a specific direction. Reversing the flow could impair their functionality, leading to potential malfunctions. Safety elements such as the safety valve (392) and automatic bleeder (372) are positioned to work with the intended flow direction. Reversing the flow would compromise their effectiveness, increasing the risk of overheating or pressure buildup. The design, including the placement of ribs (205, 209) and the rib ring (207), is intended to strengthen and stiffen the module while aiding in heat dissipation. Reversing the flow would also affect the heat dissipation efficiency, potentially leading to overheating. Also the structural elements, such as the welding flange (250) and the fastening lug (270), are designed to support the module under specific flow conditions. Reversing the flow would place unexpected stress on these components, potentially compromising the module’s integrity. In addition, the hydraulic module has a specific design which is not suitable for retro- compatibility with a standard stainless steel backup heater. The standard stainless steel backup heater is typically cylindrical. This shape is commonly used because it provides a robust and compact design, which is efficient for heat distribution and easy to install in various settings. The cylindrical form also helps in evenly distributing the heat generated by the heating elements inside the heater. The hydraulic module of EP2312224B1 has a complex and voluminous housing and overall form which is not designed to be retrofittable.

[0009] KR1020190033229A is directed to providing a water heater capable of receiving double or triple heating of the heating rod by continuously receiving the heating of the heating rod and discharging it through the upper exposure outlet, thereby significantly reducing waste of electric energy through maximization of thermal efficiency. To achieve this, KR1020190033229A discloses a first tubular heating body (100): This is the main part of the water heater, sealed at the bottom (100a) and top (100b). It has a lower inlet (100c) for water to enter. A second tubular heating body (200): This is positioned inside the first tubular heating body. It is fixed to the bottom (100a) and extends upwards, maintaining specific clearances (T1 and T2) from the bottom. It has a lower submerged outlet (41) and an upper exposed outlet (42), along with a hollow hole (43) to hold a heating rod (10). A hollow rod (40) is fixed by an upper coupling hole (K) and houses the heating rod (10) which heats the water. The design can include third and fourth tubular heating bodies to further enhance thermal efficiency. These additional bodies guide the water through multiple layers, maximizing heat transfer. When the second tubular heating body (200) is present, water enters through the lower inlet (100c), passes through the first tubular heating body (100), and is heated by the heating rod (10) in the hollow rod (40). The heated water then exits through the upper exposed outlet (42). If the second tubular heating body (200) is not present, water would simply flow through the first tubular heating body (100) and is heated by the heating rod (10). In the water heater of KR1020190033229A thermal stratification plays a crucial role in the efficiency of the water heater. Water enters the system through the lower inlet (100c) and flows into the first tubular heating body (100). The heating rod (10) inside the hollow rod (40) heats the water as it passes through. As the water heats up, it becomes less dense and rises to the top of the first tubular heating body (100). The cooler, denser water remains at the bottom, creating distinct layers of water at different temperatures. This body is positioned upright within the first tubular heating body and has specific clearances (T1 and T2) from the bottom (100a). It helps guide the water through multiple layers, ensuring that the water is heated evenly. The lower submerged outlet (41) and upper exposed outlet (42) facilitate this process by allowing the heated water to rise and the cooler water to be heated. The design, including the potential addition of third and fourth tubular heating bodies, is intended to maximize thermal efficiency by guiding the water from the lower side to the upper layers and then back down again. This repeated movement ensures that the water is thoroughly heated and that the heat is distributed evenly throughout the system. By maintaining thermal stratification, the system ensures that hot water is available at the top for use. This reduces the need for constant reheating and conserves energy. The water heater of KR1020190033229A relies on proper sealing to prevent leaks. Components like the O-ring (22) and the rotating body (24) for ensuring a tight seal around the heating rod (10) and preventing water leakage are disclosed for this purpose. The design ensures that water flows through the first and second tubular heating bodies (100 and 200) in a controlled manner, maximizing contact with the heating rod (10) for efficient heating. However, if there is a leak at the lower submerged outlet (41) or after the outlet, in other words in the heat pump system, the leakage could disrupt this flow.

[0010] In addition, the heating rod (10) is located within the hollow rod (40). This leads to a low volume of water in the hollow rod (40) and a relatively large surface of the heating rod in contact with a relatively low volume of water. This larger surface area allows more efficient transfer of heat from the element to the water, leading to fast heating, the overall goal of KR1020190033229A. The heating rod (10) within the hollow rod (40) is intended to provide good heat distribution. However, in case of a leak in the system and as a result low water volume in the hollow rod (40) would lead to inadequate cooling of the heating rod (10) in view of the insufficient water volume. Thus, the heater of KR1020190033229A would not be adequately cooled which would eventually lead to an overheating of the heating element if the heater is on for as little as another minute or two. With a low water volume, the energy from the heating rod (10) would be concentrated on the lower water volume. This would lead to rapid heating of the water volume and to the formation of steam and excessive pressure buildup in the hollow rod (40). This pressure buildup could lead to a siphon effect through the hollow rod (40). Steam formation would lead to the water vapor escaping from the water heater which can create a negative pressure in the hollow rod (40) which would lead to drawing more water into the hollow rod (40) and sustaining the siphon effect. If the water level drops too low, parts of the heating rod (10) would be exposed to air. The heating rod (10) of KR1020190033229A would then be at risk of “dry fire”, leading to damage and possible burnout of the water heater within mere tens of seconds. However, there is no disclosure of the safety means of the water heater of KR1020190033229A which the same would rely on to maintain sufficient water in case of a leakage.

[0011] In addition, due to the fact that the disclosed water heater relies on the thermal stratification, a reversal of flow direction can lead to overheating or pressure build-up due to several factors: The heating rod (10) is designed to transfer heat to water flowing in a specific direction. If the flow is reversed without proper adjustments, the water might not circulate efficiently, causing localized overheating around the heating rod (10). The design ensures that water flows smoothly through the clearances (T1 and T2) and tubular heating bodies (100 and 200). Reversing the flow could create bottlenecks or areas where water gets trapped, leading to increased pressure in those sections. Water expands when heated. If the flow direction is reversed, the expansion would not be managed effectively, causing pressure to build up in certain areas, which could stress the structural components. The water heater's safety mechanisms, such as the position of the O-ring (22) , are positioned based on the original flow direction. Reversing the flow without reconfiguring these mechanisms could render them ineffective, increasing the risk of overheating or pressure-related failures. So, in the case of KR1020190033229A ensuring the one correct flow direction is crucial for maintaining the water heater's efficiency and safety. In addition, there is no disclosure of a thermostat, fuse or other safety feature in accordance with NF EN 60335-2-40: 2024 - Particular requirements for electrical heat pumps, air-conditioners, and dehumidifiers. The norm requires in section 22.102.2 having a thermostat with manual (not automatic) reset is mandatory when using a (back-up) heater.

[0012] US20160201901A1 is directed to a heater device for heating liquified gas in liquefied gas-utilization equipment. For example, liquefied gas-utilization equipment such as a generator and the like using cassette gas, a heater device to heat and atomize the liquefied gas is provided. In particular, in this type of utilization equipment, even when the use environment of the equipment is at low temperatures, it is necessary to efficiently heat the liquefied gas and atomize it. US20160201901A1 is in particular directed to obtaining a heater device for heating liquefied gas which can heat and atomize the gas efficiently even at low temperatures by increasing the flow path length and the transit time to be heated by the heater, and by spending time to heat the liquefied gas. To achieve the same US20160201901A1 discloses a vaporizer designed to heat liquefied gas, making it easier to use at low temperatures. The vaporizer comprises a heater device that heats the liquefied gas as it flows through a gas path. This helps in turning the liquid gas into vapor. The heater device is housed in a cup-shaped device housing that contains a cylindrical heater. This housing is attached to a regulator, which controls the gas flow. Inside the housing, there’s a hollow section that forms part of the gas flow path. The gas flows through this path in a winding manner, passing along both the outer and inner surfaces of the cylindrical heater. Within the hollow section, there’s a partition wall that acts as a heating wall. This wall has the heater inside it and creates two gas flow paths on either side. These paths allow the gas to flow around the heater, getting heated as it moves. The gas enters through an intake pipe and flows through the inner and outer gas paths formed by the partition wall. These paths are connected at the end, allowing the gas to circulate and heat up efficiently. The heater can be made of materials like ceramic or PTC (Positive Temperature Coefficient) elements, which are effective at heating the liquefied gas. When the heater is powered, it heats the gas as it flows through the winding paths. The design ensures that the gas spends enough time in contact with the heater, maximizing the heating efficiency and ensuring the gas is properly vaporized even at low temperatures. The vaporizer of US20160201901A1 is unrelated to backup heaters for water in a heat pump system. The vaporizer is specifically designed to heat and vaporize liquefied gas, not water. Its components and flow paths are optimized for gas, which has different thermal properties compared to water. The efficiency of the vaporizer is based on heating gas as it flows through meandering paths around the heater. Water has a higher heat capacity and different flow characteristics, which could lead to inefficient heating and potential overheating of the vaporizer components. The materials used in the vaporizer might not be suitable for prolonged contact with water. This could lead to corrosion or other material degradation, compromising the safety and longevity of the device. The safety features and sensors in the vaporizer are designed for gas. Using it with water could bypass these safety mechanisms, increasing the risk of overheating or other failures.

[0013] US20120237191A1 is directed to providing hot water delivery systems that can provide hot water in a more energy efficient manner than storage tanks systems yet still deliver hot water at the higher flow rate associated with storage tank systems. To achieve this goal US20120237191A1 discloses an instantaneous on-demand hot water system which passes the incoming water through at least one surrounding continuous flow chamber. Each chamber includes an electric heating element which is activated as the continuous flow of water enters the chamber. Cold water enters the system through the cold- water inlet and passes through a flow control valve and a one-way check valve. This ensures a constant flow rate and prevents backflow. The cold water first enters the outermost tube, which is part of the first heating chamber. This tube surrounds the cartridge heating element. The water flows upwards in the inlet passage between the outermost tube and the first inner tube. When it reaches the top, it spills over into an opening and begins to flow downwards in the return passage between the first and second inner tubes. This change in direction creates turbulence, which enhances heat absorption from the heating element. The water flows upwards in the inlet passage between the outermost tube and the first inner tube. When it reaches the top, it spills over into an opening and begins to flow downwards in the return passage between the first and second inner tubes. This change in direction again creates turbulence, which enhances heat absorption from the heating element. After flowing downwards in the return passage of the first chamber, the water is diverted by the base cover and forced through another opening into the second chamber. In the second chamber, the water flows upwards in the incoming passage between the second and third innertubes. Again, it reaches the top and spills over into another opening, flowing downwards in the return passage between the third inner tube and the cartridge heating element. This repeated change in direction and turbulent flow further enhances heat absorption. The water continues to absorb heat as it flows downwards in the return passage of the second chamber. It then exits through the hot water outlet, monitored by a temperature probe, and is delivered to its final destination. The system further comprises a temperature probe and a flow control valve which communicates with the control unit. The temperature probe can incorporate a high temperature cut-off switch for added safety protection. The temperature probe is located at the hot water outlet of the water heating system. Specifically, it monitors the temperature of the heated water as it flows out of the sealed tubular housing. This placement ensures that the probe accurately measures the temperature of the water being delivered to its destination, allowing for proper control and regulation of the hot water temperature. The tubes disclosed in US20120237191A1 are designed to enhance heat distribution and absorption through turbulent water flow. In case the water flow is too low, the tubes alone are not sufficient to prevent overheating. The tubes of US20120237191A1 are designed to help distribute heat evenly and create turbulence for better heat absorption. However, the tubes rely on a certain flow rate to function effectively. So, in case the flow rate is too low, the water would not move quickly enough to absorb and dissipate the heat efficiently. Without sufficient water flow, the heating element could overheat because the water isn't carrying away the heat as intended. This would lead to damage to the heating element or other components of the system. Essentially the same problems would arise as in the disclosure of KR1020190033229A regarding overheating of the heating element and the risk of dry fire. Without adequate water flow, the heating elements do not have enough water to cool them down, causing the temperature to rise beyond safe operating limits within tens of seconds. Dry fire occurs when the heating elements are activated without sufficient water flow, causing the elements to heat up rapidly without any medium to absorb and carry away the heat which can lead to a siphon effect leading in the extreme to the heating element being exposed to air. The heating elements can reach dangerously high temperatures, potentially leading to excessive heat build up which can pose a fire risk if the surrounding materials are flammable and in particular prone to melting. The temperature probe at the hot water outlet is located such that it is not ensured to detect the immediate temperature increase within the heating chambers of the cartridge heater if there is insufficient water to transfer the heat to the outlet. Given that the delay time for such a measurement is within tens of seconds, the disclosed location of the thermostat can cause a delay in the thermostat detecting the high temperature, which can result in overheating and damage before the cut-off switch is activated. This problem is not contemplated by US20120237191 A1 . In addition, the disclosed check valve does not allow for a bidirectional use of the disclosed hot water delivery system. The disclosed check valve could create pressure differences within the system both in case or reversing the flow direction and also in case of a leakage. In particular if the water supply is low and the check valve restricts flow, there might be an inconsistent water volume, leading to uneven heating and potentially even severs dry fire conditions.

[0014] DE7633515U1 is directed to designing a hot water heater in such a way that that the heating process for a limited amount of water can be achieved more quickly. In order to achieve this goal, DE7633515U1 discloses that the water tank of the hot water heater has at its lower boundary a heating flange with an electric heating element attached and with a water inlet and an outlet pipe. Mounted on the heating flange is a double pipe with an inner riser pipe that accommodates the heating element and a coaxial outer downpipe, whose funnel-shaped expanded pipe opening is located slightly above the middle of the tank. The open end of the riser pipe is located in the upper tank area of the hot water heater and extends above the downpipe. The water, which is already heated after a relatively short time during operation, flows downwards in the downpipe from the upper tank area and enters the riser pipe through pipe openings distributed around the circumference of the riser pipe. The water flows past the heating element and rises through natural convection, reaching the upper tank area. A temperature sensor of a temperature controller, which is not further illustrated, is located in the flow area of the water to be heated. This temperature sensor is positioned directly above the expanded pipe opening of the downpipe. According to DE7633515U1 it is thus possible to provide a limited amount of warm or hot water in a very short time, the natural convection process plays a significant role in the functioning of the hot water heating system. The double pipe design in the hot water heating system helps distribute heat and create turbulence for better heat absorption. However, if there is not enough water volume in the system due to a leak, the double pipe alone would not be sufficient to prevent overheating. The double pipe design helps in distributing heat evenly, it relies on a certain amount of water flow to function effectively. Without enough water, the heat cannot be adequately absorbed and distributed. The natural convection occurs when the water near the heating element gets heated, becomes less dense, and rises. Cooler, denser water then moves in to take its place, creating a continuous cycle of water movement. The double pipe design leverages natural convection to distribute heat. As the heated water rises through the inner riser pipe, it creates a flow that helps in evenly distributing the heat throughout the water in the tank. The design of the tubes, with their multiple passages and changes in direction, enhances turbulence. This turbulence improves the water's ability to absorb heat from the heating element. However, if there is not enough water in the system due to a leak, the natural convection process would be disrupted. Without sufficient water, the convection currents cannot form properly, and the water may not flow effectively through the tubes. This could lead to localized overheating around the heating element, as the heat is not being carried away efficiently. Therefore, while natural convection helps in normal operation, it is not sufficient on its own to prevent overheating in the case of low water volume. So, additional safety features are crucial for this purpose. DE7633515U1 discloses that the water tank heating system relies on natural convection. Natural convection is disrupted in case of too low water volume. In order to achieve natural convection, DE7633515U1 discloses that the heating element is within a small volume of water to ensure the natural convection movement needed for the water tank system to work as intended.

[0015] In order to illustrate the problems of the double pipe design: In case there is not enough water to flow to the top of the downpipe (7) and its opening (8), then the water is flowing through the riser pipe (6) past the heating element (3) through openings 9 at the bottom of the riser pipe (6), however, between the downpipe (7) and the riser pipe (6), the heating element (3) is only directly cooled via the water flowing into the riser pipe (6) through the openings (9). In case of a leakage and the natural convection being disrupted, the indirect cooling of the heating element (3), by the water in the downpipe (7) would leave a very low volume of water to heat before the heating element (3) would start being exposed to air very quickly. The indirect cooling is due to the wall of the riser pipe (6) which has only a small openings (9) at the bottom to ensure natural convection and reduces the contact of the heating element (3) to a small direct amount of water. This is the essence of the double pipe construction of DE7633515U1. So, in case of a leakage and siphon effect, the low water volume in the water tank as such would lead to insufficient retention of adequate water in the system to directly cool the heating element (3) as the surface of the heating element (3) to direct water cooling ratio is designed for the natural convection which requires low water volume relative to the heating element surface area for the natural convection to work sufficiently. This would lead to fast steam generation which would reduce the water volume at the opening 8 which would disrupt the natural convection principle even further. If the heating element (3) has to heat a too big volume of water, the natural convection will no longer work as intended. The heat would be concentrated around the heating element (3) which would reduce the overall temperature gradient that drives the natural convection. The convection currents would diminish, and the convection currents would be weakened during normal operation, so normal operation would no longer work as intended. In addition, the water tank of DE7633515U1 cannot be operated bidirectionally in view of the principle of natural convection being at the core of the design. In addition, the design of DE7633515U1 with the flow paths disclosed is in particular prone to the siphon effect also in view of the openings (11) which are disclosed as improving the efficiency of heating the water in the water tank. These openings (11) but also already the in case of a leakage within the system which would not retain water in the water tank. Overall, with or without the openings (11) the water tank of DE7633515U1 shows similar problems to KR1020190033229A in case of leakage in the system. Thus, the water tank of DE7633515U1 is not safe in case of a low water volume in the water tank due to leakage in the system.

[0016] CN1414319A is directed to providing an instant heating electric boiler, wherein there is direct contact between the electric heating body and the water and a control of the flow rate of the water flowing through the electric heating body, so as to heat the water more effectively and at the same time lengthen the flow of water flowing through the electric heating body before and after, thereby increasing the water resistance, to reduce the leakage current of the electric heating body, and ensure the safe use of instant heating electric boilers. In order to achieve this goal CN1414319A discloses an electric boiler consisting of a furnace body made of heat-resistant insulating material, an electric heating element, and several baffles arranged in a staggered manner to create different temperature and flow velocity distributions. The furnace body is made of synthetic resin insulating material to ensure safety against electric heating element leakage. The furnace body has a low-temperature water inlet and a high-temperature water outlet. Low-temperature water flows into the furnace body and is heated by passing through specific flow-blocking passages formed by the baffles. These passages create layers with different temperature and flow velocity distributions. The layers include an inflow layer, first insulating layer, heating layer, second insulating layer, and outflow layer. The inflow and outflow layers have wider intervals, allowing for larger water volume and slower flow, which helps discharge air bubbles. The heating layer has a wider space to slow down water flow, ensuring smooth heat dissipation and conduction. The insulating layers have narrower intervals, speeding up water flow for rapid heating. The furnace body has a symmetrical structure, making the layers interchangeable. The furnace body assembly is composed of five sections, each forming different layers. The heating element is housed in the heating layer and directly contacts the water to improve heating efficiency. It can be insulated with synthetic resins to reduce leakage current. The electric boiler further comprises a pump at the water outlet which supplies high-temperature water or circulates it. An exhaust pipe connects the outflow layerto a replenishing water tankto discharge air bubbles. A liquid flow switch controls the power supply to the heating element, and all components are connected to a control box. The design of the electric boiler, with its staggered baffles and layered structure, helps in distributing heat and creating turbulence for better heat absorption. However, if there is too low water volume in the system due to a leak, the design alone may not be sufficient to prevent overheating. The baffles and layers rely on a certain amount of water flow to function effectively. Without enough water, the heat cannot be adequately absorbed and distributed, leading to potential overheating. Insufficient water flow means that the heating element could overheat because the remaining water isn't sufficient to carry away the heat. This could cause localized overheating around the heating element, potentially damaging it and other components. The water in the system might become stagnant, especially in areas where the flow is minimal. This stagnation can lead to further inefficiencies and potential overheating. So, the electric boiler thus applies to the principle of natural convection and water flow turbulence. The water is heated by the electric heating element. As the water heats up, it becomes less dense and rises. Cooler, denser water then moves in to take its place, creating a continuous cycle of water movement. The electric boiler design includes multiple layers and baffles that create specific flow-blocking passages. These passages help in distributing the heat evenly and enhancing the natural convection process. The staggered arrangement of the baffles creates different flow velocities and temperature distributions within the boiler. This ensures that the water is heated efficiently and uniformly as it moves through the various layers. The design slows down the flow of water in certain layers, allowing more time for the water to absorb heat from the electric heating element. This enhances the overall efficiency of the heating process. The flow blockages in the electric boiler are created by the staggered arrangement of baffles within the furnace body. These baffles form specific flow-blocking passages that control the movement and distribution of water. The baffles divide the furnace body into multiple layers, each with different temperature and flow velocity distributions. These layers include the inflow layer, insulating layers, heating layer, and outflow layer. The baffles create narrow passages in the insulating layers, which increase the flow velocity of the water. This ensures that the water moves quickly through these layers, allowing for rapid heating. In the heating layer, the baffles create wider passages, slowing down the water flow. This allows the water to absorb more heat from the electric heating element, ensuring efficient heat transfer. The staggered arrangement of the baffles creates turbulence in the water flow. This turbulence enhances mixing and heat absorption, preventing hotspots and ensuring uniform heating. The wider passages in the inflow and outflow layers allow for slower water flow, which helps in discharging air bubbles that may form during heating. The turbulence design is dependent on correct water flow. Turbulence occurs when water flows at a certain rate, creating chaotic and irregular movement. If the flow rate is too low, the water may not achieve the necessary speed to create turbulence, resulting in laminar (smooth) flow instead. Turbulent flow enhances heat absorption because it increases the contact between the water and the heating element. This helps in distributing the heat more evenly and efficiently. Turbulence promotes better mixing of the water, preventing hotspots and ensuring uniform temperature distribution throughout the system. The design of the baffles and passages in the boiler relies on a specific flow rate to create the desired turbulence. If the water flow is too low, the design may not function as intended, leading to reduced heating efficiency and potential overheating. Thus, a leakage in the instant heating electric boiler described in CN1414319A would lead to insufficient water volume, reduced heat absorption, water stagnation, interrupted natural convection, and diminished turbulence. In particular the narrow passages created by the insulation layers that These issues would result in localized overheating, potential damage to the system components, and compromised safety. The furnace body of CN1414319A would also not allow for reversing he flow direction in the instant heating electric boiler. Reversing the flow would significantly alter the operation of the instant heating electric boiler. The staggered baffles design which creates flow-blocking passages and narrow passages to increase the flow velocity of the water would be disrupted. Reversing the flow direction would negate the effects created in the design which can lead to uneven heating or hotspots. The system uses natural convection, where heated water rises and cooler water replaces it, creating a continuous cycle, this would be disrupted and could lead to stagnation of the water in the furnace body which would disrupt normal operation. Reversing the flow could also lead to air bubble formation which can lead to blockages. The system is designed to reduce leakage current by maintaining sufficient water flow. Low water volume can increase the risk of electrical leakage. Overheating due to low water volume can compromise the synthetic resin insulation, leading to potential electrical hazards.

[0017] The baffles and flow-blocking passages rely on adequate water flow to function effectively. Stagnant water reduces the system's overall efficiency, as it fails to distribute heat evenly. The liquid flow switch controls the power supply to the heating element. If the switch fails to detect the low water flow accurately, the heating element may continue to operate, exacerbating the overheating risk. With low water volume, there is a higher risk of dry fire, which can cause the heating element to overheat and become damaged. The temperature sensors may not detect the immediate temperature rise within the heating chambers, leading to delayed activation of safety cutoffs.

[0018] SK9745Y1 is directed to overcoming challenges of auxiliary electric heaters for heat pump systems, particularly those using PTC (Positive Temperature Coefficient) elements, with complex heat transfer structures and limited power variability, requiring multiple cartridges for high power outputs and risking overheating due to high temperature resistance coils. PTC elements are elements that are electrically conductive at low temperatures, but after a certain temperature barrier is reached, the electrical conductivity drops sharply. Thus, when the temperature of the PTC element increases, its resistance increases. Negative feedback is used, acting on the actual self-heating of the PTC element due to the passing current. When powered from a constant voltage source, this will cause a drop in power on the PTC element, counteracting further temperature rise. The result is simple heating with stabilization of the temperature of the heated space. In order to achieve this goal, SK9745Y1 discloses a modular and expandable assembly. A PTC electric heater of smaller power will have smaller dimensions than a more powerful PTC electric heater. The modular assembly will be easily expandable while ensuring the required watertightness of the assembly. The modular PTC electric heater is equipped with a heated liquid inlet and a heated liquid outlet. The modular PTC electric heater is assembled from at least two structurally identical modules. The module is designed as a cylindrical or cuboid container with a cavity water tightly closed by two flanges or plugs, while the module has a tubular inlet opening and a tubular outlet opening, both openings are connected with the container cavity. The modules are interconnected by a connecting part provided with two sealings and each module is equipped with at least one PTC cassette heating element fixed on at least one flange or plug. The modular design provides high power output variability while maintaining compact dimensions, ensuring efficient and uniform heat transfer with reduced risk of overheating, allowing for scalable solutions from 400W to virtually unlimited power, suitable for heat pump systems and domestic hot water heating.

[0019] Thus, there is a need for a (back-up) heater which has a reduced risk of melting, electrical hazards and fires, in particular in case of being made of thermoplastic material, while allowing for flexibility in design. There is in particular a desire to retain enough water to prevent exposing parts of the heating element to air in case of low water volume due to leakage within the response time of existing safety measures leading to failure of existing safety mechanisms. This exposure can lead to “dry fire” when the heating element operates without being fully submerged in water leading to damage and possibly even burnout of the (back-up) heater. In addition, there is a need for the (back-up) heater having a profile comprising at least: conformity of the (back-up) heater with the thermostat requirements of NF EN 60335-2-40 having a thermostat which manual reset, one design of the (back-up) heater allowing for a safety design of the (back-up) heater which allows selectively for upflow or downflow operation of the (back-up) heater, and with a safety design of the (back-up) heater that allows for a plastic housing, and for glycol addition, where the housing is kept intact and is still fully functional even in case of a leakage whilst reducing or preferably preventing unwanted tripping of the safety functionality during normal operation. Additionally, the heater can selectively have the simple, in particular cylindrical design, of existing standard steel heater so that it can be retrofitted to replace standard steel heaters.

[0020] Hence, the above improvements as to the reliability, versatility, and safety of a (back-up) heater are desired compared to existing metal and thermoplastic (back-up) heaters. These improvements may not be limited to a back-up heater but may also apply to a heater in general.

[0021] Summary of the invention

[0022] The present invention relates to a heater for connection to a liquid circuit and configured to heat liquid (of the liquid circuit), in particular as a back-up heater in addition to a heat pump unit, the heater comprising a tank for receiving liquid and having a tank wall extending (basically) in a longitudinal direction from a tank end and defining a liquid tank volume, the longitudinal direction (basically) running opposite or parallel to gravity, when the heater is installed on site, a first port and a second port for passage of liquid of the liquid circuit into and out of the tank, respectively, or out of and into the tank, respectively, at least one heating means having at least one heating element inside the tank for heating liquid inside the tank, the heating means extending at least partially in the longitudinal direction), and at least two hollow elements provided inside the tank and extending in the longitudinal direction, the at least two hollow elements each defining a (respective, e.g. different) liquid subvolume (and for substantially separating the liquid sub-volume from the remaining liquid tank volume along the longitudinal direction), wherein the at least two hollow elements are in fluid communication with the remaining liquid tank volume at an opening of the respective hollow element, wherein the heater is configured, when installed on site, that one of the at least two hollow elements guides liquid along at least a part of the respective hollow element for flow of liquid along the heating elements of the heating means in the longitudinal direction, and the other one of the at least two hollow elements retains liquid in the tank in case of shortage of liquid in the tank, such that at least a part of the heating means remains immersed in liquid in the tank.

[0023] A hollow element modifies / impacts the accommodation and / or flow of liquid in the tank. Specifically, a hollow element defines a liquid sub-volume and changes the direction / passage of the liquid e.g. by substantially surrounding the sub-volume and / or substantially guiding a flow of the liquid sub-volume, relative to the first and second ports, i.e. inlet port or the outlet port. This may be seen as increasing the minimal distance travelled by the liquid from the inlet to the outlet port. Accordingly, the passage of the liquid compared to when there is not such hollow element is changed.

[0024] A hollow element may direct a sub-volume away from the input port so as to improve the liquid flow as to the heating by the at least one heating means and, hence, the heater may more efficiently and / or more homogeneously heat liquid inside the tank. In other words, a guiding tube may make the journey / path of the liquid along the heating means longer compared to when there is no such tube, which supports more stable and homogeneous heating of the liquid. Hence, by guiding liquid by way of a hollow element towards or along the heating means, the positions of the first and second ports can more freely be chosen and need, for example, no longer be on opposite ends of the tank in the longitudinal direction, as a hollow element may change the flow direction and flow path of the liquid, compared to a direct and less favorable passage of fluid with less heat absorption. For example, by way of a hollow element, more uniform liquid flow distribution may be achieved, resulting in less pressure drop in heater. More specifically, the guidance may help to direct liquid around the heating surface of the heating elements of the heating means along a "long path" so it ensures a better homogeneity of the heat exchange. In particular, it may reduce dead areas, i.e. areas which are not effective for heating (for example tourbillon in corners which are not heated).

[0025] The tank may store a liquid sub-volume before the liquid would excessively drain through the outlet port, which would possibly expose the heating means. In this case, liquid would be missing in the tank. In particular, the amount of liquid may fall below the normal level of liquid, e.g. below the level of liquid defined by the sub-volume. This may be regarded as the tank being short of liquid. Reasons for liquid missing in the tank may be leakage of liquid outside the tank and, thus, loss of liquid through a port, hence, emptying the tank; or a non-working pump for pumping liquid in the liquid circuit, leading to no circulation and lowering of the level of liquid in the tank. If the liquid circuit is interrupted, i.e. if insufficient liquid is supplied via the inlet port, e.g. due to a liquid leak in the hydraulic system, an on-going draining of the hydraulic system or an interruption of the liquid circulation in the hydraulic system, this may lower the liquid received in the tank. These issues may all lead to the level of liquid falling below the normal level, in particular the tank volume not being sufficiently filled with liquid and may be summarized as an (abnormal) shortage of liquid in the tank.

[0026] For example, when a hollow element guides liquid away from the inlet port, the hollow element may be in fluid communication with the inlet port, so as to direct the inlet flow of fluid. The position of the opening of a hollow element may, in this case, represent a shifted inlet port. When a hollow element retains liquid from draining of the outlet port, at least a sub-volume of liquid is retained in the tank. The position of the opening of the hollow element may, in this case, determine the maximum level and, hence, the sub-volume, of the liquid retained in the tank. A hollow element may, in this case, be in fluid communication with the outlet port but the hollow element may, alternatively, be located within the tank without direct fluid communication to the outlet tank. Of course, a hollow element may, at the same time, fulfill both functions, namely guiding liquid away from the inlet port and preventing liquid from flowing through the outlet port.

[0027] This may help to ensure that at least a part of the heating means, i.e. the heating elements having the heating surfaces, remains immersed in liquid and, thus, avoid that the heater is overly heating, as heat is not absorbed by the liquid, which may lead to damage such as melting e.g. of the tank, in particular if the tank is made of plastics. Hence, in case of shortage of liquid in the tank, e.g. leakage of liquid in the hydraulic system, it may be prevented that the heating element is not immersed. Rather, due to remaining immersion in liquid, at least a part of the heat may be transferred away from the heating means and, thus, be blocked e.g. from the tank walls. Often, as the liquid volume may be rather low (e.g. a few liters) with respect to the power of the heating element (e.g. a few kW such as 3, 6 or 9 kW), the liquid may not be able to absorb all the energy and may evaporate relatively quickly.

[0028] The retained liquid will need some time to evaporate (e.g. tens of seconds) so it offers time for the safety counter-measure of the present invention in the heater to react and prevent further failure or risk: switch off the power, for example. Without retained liquid, no such “delay” would occur and the heater would heat the surrounding air very quickly and, within a few seconds, temperature would rise for hundreds of degrees and could burn the plastic, with a further risk then that the structure of the heater may fall down and / or that the heating surface of the heating elements of the heating means may be in contact with a flammable surface, such as a building, or even with people.

[0029] According to the invention, the first hollow element may guide liquid away from the inlet port, and the second hollow element may prevent liquid from draining of the outlet port or slow the process, so that both configurations pertaining to a hollow element are combined in the heater.. The heater may be installed upside-down, i.e. its orientation may be swapped, meaning that it may be installed in the opposite direction: With two hollow elements, the first hollow element may have a retention / trap-function, while the second hollow element provides a guiding function. In opposite installation, the second hollow element may have the retention / trap-function, and the first hollow element may function as a guide. Hence, versatility of the heater is improved. For example, the safety design constructively easily allows for bidirectional liquid flow, which allows for selectively operating the heater in upflow or downflow direction.

[0030] The heating means is, at least partially along the longitudinal direction, located between the first hollow element and the second hollow element, seen in the transverse direction. Hence, the first and second hollow elements have a strong impact on the liquid, e.g. the presence of the liquid and the flow of the liquid, in relation to the heating means. For example, guiding the flow of liquid and retaining liquid close to the heating means may both be improved if the first and second hollow elements sandwich the heating means. Accordingly, the liquid may more precisely be directed with respect to the heating means. In particular, if the heating means (heating element) is positioned between an outer and an inner hollow element, at least in parts, this may help to reduce undesired energy transfer from the heating means to the tank wall, which tank wall may be sensitive to heat, in particular if the tank wall is made of (thermoplastics.

[0031] The heater further comprises a thermostat to measure the temperature of the liquid inside the tank, optionally located within at least one of the first hollow element and the second hollow element, further optionally at the tank end.

[0032] The thermostat measures the temperature of the liquid and can, as such, serve as an indicator of abnormal situations. If the thermostat is located at the end of the tank and / or at a first hollow element or a second hollow element, it may remain preferably immersed in liquid, even in case of leakage of liquid through the second / outlet port of the tank and, accordingly, continue to provide reliable temperature measurement results. In particular, if the thermostat is immersed in liquid, thermal conduction and, thus, temperature detection, is quick. However, it is not necessary to have the thermostat immersed in the liquid in case of a leakage: The thermostat may be above the retained liquid level and may nevertheless identify leakage because of the steam generated by the heating means (heating elements). So, in other words, the thermostat would measure the temperature of the steam generated. Further, the thermostat is not necessarily in direct contact with the liquid, but it may be included in a sleeve immersed in the liquid, to provide for an indirect measurement.

[0033] Hence, the invention is characterized by at least two hollow elements, i.e. at least first and second hollow elements and the thermostat for measuring the temperature of the liquid inside the tank, forming a hydraulic trap for sufficient retention of water in the tank of the heater to allow for sufficient response time to stop the heater and / or the heat pump system comprising the heater in sufficient time to prevent melting of the housing of the heater, in particular the tank wall of the heater.

[0034] In order to elucidate the severity safety concern of a (back-up) heater without the hydraulic trap of the present invention and to illustrate the safety impact of the hydraulic trap according to the invention - which allows for combination of the needed sufficient delay of delay dry condition and the speedy response time - the dry conditions due to leakage and the resulting consequences are explained for a heater without the inventive hydraulic trap:

[0035] Starting from a standard stainless steel (back-up) heater where only the housing was replaced by a plastic housing, in other words in particular the tank wall of the heater is made of (thermo-)plastic. Even in case as is commonly provided, a thermostat is incorporated which monitors the temperature of the heating element of the (back-up) heater. The thermostat monitoring the heating element would show the following behavior in case the (back-up) heater has no water inside and has a heating power of 0 to 9 kW. The thermostat controlling standard heating means trip off in such a situation after 45 seconds. At this time, the heat in the (back-up) heater is so high that the plastic housing, in other words the tank wall, is permanently damaged. In addition, the water boiling temperature increases with pressure. At 3 bara, the boiling temperature is 130 °C. The local maximum temperature at 3 bara is 130 to 160 °C. The situation leads to steam generation and siphon effect and leads to the (back-up) heater being depleted of water. The (back-up) heater can operate in this dry condition for 15 seconds until reaching the melting temperature of a common plastic housing (280 °C). The surface temperature of the heating element rises at a pace of about 250 °C every 10 seconds. This means that within 30 seconds temperatures of around 700 °C are reached. Even if it is taken into consideration, that the heating element would start at a third of the 9 kW (maximum power of this elucidating example), so for example at 3 kW the maximum time before the plastic housing will be permanently and severely damaged is not more than 30 seconds. Thus, within 15 to 30 seconds - depending on the operating conditions at the time of leakage - the plastic housing, in other words the tank wall, would be severely and permanently damaged, if the plastic housing of a standard stainless steel (back-up) heater is replaced by a plastic housing without additional safety measures to protect the plastic housing, in other words the tank wall of the heater.

[0036] Starting from the prior art disclosed in KR1020190033229A the design is explained in its response to the heating rod (10) heating with still water (in other words no water flow). The disclosure of KR1020190033229A does not comprise a thermostat or fuse or other safety feature to prevent overheating. The disclosure of design of KR1020190033229A only discloses the O-ring (22) to rely on to prevent leakage of water. If there is, however, a leak in the heating system, the water volume and water flow could be insufficient and the water flow could even cease. Without at least a fuse the following would occur:

[0037] The heating rod (10) would continue to heat the water, emitting radiative heat and natural convection heat, and would within seconds start to generate steam. The steam is generated all over the heating rod (10) surface creating a high thermal resistance. The water steam increases the pressure in the hollow rod (40). Water steam increases the pressure. Water pressure may remove all water in the rod vicinity. The hollow rod (40) will likely reach very high temperatures far above 300°C within the first 30 seconds. KR1020190033229A discloses molybdenum material for the heating wire. Molybdenum has a melting point above 2000°C. Thus, the heating wire will heat up rapidly and far surpasses the melting temperature of a plastic housing, in other words tank wall, of a heater. The building pressure may further damage the system.

[0038] Looking at the heating bodies of the design, in particular the heating body (200) does not retain the water in case of leakage event since there is continuity of the streamline of pressure and a siphon effect can easily happen in case of leakage. The heating rod (40) can then be exposed to air in case of leakage. In fact, both the heating body (100) and (200) cannot keep the heating rod (10) from being exposed to air in case of a low water volume due to leakage. KR1020190033229A discloses that when the hollow rod (40) is exposed to the outside to provide the upper exposure outlet (42), heat loss can easily occur and even the heat of the heating rod (10) is directly lost through the upper exposure outlet (42). In the disclosed design, the tubular buffer body (300) is fixed to the hollow rod (40) so as to surround the upper exposure outlet (42). The buffer body (300) is positioned so that the heating rod (10) is formed along with the hollow rod (40) around the upper exposure outlet (42) is partially surrounded by the buffer body (300). This design is intended to minimize energy loss by protecting the hollow rod (40) and the heating rod (10) by way of the buffer body (300). This means, that the heating rod (10) in the hollow rod (40) cannot be covered by the water in the heating bodies (100, 200) but rather the upper part of the heating rod (10) is indirectly cooled by the comparatively small buffer body (300). This means that as the water in the small volume of the hollow rod (40) is heated to the point of steam, the only water that would work as a coolant on the upper part of the heating rod (10) is the buffer volume in the buffer body (300). So, with a low water volume, the energy from the heating rod (10) would be concentrated on the lower water volume. This would lead to rapid heating of the water volume and to the formation of steam and excessive pressure buildup in the hollow rod (40). This pressure buildup could lead to a siphon effect through the hollow rod (40). Steam formation would lead to the water vapor escaping from the water heater which can create a negative pressure in the hollow rod (40) which would lead to drawing more water into the hollow rod (40) and sustaining the siphon effect. If the water level drops too low, parts of the heating rod (10) would be exposed to air. The heating rod (10) of KR1020190033229A would then be at risk to “dry fire”, leading to damage and possible burnout of the water heater very quickly. So, the water heater of KR1020190033229A has to rely on maintaining sufficient water flow and relies as disclosed on the sensor readings for the safety of the appliance. So, based on the disclosure of KR1020190033229A the heating rod (10) would keep heating until the failure of the (back-up) heater integrity. The water heater of KR1020190033229A thus has no possibility of a reset in such a case.

[0039] In addition, if the design disclosed in KR1020190033229A would have to rely on the indirect cooling of the buffer body (300) in case the heating rod (10) starts generating steam, the upper part of the heating rod (10) cannot be cooled by the heating bodies (100, 200) regarding the upper part of the heating rod (10) as the heating bodies (100, 200) are not in direct contact with this upper part of the heating rod (10). This means that there would be a pressure and heat build-up in the hollow rod (40) which would only be surrounded by the buffer body (300). This buffer body (300) is designed to minimize heat loss and is designed to be insulating. This means that it has relatively little cooling capacity. The heating rod would heat up several hundred degrees per every 10 seconds which would lead to the water in the buffer body (300) also rapidly evaporating. So the buffer body (300) would add to the heat and pressure build-up in case of a leakage. The buffer body (300) would therefore through the indirect contact with the heating rod (10) potentially lead to an even more hazardous situation in case of low water volume in the heating system due to a leak. It is conceivable that the conditions in the hollow rod (40) might eventually lead to superheated steam which would lead to even more excessive heat and pressure buildup in the system with also the risk of rupture or explosion if the (back-up) heater is not shut off in time.

[0040] It needs to be noted that the generated heat will not dissipate as fast as it builds up, so that a safety design needs to respond early enough to avoid the heat build up before the heat reaches dangerous temperatures, above the melting point of the housing of the (back-up) heater. In case the (back-up) heater housing is made of plastic, the melting point of the housing will be around 280 °C. It is therefore not surprising that KR1020190033229A does not disclose a plastic housing, as the design of KR1020190033229A would be inherently unsafe.

[0041] Essentially the same arguments apply to US20120237191A1 and DE7633515U1 regarding the dry fire risk in case of leakage in the system.

[0042] In comparison to the severe damage of a heater according to the prior art without the hydraulic trap according to the invention, the advantage of the hydraulic trap and its elements can be described illustratively as follows:

[0043] The hydraulic trap within the meaning of the application means a trap comprised of at least two hollow elements, i.e. at least first and second hollow elements and a thermostat for measuring the temperature of the liquid inside the tank of the heater. This hydraulic trap allows for a sufficient retention of the liquid in the tank and a much earlier safety trigger parameter, namely the liquid temperature, allowing for a much faster response time and thus a faster control signal of the thermostat, which can in other words be described as an emergency control signal of the thermostat. The control signal of the thermostat will prevent the overheating and damage of the heater. At the same time the inventive heater comprising the hydraulic trap allows for normal heating of the liquid during normal operation without unwanted disruption of operation due to unwanted triggering of for example an emergency stop signal by for example the thermostat of the heater. In other words, the control signal of the thermostat comprises a command to adjust or stop the operation of the heating means.

[0044] In other words, the hydraulic trap allows that the heater remains sufficiently immersed sufficiently long for the quick safety response time of the thermostat of the hydraulic trap in any leakage scenario, such as liquid leakage of the fittings and / or flask and / or hydraulic circuit. In particular, the level of immersion can be implemented in a predefined way by way of constructively simple dimension choices at the design and assembly stage, taking into account the performance of the heater. The hydraulic trap also allows that the thermostat of the hydraulic trap can simply be reset after the leak situation has been resolved, literally at the push of a button.

[0045] In other words, the volume liquid retained in the tank can be chosen in a predefined way to ensure that the heating means remains immersed in liquid in the tank to the extent needed to prevent dry fire for a predefined period of time, wherein the time can be chosen such that the heater operation is stopped in sufficient time before the melting temperature of the plastic housing is reached so that dry fire and or other heat damage to the heater can be prevented making use of the fast response time of the thermostat of the hydraulic trap.

[0046] So illustratively, the hydraulic trap retains sufficient water in the heater that in case of a leakage in the heat pump system which leads to low water volume in the heater so that the heating element is sufficiently stopped and will not heat beyond the melting point of the (thermo-)plastic housing. In other words, the hydraulic trap allows for sending a stop signal in a very short response time based on the temperature of the liquid in the heater and allows that during normal operation and with sufficient water in the system, the heating element is not shut off erroneously.

[0047] As an example to illustrate the principle of the hydraulic trap: a predetermined setpoint temperature can be predefined fitting the parameters of the respective heater regarding the hollow element dimensions and regarding the thermostat parameters, in particular the setpoint of the thermostat of the hydraulic trap according to the invention. The thermostat setpoint can be set to trigger, in other words generate a control signal of the thermostat, such as for example a command for an emergency stop of the operation of the heating means if the detected temperature reaches or exceeds for example a temperature selected from the range of degrees comprised in 80 °C to 120°C, in particular 80°C to 115 °C, in particular 85°C to 115 °C. The degrees can be chosen in said range per degree, thus for example 80°C or 81 °C until 120°C, or for example in tenth of a degree steps between 80°C and 100°C, or for example in half degree steps, such as 80 °C, 85°C and so on until 100°C or in 5 degree steps, such as 80 °C, 85°C continued until to 120 °C, all steps in the example sequences are included and each temperature within the sequences can be chosen as appropriate for the respective heater. The setpoint temperature can include a tolerance, for example ± 6°C. For example, 85 °C with a tolerance of ± 6°C is chosen. At the higher end, for example also a setpoint temperature of 115 °C can be chosen. The response time of the thermostat of the hydraulic trap depends on this selection and is typically between 7 and 15 seconds. Fast thermostats can have a response time of 3 to 5 seconds. The formula for determining the setpoint can be formulated as performance of the heater and taking into consideration the temperature of the water T initially Tini and needs to fulfill Tplastics < T melting as long as the heating time t is < time to reach the maximum allowed heater temperature tmax in dry conditions. For example, in case of a 9 kW heater: initial Temperature 30°C, the tmax would be < 15 seconds. So, for the example 9 kW heater, Tini 30°C, T plastics < T melting if tmax 15 seconds. At the time of reaching the predetermined setpoint temperature, the temperature of the heating means is under the 400 °C of the dry heating curve as shown for example in Fig. 9a for typical heating means for heaters according to the invention due to the cooling effect of the retained water in the system and / or in case the heater is not heating at full capacity. For example, if the heater is heating at only 3kW the time to reach the maximum heater temperature under dry conditions is for example < 15 to 30 seconds. So, the system remains intact in case of a leakage leading to a too low water volume in the tank of the heater.

[0048] So, the setpoint can for example be predefined based on the performance of the heater (Pheater in kW), the initial temperature (Tini), the time (tmax) at which the maximum allowed surface temperature of the heater (Tmaxheater) in dry conditions is reached to ensure that the temperature (Tplastics) of the tank wall is remains below the melting temperature (Tmelting) of the tank wall, in case the tank wall is made of (thermo-)plastics. In addition, the influence of the geometry of the heating means on heat transfer by radiation and natural convection can be taken into account. This allows configuring the setpoint (Tthermostat) such that the heater’s performance under normal conditions is not affected taking the parameters of the respective heater and heat pump system into consideration and ensuring the safety of the plastic components of the heater.

[0049] Thus, the heater according to the invention has a reduced risk of melting, electrical hazards and fires, in particular in case of being made of thermoplastic material, while allowing for flexibility in design. The heater can be designed in a constructively simple way to predefined prevent exposing parts of the heating element to air in case of low water volume due to leakage. This prevents “dry fire” when the heating element operates without being fully submerged in water leading to damage and possibly even burnout of the (back-up) heater. In addition, the heater has a profile comprising conformity of the heater according to the invention with the thermostat requirements of NF EN 60335-2-40 having a thermostat which allows manual reset. The heater according to the invention has a safety design which allows selectively for upflow or downflow operation of the (back-up) heater.

[0050] The heater according to the invention further has a safety design that is so versatile that it allows for a plastic housing and furthermore for glycol addition to the liquid wherein the hydraulic trap ensures that the housing of the heater, in particular the tank wall, is kept intact and is still fully functional even in case of a leakage. The hydraulic trap also reliably prevents unwanted tripping of the safety functionality during normal operation in this case.

[0051] Adding glycol to water in a heat pump system with a backup heater has several consequences: Glycol is added to the water for freeze protection. Glycol lowers the freezing point of the water-glycol mixture, preventing the system from freezing in cold temperatures. This is particularly useful in climates where temperatures can drop below the freezing point of water. Glycol solutions often contain corrosion inhibitors that protect the system components from corrosion, extending their lifespan.

[0052] However, glycol has lower thermal conductivity compared to water, which means the heat transfer efficiency is reduced. This may require adjustments in system design to maintain optimal performance. Glycol increases the viscosity of the mixture, which can affect the flow rate and pressure drop in the system. This may necessitate larger pumps and pipes to ensure adequate flow. Due to the reduced thermal conductivity, a larger heat source (e.g., heat pump, boiler) may be needed to compensate for the loss in heat transfer efficiency. Larger recirculation pumps may be required to handle the increased viscosity and friction losses. Larger pipe sizes may be necessary to minimize pressure drops and ensure optimal flow rates. Glycol solutions need to be monitored regularly to ensure the correct concentration and to check for any signs of corrosion or degradation. However, the present invention allows in a constructively easy and reliable manner to provide for a heater which is versatile and safe also with regard to allowing for the addition of glycol. The heater according to the invention is thus versatile also in view of allowing for the addition of glycol whilst ensuring safe functionality.

[0053] The heater according to the invention can also have the same overall design of a standard stainless steel heater and can thus easily be used to replace such a standard heater. The heater according to the invention is thus a safe, versatile and sustainable heater having the characteristics above, in particular being safe with regard to the risk of dry fire even in case of a leakage,

[0054] Accordingly, the safety and reliability of the heater as to the heating may be improved and the heater may in particular be safer in case of shortage of fluid in the tank and, thus, more reliable than a prior art heater, while offering versatility.

[0055] Whenever reference is made “a” hollow element in the following, this may relate to only the first or second hollow element and, alternatively, to both the first and second hollow elements.

[0056] Heating means may refer to one heating element or may include several heating elements, which are preferably similar to each other. The function of a heating means (heating elements) is to heat the liquid in the tank.

[0057] A hollow element extends (basically) in the longitudinal direction, which means that the hollow element may exclusively extend in the longitudinal direction or that the hollow element may extend i.e. in a direction which has at least a component in the longitudinal direction.

[0058] The longitudinal direction may basically run in a direction opposite or parallel to the direction of gravity. The orientation may not be of relevance. However, in the drawings, the longitudinal direction is indicated as running opposite to gravity.

[0059] Preferably, the longitudinal direction may (substantially) run opposite to the direction of gravity. The orientation of the longitudinal direction is opposite to gravity, while the longitudinal dimensioning is parallel to gravity. The longitudinal direction may (substantially) be the vertical direction. As such, the heater may preferably be installed vertical on site, but a small angle to the vertical is also possible. The opening of each hollow element may be open in basically a transverse plane perpendicular to the longitudinal direction, meaning that the opening faces the longitudinal direction or its opposite direction. The opening of the hollow element may be seen as a termination (of the impact) of the hollow element with respect to the sub-volume. The opening of the hollow element may represent an open end of the hollow element. The opening fluidly connects the liquid sub-volume as defined by the hollow element with the remaining liquid volume in the tank.

[0060] For separation of the liquid sub-volume from the remaining liquid tank volume upstream the opening, the respective hollow element is preferably configured so as to substantially exclude fluid communication with the remaining liquid in the tank except for at the opening. In particular, substantial separation of fluid may mean that the fluid in the hollow element does substantially not mix with the remaining fluid in the tank outside the hollow element. For example, if a hollow element is connected to the first port, fluid communication with the remaining liquid in the tank upstream the opening may substantially be excluded. It is preferable that the end of a hollow element opposite the opening is substantially sealed relative to the remaining liquid sub-volume. However, it is not excluded that there is a hole or channel or bypass for pressure equalization in the hollow element at the end opposite the opening of the hollow element. Accordingly, the separation of the sub-volume from the remaining volume of the tank is not necessarily a complete separation, but a substantial separation to the effect that the substantial flow of liquid is substantially guided / received by the hollow element. The separation of the sub-volume from the remaining liquid volume allows for guidance of the subvolume and / or for retention of the liquid from draining off the lower port of the tank. Hence, the separation allows to avoid undesired emptying of the tank and, thus, exposure of the heating means, and / or for guidance of substantial liquid along the desired path, i.e. along the heating elements for absorption of heat by the liquid. A hollow element defines a cavity for receipt / guidance of liquid, here in particular of basically a sub-volume of the liquid in the tank. Each of the hollow elements may be a tube. Liquid may be stored in the cavity or may flow through the cavity. The hollow element does not need to define a cavity which is closed except for the opening. Rather, the hollow element may be configured to guide fluid along a longitudinal extension of the cavity.

[0061] The hydraulic / liquid circuit refers to the circuitry of a heating system with liquid (water) to be heated. The liquid may not need to be pure water, but may include additives, such as glycol.

[0062] When the heating system is a heat pump, this liquid is heated by the refrigerant and will be used via a closed-loop system to heat domestic hot water or for heating applications (floor heaters...). When the heating system is an electric heater, in particular instantaneous electric water heater, this liquid is heated by the electric heater and is consumed via an opened-loop system for domestic applications (showers...). Specifically, in constellations in which the tank would empty basically completing through drainage of liquid through the lower port upon leakage of liquid in the liquid circuit, a hollow element may be configured and located within the tank so that the tank does not empty completely via the lower port. Here, in case of leakage in the circuit and / or pump in the circuit off or damaged, the liquid will use the port which is the lowest (which can be the inlet or outlet port). In other words, the first and second ports may be used as inlet or outlet port, while, with respect to drainage and emptying of the tank, the lower port of the first and second ports, when installed on site, would function as drainage port (and, thus, as outlet port, irrespective of whether the lower port had been used as outlet or inlet port during the normal use). The first and second ports may serve for passage of liquid of the liquid circuit into and out of the tank, respectively. Alternatively, they may serve for passage of liquid of the liquid circuit into out of and into the tank, respectively. In some embodiments, the first and second ports may have both functions, but it is conceivable that in some embodiments each port may only have a single function, namely as outlet or inlet. An embodiment of the invention may optionally be described as follows: At least the first hollow element retains liquid in the tank at a position higher than the lowest point of the heating surface of the heating elements, when the heater is installed on site, and prevents liquid from draining off the lowest port of the first and second ports of the tank. A second hollow element may guide fluid along at least a part of the heating element(s), to avoid direct flow of the fluid to the outlet port, possibly avoiding at least a part of the heating means.

[0063] The present invention is directed to a heater for heating liquid, in particular water. A preferred configuration of the invention is a back-up heater for heating water. A back-up heater is a heater used as a back-up solution to heat liquid / water in the event of heat pump unit insufficiency (e.g. in case of insufficient power, outside the heat pump's operating range, very cold temperatures outside, heat pump breakdown / fault heat etc.).

[0064] The heating means may, at least most of its parts, be in direct or indirect contact with the liquid to be heated. For example, the heating elements of the heating means may completely be surrounded by the liquid, e.g., immersed in the liquid.

[0065] Optionally, the heater is configured such that the at least two hollow elements and the first and second ports are arranged such that the lower port of the first and second ports or the opening of at least one of the at least two hollow elements is located above at least 70%, preferably 90% of the length of the heating elements in the longitudinal direction, when the heater is installed on site, so that at least a part of the heating means remains immersed in liquid in the tank, in case of shortage of liquid in the tank; and / or at least one of the at least two hollow elements has a length in the longitudinal direction of at least 50%, preferably of at least 80% of the length of the heating element(s) in the longitudinal direction, and the heater is configured such that the one of the at least two hollow elements and the heating means are arranged such that the one of the at least two hollow elements guides liquid (at least a part of the liquid; preferably substantially the entire liquid) along at least a part of the respective hollow element for flow of liquid along the heating element(s). Optionally, the opening of the one hollow element is above the lowest point of the at least one heating element, when the heater is installed on site.

[0066] Optionally, the first hollow element and the second hollow element at least partially overlap each other in the longitudinal direction and are distanced from each other in a transverse direction perpendicular to the longitudinal direction, and / or optionally extend in parallel and / or are tubular (cylindrical) or conical and / or are concentrically arranged. If the first and second hollow elements overlap each other at least partially in the longitudinal direction, the flow of liquid and storage of liquid may be improved as to the efficiency / protection of the heating means. Preferably, the first and second hollow elements are distanced from each other in the transverse direction, which is perpendicular to the longitudinal direction, so as to allow for flow of liquid in and outside of the hollow elements. In particular, the flow of liquid may specifically be guided by the hollow elements.

[0067] In one embodiment, the first hollow element and the second hollow element may represent an inner and an outer hollow element, respectively, or vice versa. This means that the first hollow element may be located at least partially inside the second hollow element, or vice versa, and / or that the first hollow element may have a smaller cross-sectional area (e.g. smaller diameter) than the second hollow element (e.g. larger diameter).

[0068] The hollow elements may each represent a tube, optionally extending in the longitudinal direction. The first hollow element and the second hollow element in a tubular configuration may have the same or preferably different radii. A first tube having a smaller radius may, seen in the radial direction, i.e. the transverse direction, be within the outer tube having a larger radius. Optionally, the tubes may extend at least partially, in parallel relative to each other and in the longitudinal direction. The first and second tubes may be arranged concentrically, in particular concentric relative to the center of the tank, seen in the transverse direction.

[0069] An embodiment of the invention may optionally be described as follows: The first hollow element may retain liquid in the tank at a position higher than the lowest point of the heating surface of the heating elements and prevent liquid from draining off the lowest port of the tank. Optionally, in the inverted orientation of the heater, the second hollow element may retain liquid in the tank at a position higher than the lowest point of the heating surface of the heating elements and prevent liquid from draining off the lowest port of the tank.

[0070] The flow direction in a heater according to the invention can also be defined by upflow and downflow orientation. Upflow and downflow orientation in the context of a (back-up) heater refer to the direction in which the fluid, such as a liquid, for example water, flows through the heating system. Upflow in this context thus refers to cold water entering the system at a lower point, typically near the bottom of the (back-up) heater, the water flows upwards through the heating chamber surrounding the heating element(s). The water absorbs the heat from the heating element(s) increasing in temperature while the water flows upwards. The heated water reaches the top of the tank and is then directed to the hot water outlet. Downflow refers to cold water entering the (back-up) heater at a higher point, such as near the top of the (back-up) heater. The fluid, such as a liquid, for example water, flows downwards through the heating chamber surrounding the heating element(s) of the (backup) heater. As the fluid flows downward, it absorbs heat from the heating elements, increasing in temperature. The heated fluid, such as water, reaches the bottom of the (back-up) heater and is then directed to the hot fluid outlet, e.g. the hot water outlet. The upflow design takes advantage of the natural tendency for the hot fluid, such as water, to rise, ensuring that the hottest fluid, such as water, is available at the top of the (back-up) heater. The downflow design might be used in systems where the hot fluid outlet, for example water outlet, is located at the bottom. The downflow design ensures that the fluid, such as a liquid, for example water, is evenly heated as it flows downward. The flow direction is crucial for optimizing the efficiency and performance of the (back-up) heater.

[0071] A design of a (back-up) heater which allows to be used for upflow and downflow can also be referred to as bidirectional.

[0072] Optionally, the first port may be located in a first end section of the tank, and the second port may be located in a second end section of the tank, substantially opposite to each other, seen in the longitudinal direction, and lower than the first end section, when seen in the longitudinal direction, when the heater is installed on site.

[0073] A hollow element may extend from the second port and / or the tank base / bottom (upwards) in the longitudinal direction to a retaining level at which the opening is located, when viewed in the longitudinal direction, when the heater is installed on site, wherein the heater may be configured such that the outside of the first hollow element and an outer counterpart (e.g. tank wall) retain liquid in the tank and prevent liquid from draining off the outlet / second port. Accordingly, a structural element of the hydraulic trap may be formed between a hollow element and the counterpart, such as the tank wall. Here, the first hollow element may represent an (upward) extension or elongation from the outlet / second port to the opening. Accordingly, liquid is retained from draining through the outlet port up to the height of the opening of the first hollow element.

[0074] Optionally, the heating means is at least partially located below the retaining level and seen in the transverse direction, between the first hollow element and the outer counterpart, optionally the tank wall, such that the heater is configured to at least partially keep the heating means immersed in liquid, if the liquid circuit is interrupted and liquid drains off the outlet port. Accordingly, direct transfer of heat from the heating means to the liquid can at least partially be maintained, so as to avoid overheating of the heating means within any response time required by a safety mechanism. In this case, a hollow element is in direct fluid communication with the outlet / second port, wherein the outer surface of the first hollow element, together with the outer counterpart act as structural elements of the hydraulic trap retaining liquid from draining of the outlet / second port in the height of the opening.

[0075] Optionally, the opening of the second hollow element defines a guiding level, when viewed in the longitudinal direction, when the heater is installed on site, wherein the second hollow element is at least partially, seen in the longitudinal direction, located between the first hollow element and the tank wall, seen in the transverse direction, and the heater is configured to guide flow liquid in the tank from the first / inlet port downwards along at least a part of the second hollow element to the guiding level, then upwards at least to the retaining level along at least a part of the heating means and along at least a part of the first hollow element and the second hollow element, and then downwards and inside the first hollow element to the outlet / second port. Accordingly, by way of the first and second hollow elements, the liquid in the tank is guided, so as to improve transfer of heating from the heating means to the liquid and to allow for liquid to be retained in the tank at least partially surrounding the heating means in case of leakage through the outlet / second port.

[0076] In another embodiment, when the first / inlet port may optionally be located in a first end section of the tank and the second / outlet port is located in a second end section of the tank substantially opposite to the first end section, seen in the longitudinal direction, and lower than the first end section, when viewed in the longitudinal direction, when the heater is installed on site, the first hollow element extends upwards from the tank bottom in the longitudinal direction and has the opening, which defines a retaining level, when viewed in the longitudinal direction, when the heater is installed on site, wherein the heater is configured to retain liquid in the tank at the retaining level inside the first hollow element, if the liquid circuit is interrupted and liquid drains off the second / outlet port. Accordingly, the inside of the first hollow element acts as a part of the hydraulic trap. In this case, the second / outlet port is not in direct fluid communication with the inside of the first hollow element.

[0077] Optionally, the heating means is at least partially located below the retaining level and, seen in the transverse direction, inside a hollow element, such that the heater is configured to at least partially keep the at least one heating means, preferably all heating elements, more preferably the entire heating means, immersed in liquid in case of shortage of liquid in the tank. This helps to void situation in which the heating means basically ceases to transfer heat to the liquid and transfers heat to air within the tank.

[0078] Optionally, the heater comprises a second hollow element, and the opening of second hollow element represents a guiding level, when viewed in the longitudinal direction, when the heater is installed on site, wherein the second hollow element is at least partially, seen in the longitudinal direction, located inside the first hollow element, seen in the transverse direction, and the heater is configured to flow liquid in the tank from the first / inlet port downwards along at least a part of the second hollow element to the guiding level, then upwards at least to the retaining level along at least a part of the heating means and then downwards and outside the first hollow element to the second / outlet port. Accordingly, by way of the first and second hollow elements, the liquid in the tank is guided, so as to improve transfer of heat from the heating means to the liquid and to allow for liquid to be retained in the tank at least partially surrounding the heating means in case of leakage, e.g. through the second / outlet port or somewhere else in the hydraulic circuit.

[0079] Optionally, the tank wall and / or the tank end may comprise at least two guiding features / shapes such as recesses, extruded parts and / or cavities, to (further) improve liquid flow distribution inside the tank by reducing / avoiding dead zones. An example of such an embodiment may be an entity comprising first and second hollow elements. The first and second hollow elements are substantially tubular, wherein the second hollow element represents an inner tube, and the first hollow element represents an outer tube, in which the inner tube is concentrically received. The inner tube is sealed to the tank end. At the interface between the tank end and the tank wall, guiding features with e.g. spoon-shaped buckets are provided to allow for better whirling of the fluid inside the tank. Thanks to these (optionally: three) guiding features / shapes (for instance extruded parts, recesses or cavities), a better flow distribution inside the tank may be achieved and the path of the liquid along the heating means may be increased.

[0080] Optionally, an end of a hollow element opposite to the opening in the longitudinal direction abuts an end of the tank and is optionally sealed to the end. While the tank end may be at an upper or lower end of the tank, an end of a hollow element may about the tank end, in particular in the transverse direction, i.e. perpendicular to the longitudinal direction. The tank end may be understood as tank bottom or tank head.

[0081] Optionally, the tank end may be included in the flange of the heater. Optionally, the heating means may extend from the flange towards the inside of the tank. Optionally, the end of a hollow element is sealed to a tank end, in particular to the effect that the opening of a hollow element represents the substantially only fluid communication with the remaining liquid for the subvolume.

[0082] Optionally, in particular preferably, the tank according to the invention is made of (thermo)plastics, preferably PPS, and / or the first and / or second hollow elements are made of metal, in particular stainless steel or copper. Further optionally, a hollow element (the first or second hollow element, e.g. the outer hollow element) may be made of stainless steel, as this allows for welding to the flange (which also comprises stainless steel), and the second hollow element (first or second hollow element, e.g. the inner hollow element) may be made of copper, which may be preferable in terms for manufacture. The outer hollow element may be understood here as a hollow element closer to the tank wall than to the inner hollow element. In general, making the tank and / or the first hollow element and / or the second hollow element of plastics may reduce the costs and may allow for versatile functions. The tank and / or at least a part of the flange may be made of plastics. The tank may be formed as a single piece of plastics, at least the tank shell and the tank end. A “cap” of the tank, e.g. flange, may be separate from the remaining tank. Nevertheless, temperature restrictions need to be borne in mind. The outer hollow element has also a benefit in terms of safety, as it reduces the risk of tank melting in case of temperature rise inside the tank in particular if the tank is made of plastics. Indeed, the hollow elements acts as a physical barrier between the heating means and the tank wall in particular if the tank is made of plastics.

[0083] Optionally, the heating means is a resistive heating means. In other words, the heating means may be an electrical heating means. The heating means may have helicoidal or U-shaped heating elements, wherein this may in particular relate to the resistive path.

[0084] However, the heater of the invention is optionally adaptable to both electric and hydraulic back-up configurations. This means that, instead of an electric, i.e. resistive, heating means, the heating means may rely on hydraulic heating. The latter may mean that the liquid (water) of the main circuit, i.e. entering / exiting the first and second ports, may mix with water of a secondary (hot water) circuit or that the secondary (hot water) circuit is also heated by the electric heating means.

[0085] Optionally, the heater may comprise an external insulation layer.

[0086] The liquid circuit may represent a main water flow circuit. A secondary water flow circuit may flow between secondary water flow ports also through the tank and may also be heated by means of the heating means.

[0087] Optionally, one of the first and second ports is at the (side) wall (shell) of the tank and the other one of the first and second ports is at a tank end. This allows for a simple shape of the hollow elements, e.g. as straight tubes and, thus, for easy manufacture. For example, no bending or corners of the tube may be necessary.

[0088] The invention is also directed to a heat pump system comprising the heater of the invention. Specifically, the heat pump system of the invention comprises a liquid circuit comprising a heat pump unit for heating (optionally also for cooling liquid) and further comprising the heater of the invention, wherein the heater of the invention optionally represents a back-up heater for (selective) additional heating of the liquid of the liquid circuit.

[0089] The invention, in general, refers to any kind of heat pump system, including at least one ground source (water)- and / or air source-heat pump unit. The heat pump units may be indoor or outdoor units. A heat pump unit of the heat pump system may be a split unit or a monobloc unit. The heat pump system may be configured to heat air or water (of a closed loop or of an open loop, such as domestic hot water), while the heater as part of the system may heat liquid (e.g. water) directly, possibly for later heat exchange with air or another liquid.

[0090] The invention further relates to a method of operating a heat pump system according to the invention comprising a heater according to the invention, wherein the heat pump system comprises a control unit and / or a remote control unit and wherein a thermostat of the heater is configured to communicate with the control unit and / or the remote control unit, the method comprising the steps of: monitoring a temperature of a liquid in the heater via the thermostat, generating a control signal of the thermostat in case the monitored temperature reaches a predetermined setpoint, transmitting the control signal of the thermostat to the control unit and / or the remote control unit, receiving the control signal of the thermostat at the control unit and / or the remote control unit, and generating a further control signal of the control unit and / or remote control unit to activate an adjustment or shutoff mechanism via the control unit and / or the remote control unit in response to the control signal of the thermostat for adjusting or stopping the operation of the heater and / or of the heat pump system.

[0091] Monitoring the temperature of the liquid in the heater via the thermostat within the meaning of the application means that a temperature sensor of the thermostat is in direct or indirect contact with the liquid in the heater. This sensor detects the current temperature of the liquid either directly or indirectly. If the thermostat is located at the end of the tank and / or at a first hollow element or a second hollow element of the heater, it may remain preferably immersed in liquid, even in case of leakage of liquid through the second / outlet port of the tank and, accordingly, continue to provide reliable temperature measurement results. In particular, if the thermostat is immersed in liquid, thermal conduction and, thus, temperature detection, is quick. However, it is not necessary to have the thermostat immersed in the liquid in case of a leakage: The thermostat may be above the retained liquid level and may nevertheless identify leakage because of the steam generated by the heating elements. So, in other words, the thermostat would measure the temperature of the steam generated. Further, the thermostat is not necessarily in direct contact with the liquid, but it may be included in a sleeve immersed in the liquid, to provide for an indirect measurement. The temperature sensor of the thermostat, preferably continuously, measures the temperature and sends, preferably real-time, data to a control unit. The thermostat can comprise a control unit of the thermostat. in case the monitored temperature reaches a predetermined setpoint, the thermostat generates the control signal, in other words an emergency control signal of the thermostat, which will prevent the overheating and damage of the heater. At the same time the inventive heater comprising the hydraulic trap allows for normal heating of the liquid during normal operation without unwanted disruption of operation due to unwanted triggering of such an emergency stop signal. In other words, the control signal of the thermostat comprises a command to adjust or stop the operation of the heating means.

[0092] The term control unit refers to a local control unit of the heat pump system. The local control unit can be a control unit of the heater and / or a control unit of the heat pump system.

[0093] The control unit and / or the remote control unit compares the measured temperature to a predetermined setpoint for example a desired temperature. Based on the comparison, the control unit and / or the remote control unit generates the further control signal. If the liquid temperature deviates from the setpoint, the further control signal of the control unit and / or the remote control unit prompts adjustments to or the stop of the heater and / or the heat pump system. In other words, the further control signal of the control unit and / or the remote control unit comprises at least a command to adjust or stop the operation of the heating means and / or deactivates the heater and / or adjusts or stops the operation of the heat pump system. In other words, the further control signal of the control unit and / or the remote control unit can illustratively be called a thermal adjustment or stop command.

[0094] The control unit can comprise a printed circuit board or be a printed circuit board (PCB). The control unit can further comprise a microprocessor or processor, which can be described as the brain of the control unit which processes data and executes control algorithms. The control unit further comprises sensors and input devices like temperature sensors, such as the thermostat 13, buttons or switches which input data into the control unit. The control unit can further comprise actuators and output devices, thus components such as relays, motors, or LEDs which the control unit activates to perform specific tasks. The control unit can further comprise communication interfaces, such as modules for wired communication (for example ethernet, or RS232) or wireless communication (for example Wi-Fi or Bluetooth). The control unit, in particular the PCB, further can comprise conductive pathways connecting the various components, enabling the flow of electrical signals. The control unit can further comprise components to regulate and distribute power to the different parts of the control unit. For example, voltage regulators can ensure a stable power supply for the control unit, such as to the microcontroller and other components. The microcontroller on the control unit can further run software code which governs the operation of the control unit. The software can comprise logic for data processing, decision making, and control actions.

[0095] The setpoint can be a predetermined temperature, and / or a temperature range and / or the setpoint can also be a temperature development over time, optionally also considering the performance of the heater. In other words, the temperature development can for example be a temperature increase over time which looks at the relative behavior of the temperature. To illustrate this, in case the temperature starts to rise at a predetermined speed, the speed of temperature increase can be a predefined setpoint. Thus, the setpoint can be an absolute value, a value range or a relative value. In case of the setpoint being a temperature range, the setpoint can be reached in case the detected temperature of the liquid inside the tank of the heater falls outside of a predefined temperature range. In other words, the temperature deviates from the predefined range which can be too high or too low, however for the purpose of the invention will typically be chosen to fall outside the range when the temperature is too high. The thermostat detects this discrepancy. Upon detecting that the temperature is outside the desired range, the thermostat generates the control signal of the thermostat. The control signal of the thermostat can illustratively be called an emergency signal.

[0096] As an example for a predetermined setpoint temperature, the thermostat setpoint can be set to trigger, in other words generate the control signal of the thermostat, such as for example a command for an emergency stop of the operation of the heating means, if the detected temperature reaches or exceeds for example a temperature selected from the range of degrees comprised in 80 °C to 120°C, in particular 80°C to 115 °C, in particular 85°C to 115 °C. The degrees can be chosen in said range per degree, thus for example 80°C or 81 °C until 120°C, or for example in tenth of a degree steps between 80°C and 100°C, or for example in half degree steps, such as 80 °C, 85°C and so on until 100°C or in 5 degree steps, such as 80 °C, 85°C continued until to 120 °C, all steps in the example sequences are included and each temperature within the sequences can be chosen as appropriate for the respective heater. The setpoint temperature can include a tolerance, for example ± 6°C. For example, 85 °C with a tolerance of ± 6°C is chosen. At the higher end, for example also a setpoint temperature of 115 °C can be chosen. The response time of the thermostat of the hydraulic trap depends on this selection and is typically between 7 and 15 seconds. Fast thermostats can have a response time of 3 to 5 seconds. The formula for determining the setpoint can be formulated as performance of heater and taking into consideration the temperature of the water T initially Tini and needs to fulfill Tplastics < T melting as long as the heating time t is < time to reach the maximum allowed heater temperature tmax in dry conditions. For example, in case of a 9 kW heater: initial Temperature 30°C, the tmax would be < 15 seconds. So, for the example 9 kW heater, Tini 30°C, Tplastics < T melting if tmax 15 seconds. At the time of reaching the predetermined setpoint temperature, the temperature of the heating means is under the 400 °C of the dry heating of Fig. 9a for typical heating means for heaters according to the invention due to the cooling effect of the retained water in the system and / or in case the heater is not heating at full capacity. For example, if the heater is heating at only 3kW the time to reach the max heater temperature under dry conditions is for example < 15 to 30 seconds. So, the system remains intact in case of a leakage leading to a too low water volume in the tank of the heater.

[0097] So, the setpoint can for example be predefined based on the performance of the heater (Pheater in kW), the initial temperature (Tini), the time (tmax) at which the maximum allowed surface temperature of the heater (Tmaxheater) in dry conditions is reached to ensure that the temperature (Tplastics) of the tank wall is remains below the melting temperature (Tmelting) of the tank wall, in case the tank wall is made of (thermo-)plastics. In addition, the influence of the geometry of the heating means on heat transfer by radiation and natural convection can be taken into account. This allows configuring the setpoint (Tthermostat) such that the heater’s performance under normal conditions is not affected taking the parameters of the respective heater and heat pump system into consideration and ensuring the safety of the plastic components of the heater.

[0098] The dry heating curve of a suitable heating means is shown for example in Fig. 9a. The thermostat which commonly monitors the heating means cannot trigger at the temperature reached after 15 seconds as this would prevent the functionality of the heating means. The thermostat which monitors the heating means will typically trigger after 45 seconds, at which time the heating means has a surface temperature just under 1000 °C in dry conditions. This is shown in Fig. 9a and 9b.

[0099] Transmitting the control signal within the meaning of the application refers to the mode of transmission of the respective control signal, such as the control signal of the thermostat and / or the further control signal of the control unit and / or remote control unit. The transmission can be via a physical connection. So, in other words, the control signal can be transmitted through electrical wiring. The control signal travels, illustratively, from the thermostat to the control unit via dedicated wires. The transmission can also be wireless. So called smart heat pump systems can communicate using wireless communication methods such as Wi-Fi, Zigbee, or Bluetooth. In other words, as an example, the thermostat can transmit the control signal of the thermostat wirelessly to the control unit and / or to the remote control unit. The wireless communication can be in both directions or in one direction.

[0100] The control unit and / or the remote control unit receives the control signal of the thermostat of the heater. The control unit and / or the remote control unit respectively is configured to comprise a receiver which is compatible with the transmission method used by the thermostat. The control unit and / or the remote control unit in other words receives the control signal of the thermostat, processes the control signal and generates a further control signal, thus the control signal of the control unit and / or of the remote control unit. The control signal of the control unit and / or of the remote control unit can be configured to instruct the heater and / or the heat pump system to adjust, in other words increase, decrease, maintain or to stop the respective heating operation. This operation can be an adjustment of the operation of the heat pump system or stopping of the operation. The respective further control signal can be predefined to comprise a command to adjust or stop the output and / or operation of the heater to ensure the liquid does not heat further beyond a second predefined setpoint, in other words a second threshold value. In other words, the control unit and / or the remote control unit translate and execute the control signal of the thermostat comprising the command to adjust or stop the operation of the heating means. Thus, for example a control unit of the heater and / or the local control unit of the heat pump and / or the remote control unit receives the further control signal. The local control unit of the heat pump can have a master slave configuration. The further control signal can be received by the master and / or by a slave control unit to execute.

[0101] The remote control unit within the meaning of the application typically comprises an interface, such as a user interface. This is also referred to as a human machine interface. A Human-Machine Interface (HMI) is a system or device that enables interaction and communication between a human operator and a machine, system, or device. Such a user interface, in particular HMI, comprises for example buttons, a display, such as for example a screen, in particular a touch screen, which shows information. The remote control unit further comprises input devices which allow the user to input commands and interact with the control unit. The remote control unit can further comprise feedback mechanisms, audio, visual or haptic feedback to inform the user about the status or response to the input. The remote control unit can further comprise software configured to manage the interaction between the user and the machine, interpret inputs, and execute commands. Further the remote control can comprise visualization software, such as for the provision for graphical user interfaces (GUIs) which display data such as via a software application on e.g. a computer or smartphone. Furthermore, the remote control unit can comprise hardware and software for communication protocols, thus protocols and standards that enable communication between the HMI and the machine or system (for example Modbus, Enthernet / IP, or the like). This has the advantage that the thermostat can be real-time monitored and remotely controlled, for example by the user or an installer.

[0102] The remote control unit can be configured to communicate with a (local) control unit of the heater and / or the heat pump system. The control unit of the heater and / or the heat pump system receives the signal from the remote control unit, processes the command, and adjusts the heating means of the heater and / or other system components of the heat pump system accordingly. For example, the remote control unit, upon receiving for example a high- temperature alert signal of the thermostat, sends a command to the heat pump system's control unit to shut down the heater. This provides an additional layer of safety by ensuring the heater is turned off remotely if temperatures become dangerously high. As an example, for illustration, a heat pump system, in a remote location or hard-to-access area, can benefit from the method according to the invention: In case the thermostat detects an overtemperature condition. The control signal of the thermostat, which in other words can also be called a high-temperature alert, is sent to a remote control unit, which could be a smartphone app or the monitoring system of a monitoring service provider. The control signal of the thermostat can optionally also be sent to the local control unit of the heat pump system. The user and / or the monitoring service receives an immediate notification and can manually or automatically check that the further control signal of the control unit and / or the remote control unit is sent to adjust or stop the operation of the heater and / or the heat pump system. Illustratively, even the user or monitoring service could manually or automatically send the further control signal, which can be a shutdown command for the heater and / or for the heat pump system. The heat pump system's local control unit receives and / or the control unit of the heater receives the shutdown command from the remote control unit and for example adjusts or deactivates the heater, preventing further temperature rise. By incorporating remote control capabilities for safety features, the heat pump system can allow for greater protection against overheating and ensure additionally enhanced user safety. This can enhance in a straightforward way the safety, for example in case the local control unit did not process the control signal of the thermostat. By involving both the control unit and the remote control unit, a fail-safe mechanism is created. If one control unit fails to act on the control signal of the thermostat, the other control unit can still take the necessary action, ensuring that the heater and / or the heat pump system is adjusted or stopped which can comprise even shutting down, when needed. The remote control unit can provide an additional layer of monitoring. The remote control unit can continuously assess the performance and status of the heat pump system and send commands if any anomalies are detected. This ensures that the heat pump system operates efficiently and any deviations from the respective predefined setpoint are promptly corrected.

[0103] In case the control signal of the thermostat is sent only to the remote control unit or additionally to the remote control, this has the advantage that the remote control unit can be programmed with additional logic or conditions for shutting down the heater and / or heat pump system. This offers more flexibility in how the heat pump system is controlled and ensures it responds appropriately to a wider range of scenarios. The remote control unit can thus work as an extension of the local control unit of the heat pump system which may not have all the functionality. This allows for retrofitting the heater according to the invention in existing heat pump systems.

[0104] Thus, the method according to the invention can optionally allow for the ability to remotely stop or adjust the thermostat and / or the heat pump system to ensure the safety of the heater and / or the heat pump system, in particular in case of a leakage. The method according to the invention can further comprise a step of generating and executing a diagnostic tool, and / or generating and executing reports, alerts, and / or notifications relating to the leakage and the heat development, in particular the status of the heater, in the heat pump system.

[0105] The method can further comprise a step of generating a report, log and / or securing data for analysis, decision making and / or improvements to the heater and / or heat pump system.

[0106] The method according to the invention enables quick response to overheating situations, minimizing the risk of damage or accidents. The method according to the invention further allows for improved safety of a (back-up) heater in a heat pump system wherein the constructively simple design of the hydraulic trap allows for a heater which has the overall dimension of standard stainless steel heater to have a plastic housing while ensuring that the heater cannot overheat in case of a leakage, making use of the functionality of the hydraulic trap. The method according to the invention further allows controlling the heating means during normal operation as typically done, and using the thermostat required by NF EN 60335-2-40 to control the heater safety in such a way, that the heater can be designed to retro-fit stainless steel heaters. Thus, the hydraulic trap and the method according to the invention allow also for a more sustainable heater which can be retrofitted in heat pump systems replacing stainless steel (back-up) heaters.

[0107] The method according to the invention further can optionally allow users to monitor and / or control the heater from a distance, enhancing convenience and peace of mind. Thus the method optionally comprises a step of monitoring and / or remote controlling the heater and / or the heat pump system.

[0108] The method according to the invention can optionally be integrated with smart home systems for automated alerts and control, further enhancing safety and usability. Thus, the method further can comprise an integration step to transmit the control signal of the control unit and / or of the remote control unit to a smart home system.

[0109] The method according to the invention can be a continuous feedback loop, where the thermostat constantly monitors the temperature to maintain optimal heating conditions.

[0110] Optionally the method can further comprising a step of checking the heat pump system and / or a step of resetting the thermostat prior to restarting the heater and / or the heat pump system.

[0111] The method can optionally further comprise monitoring additional sensors of the heat pump system, in particular mass flow sensors, volume flow sensors, additional temperature sensor and / or pressure sensors,

[0112] The invention is further directed to a data processing device comprising means for carrying out the method according to the invention.

[0113] The invention is further directed to a computer program product comprising instructions to cause the heat pump system according to the invention to execute the steps of the method according to the invention.

[0114] The invention further relates to a computer readable data carrier having stored there on the computer program product according to the invention.

[0115] The invention further relates to a data carrier signal carrying the computer program product according to the invention.

[0116] The invention further relates to the use of a heater according to the invention in a heat pump system, in particular according to the invention, and / or in a method according to the invention and / or in controlling the safety of a heat pump system, in particular a heater within the heat pump system via a control unit of the heat pump system locally and / or remotely, in particular via a local control unit and / or via a remote control unit.

[0117] This has the advantage that the heater which is integrated into a heat pump system to provide supplementary heating when required is controlled based on the control signal of the thermostat of a hydraulic trap of the heater, ensuring fast response to a rise in temperature, in particular in case of a leakage in the heat pump system. The heater thus includes a constructively simple safety feature which allows for the safe operation of the heater and additionally can optionally allow remote monitoring and / or control, enhancing convenience and peace of mind for users. The method includes steps for generating and transmitting the further control signal of the control unit and / or remote control unit to the heater, ensuring that the heater in operation is safe even in case of a leakage and allowing for the heater to be more sustainable by allowing a heater versatile, retrofittable heater comprising a plastic housing.

[0118] Detailed embodiments and further advantages and features related to the present invention are described in the following, wherein these examples shall not be regarded as limiting the invention.

[0119] The invention also includes further developments of the inventive heater that feature characteristics described in relation to the further developments of the inventive heat pump system or the inventive method, the data processing device, the computer program product, the computer readable data carrier, the data carrier signal or the use. For this reason, the corresponding further developments of the inventive heater or the heat pump system, method, the data processing device, the computer program product, the computer readable data carrier, the data carrier signal or the use are not described again elsewhere.

[0120] The invention also encompasses combinations of the features of the described embodiments and aspects. Therefore, the invention also includes implementations that each feature a combination of the characteristics of several of the described embodiments and aspects, provided the embodiments or aspects are not described as being mutually exclusive.

[0121] Brief description of the drawings

[0122] Fig. 1 (a) shows a cross-sectional view of a heater of an embodiment of the invention, Fig. 1 (b) shows a cross-sectional view an entity of a heater of the invention, and Fig. 1 (c) shows a perspective view of the outside of a heater of the invention.

[0123] Fig. 2(a), 2(b) and 2(c) show cross-sectional views of a heater of an embodiment of the invention in a first orientation and first flow direction, wherein Fig. 2(a) shows the structure, Fig. 2(b) additionally indicates the flow of liquid, and Fig. 2(c) additionally indicates the level of liquid as retained.

[0124] Fig. 3(a), 3(b) and 3(c) show cross-sectional views of a heater of an embodiment of the invention in a first orientation and second flow direction, wherein Fig. 3(a) shows the structure, Fig. 3(b) additionally indicates the flow of liquid, and Fig. 3(c) additionally indicates the level of liquid as retained.

[0125] Fig. 4(a), 4(b) and 4(c) show cross-sectional views of a heater of an embodiment of the invention in a second orientation and first flow direction, wherein Fig. 4(a) shows the structure, Fig. 4(b) additionally indicates the flow of liquid, and Fig. 4(c) additionally indicates the level of liquid as retained.

[0126] Fig. 5(a), 5(b) and 5(c) show cross-sectional views of a heater of an embodiment of the invention in a second orientation and second flow direction, wherein Fig. 5(a) shows the structure, Fig. 5(b) additionally indicates the flow of liquid, and Fig. 5(c) additionally indicates the level of liquid as retained.

[0127] Fig. 6 shows another embodiment of a heater of the invention, wherein Fig. 6(a) represents a cross-sectional view and Fig. 6(b) is a perspective view of a lower region of the heater of an embodiment of the invention.

[0128] Fig. 7(a) shows a side view of an upper part of a tank of an embodiment of the invention, and Fig. 7(b) shows a perspective view of a tank of an embodiment of the invention.

[0129] Fig. 8 shows the embodiment according to Fig. 1a wherein Fig. 8 shows water levels and flow direction in case of a leakage. Fig. 9a shows a diagram of the surface temperature of a standard heating means in dry conditions over time.

[0130] Fig. 9b shows a heater with a standard heating means in dry conditions between 0 seconds and 60 seconds with the thermostat controlling the heating means triggered at 45 seconds.

[0131] Fig. 10 shows a diagram of an embodiment of the method according to the invention.

[0132] Detailed description

[0133] Figure 1 (a) shows a cross-sectional view of a heater 1 of an embodiment of the invention. The heater 1 may have a cylinder shape and so it may be understood that most of the shape of the whole heater 1 be imagined with a revolution of this view around the central vertical axis, with the exception of the elements on the side of the shell such as the port 11 . The tank 2 forms a cylinder. The heater 1 comprises the tank 2 for receiving liquid. The tank 2 has a tank wall 16 which runs along the longitudinal direction I and has a tank end 4 in the transverse direction t, which represents an upper end in the orientation of Fig. 2(a). The transverse direction t is perpendicular to the longitudinal direction I, and the longitudinal direction I is opposite to gravity, when the heater 1 is installed on site.

[0134] Two hollow elements, namely the tubes 14 and 15, are provided. More specifically, the hollow element 15 represents a first hollow element, and the hollow element 14 represents a second hollow element.

[0135] The heater 1 comprises first 10 and second 11 ports for passage of liquid into or out of the tank 2. By way of the ports 10, 11 , the heater 1 is connected to a liquid / hydraulic circuit (not shown), which circuit may be a primary water circuit. The first port 10 is on the top of the tank 2, wherein the second port 11 is in the tank wall 16 close to the bottom 17 of the tank 2. The first port 10 may function as inlet port and the second port 11 may function as outlet port, or vice versa, during normal use. The heater 1 further comprises heating means 3 which is basically located inside the tank 2 and heats liquid inside the tank 2. The heating means 3 extends in the longitudinal direction I and is, in the figures, an electric heater, with resistive heating elements running helically around an axis in the longitudinal direction I. In other words, a resistive helix forms the heating means 3 in figure 1.

[0136] In any embodiment, the heater 1 may have resistive heating elements 3, e.g. having a U-shape or helicoidal shape, i.e. in the form of a helix. For example, having different stages of helicoidal elements in the longitudinal direction I may be preferable compared to U-shaped elements for heating uniformity when controlled separately. Helicoidal shape may also be preferred for compacity reason compared to U-shaped elements. As the bending of tube may be limited for mechanical reasons, the helicoidal shape may offer denser presence of heating elements for the same volume.

[0137] Electrical connections 7 for the heating means 3 pass through the flange 8. The flange 8 represents the lower end of the heater 1 and includes the tank bottom 17.

[0138] The heater 1 further comprises secondary flow ports 12a, 12b, which may be connected to a secondary water circuit, liquid of which may also be heated by the heating means 3. Alternatively or additionally, the heater 1 may, by means of the secondary flow ports 12a and 12b serve as a "hydraulic mixing device", as the secondary water circuit may mix with the liquid circuit of the first and second ports, 10,11.

[0139] The heater 1 of figure 1 (a) (and of any other figure) comprises a thermostat 13 which is located inside a thermostat housing which extends in the longitudinal direction I, from the flange 8 of the tank 2. The thermostat may be located inside the thermostat housing’s outermost end, i.e. at the end of the thermostat housing opposite to the flange 8. Fig. 1a (and any other figure showing the heater 1) comprises a hydraulic trap 30. The hydraulic trap 30 comprises the outer tube 15 and inner tube 14 creates a sub-volume of liquid covering the heating means 3 as predefined. The hydraulic trap 30 further comprises the thermostat 13 which measures the temperature of the liquid in the tank 2. The hydraulic trap 30 is explained in more detail in Fig. 8 which shows the embodiment of Fig. 1a.

[0140] Figure 1 (b) shows an entity comprising first and second hollow elements 14, 15, the heating means 3 as well as the flange 8 in detail, of the heater of Fig. 1 (a). The first and second hollow elements 14, 15 are substantially tubular, wherein the second hollow element 14 represents an inner tube, and the first hollow element 15 represents an outer tube, in which the inner tube 14 is concentrically received. The outer tube 15 is sealed to the bottom 17 and, thus, to the flange 8 and extends upwards, until the termination of the tube forms an opening 15a. The inner tube 14 has a tube end 14b which is for abutting and sealing with an upper end 4 of the tank 2. The opposite termination of the inner tube 14 represents the opening 14a.

[0141] Figure 1 (c) shows a perspective view of a heater 1 of the invention, having the first port 10 on top, and the second port 11 at lower part of the tank wall 16, wherein the electrical connection 7 including the flange 8 is located at the bottom of the heater 1. This may correspond to a perspective view of the tank 2 shown in figure 1 (a).

[0142] Figures 2(a) to 2(c) show the same heater 1 as shown in Fig. 1 , wherein additionally the direction of flow of liquid is indicated in Fig. 2(b) and the resulting retained level of liquid is indicated in Fig. 2(c). A level Ir until which liquid is retained in the tank even in case of shortage of liquid is shown and almost extends along the entire length of the heating means 3. More specifically, in the orientation of the heater 1 in figure 2(a), the flange 8 of the heater is at the lower end. This corresponds to the orientation of the entity shown in figure 1 . When connected to a liquid circuit (not shown) in a first flow direction, liquid enters via the first port 10 on top of the heater and flows along the inner tube 14 until the opening 14a is reached. Accordingly, the inlet port 10 is positioned in an upper end section 6, and the outlet port is positioned in the lower end section 9. At the opening 14a, the tube 14 terminates and the termination is regarded as an approximate end of the guidance of the liquid flow inside the tube 14. Figure 2(a) indicates the corresponding level of guidance Ig. Upon exit through the opening 14a, the liquid is being heated, while flowing upwards along the outer side of the inner tube 14. At that time, the liquid is surrounded by the outer tube 15, which prevents the liquid from directly exiting the heater 1 through the outlet 11 at the lower end. Rather, the liquid has to pass along the heating means 3, which allows the liquid to absorb further heat. When the flow of liquid reaches the opening 15a of the outer tube 15, the liquid can flow downwards between the outer surface of the outer tube 15 to the second port 11. Up to the opening 15a of the outer tube 15, liquid would be stored inside the outer tube 15 if the liquid circuit is interrupted and liquid leaks. Hence, the outer tube 15 represents a hydraulic trap which captures a sub-volume of liquid in the tank 2 and prevents the sub-volume from passing through the lower port 11 .

[0143] Figures 3(a) to 3(c) show the same heater 1 as shown in Fig. 1 , wherein additionally the direction of flow of liquid is indicated in Fig. 3(b) and the resulting retained level of liquid is indicated tin Fig. 3(c). Compared to Fig. 2, the same tank 2 is used, wherein the direction of flow is inversed. When connected to a liquid circuit (not shown) in a second flow direction opposite to the first flow direction shown in Fig. 2(b), liquid exits via the first port 10 on top of the heater after flowing along the inner tube 14. Accordingly, during normal use, the outlet port 10 is positioned in an upper end section 6, and the inlet port 11 is positioned in the lower end section 9. At the opening 14a, the tube 14 terminates and the termination is regarded as an approximate end of the guidance of the liquid flow inside the tube 14. Figure 3(a) indicates the corresponding level of guidance Ig. Before entry into the opening 14a, the liquid is being heated, while flowing downwards along the outer side of the inner tube 14. At that time, the liquid is surrounded by the outer tube 15, after liquid has entered the tank 2 via the inlet port 11 at the lower end. Rather, the liquid has to pass along the heating means 3, which allows the liquid to absorb further heat. After entry through the port 11 , the liquid flows upwards between the outer surface of the outer tube 15 and the tank wall 16. Up to the opening 15a of the outer tube 15, liquid will be stored inside the outer tube 15 if the liquid circuit is interrupted and liquid leaks. Hence, the outer tube 15 represents an essential part of the hydraulic trap 30 which captures a subvolume of liquid in the tank 2 and prevents the sub-volume from draining through the lower port 11 .

[0144] Figures 4(a) to 4(c) show the heater 1 in the inverted orientation, i.e. “upside down”, in which liquid enters via the first port 10 and exits via the second port 11 in the tank bottom 17. In this orientation, the inner tube 14 ensures that liquid is retained at the level Ir and does not drain through the lower port 11 once the level Ir has been reached. Accordingly, at least two thirds of the heating means 3, seen in the longitudinal direction I, remain immersed in liquid.

[0145] The outer tube 15 runs downwards from the flange 8 and guides liquid entering the tank 2 via the port 10 downwards. Accordingly, the outer tube 15 guides liquid from the first port 10 downwards up to the level Ig at the opening 15a. The liquid is guided, by way of the outer side of the inner tube 14 and the inner side of the outer tube 15, along the heating means 3.

[0146] Figures 5(a) to 5(c) show the heater 1 in the same orientation as in Fig. 4, but with opposite flow direction. In Fig. 5, liquid enters via the port 11 in the tank bottom 17 and exits via the port 10. In this orientation, the inner tube 14 ensures that liquid is retained at the level Ir and does not drain through the lower port 11 once the level Ir has been reached. Accordingly, at least two thirds of the heating means 3, seen in the longitudinal direction I, remain immersed in liquid. The outer tube 15 runs downwards from the flange 8 and guides liquid entering the tank 2 via the port 11 downwards, after having flow upwards inside the inner tube 14. The liquid is guided, by way of the outer side of the inner tube 14 and the inner side of the outer tube 15, along the heating means 3.

[0147] When comparing the embodiments of figures 2 and 3, on the one hand, and of figures 4 and 5, on the other hand, it is evident that the inner and outer tubes 14, 15 may be seen as switching functions in that, in figure 2 and 3, it is the inner tube 14 which primarily guides the liquid and the outer tube 15 prevents the liquid from completely draining through the lower port 11. In figures 4 and 5, it is the outer tube 15 which primarily guides the liquid, and the inner tube 14 prevents liquid from completely draining through the lower port 11.

[0148] Figure 6 shows embodiments having an inner tube 14 and an outer tube 15. In figure 6(a), the inner tube 14 extends between heating elements of the heating means 3, when seen in the transverse direction t (which may be seen as a radial direction). Accordingly, on the inside and the outside of the inner tube 14, a part of the heating means 3 is present, so that liquid can be heated when passing on both sides of the inner tube 14.

[0149] Figure 6(b) shows a realization of the opening 14a in that the inner tube 14 does not terminate abruptly, but the wall of the inner tube 14 has openings between tube walls extending further in the longitudinal direction I, namely up to the end 4 of the tank 2. The extended tube wall may represent feet and a connection to the tank end 4.

[0150] Fig. 7(a) shows a side view of an upper part of a tank of an embodiment of the invention, and Fig. 7(b) shows a perspective view of a tank of an embodiment of the invention, wherein inner entities are visible. More specifically, the tank wall 16 and / or the tank end 4 comprises at least two guiding features 18, such as recesses, extruded parts and / or cavities, to (further) improve liquid flow distribution inside the tank 2 by reducing / avoiding dead zones. An example of such embodiment is an entity comprising first and second hollow elements 14, 15. The first and second hollow elements 14, 15 are substantially tubular, wherein the second hollow element 14 represents an inner tube, and the first hollow element 15 represents an outer tube, in which the inner tube 14 is concentrically received. The inner tube 14 is sealed to the tank end 4. At the interface between the tank end 4 and the tank wall 16, 3 three guiding features with e.g. spoon-shaped bucket, are provided to allow for better whirling of the fluid inside the tank 2.

[0151] Fig. 8 shows the embodiment according to Fig. 1a wherein Fig. 8 shows water levels and flow direction in case of a leakage. The leakage 22 is beyond the first port 10 which is shown at the geodetical top of the heater 1 . The liquid, in particular water, flows into the tank 2 via the second port 11 which is lower than the first port 10. The flow direction 20 of the liquid, e.g. water, is the upflow direction, cold fluid, such as water, enters the heater 1 at a lower point through the second port 11 and leaves the heater 1 at the top. Fig. 8 shows the liquid level 24 during normal operation. The tank 2 is completely filled and the liquid flow is optimized overall towards warm water flowing upward through the heating chamber, absorbing heat from the heating elements as the liquid rises. As the liquid, such as water, ascends, the liquid continues to absorb heat, increasing its temperature. So, the upward flow direction leverages the natural tendency of hot liquids such as water to rise. The upward flow maintains a consistent temperature gradient, preventing cold liquid from mixing with hot liquid, which enhances overall efficiency constructively easily. However, the heater 1 can easily also be configured in the downflow direction, if this is needed to meet the requirements of the respective heating system (not shown). The heater 1 comprises a hydraulic trap 30 which comprises the outer tube 15, the inner tube 14 and the thermostat 13. The hydraulic trap 30 has the additional advantage that the liquid flow is redirected within the tank 2, which allows for a more efficient heat transfer from the heating means 3.

[0152] So, the hydraulic trap 30 prevents that in case of a leakage 22, the heating means 3 runs dry. In order to illustrate how the hydraulic trap 30 prevents the running dry: In a heater without a hydraulic trap 30, running dry means that the liquid level 24 of normal operation can start to drop within the tank 2 in case of a leakage 22 when the heating means 3 are heating the liquid in the tank 2. The liquid, for example water, has a reduced volume to absorb the heat generated by the heating means 3. This leads to heating the liquid to the point of boiling and steam is generated. The steam can create a vacuum within the tank 2, causing more liquid to be drawn into the tank 2 from the second port 11 , leading to a continuous cycle of steam generation and water draw. The steam condenses partially or escapes which creates a partial vacuum within the tank 2. This vacuum creates a pressure difference within the heater 1 , with lower pressure at the point of leakage 22 and higher pressure within the tank 2 where the liquid still exists. The pressure difference causes the remaining liquid in the tank 2 to be drawn towards the low-pressure area where the leakage 22 is, in an attempt to equalize the pressure. The remaining liquid in the tank 2 is pulled towards the top and eventually towards the leakage 22. Thus, the liquid level 24 drops rapidly to the siphon level 26. The siphon level 26 is below the lower end of the inner tube 4. This leaves the heating means 3 to a large extent uncovered. Without adequate water to absorb the heat and the potential for pressure buildup, there are risks of dry fire and damage to the heater 1 or even hazardous situations like rupture or explosion. In other words, the liquid flow can empty the tank 2 because of the siphon effect and at the end of the siphon effect, the heating means 3 is exposed to air.

[0153] The hydraulic trap 30 which comprises the outer tube 15 and inner tube 14 creates a sub-volume of liquid covering the heating element as predefined. In Fig. 8 the hydraulic trap level 28 of the liquid in the tank 2 covers the entire heating means 3. The hydraulic trap 30 further comprises the thermostat 13 which measures the temperature of the liquid in the tank 2.

[0154] The hydraulic trap within the meaning of the application means a trap comprised of at least two hollow elements 14, 15, i.e. at least first and second hollow elements 14, 15 and a thermostat 13 for measuring the temperature of the liquid inside the tank sufficiently long for the response of the thermostat 13 of the hydraulic trap 30. This hydraulic trap 30 thus allows for a retention of the liquid in the tank sufficiently long for the response of the thermostat 13 and thus allows for a much earlier trigger parameter, namely the liquid temperature which in turn allows for a much faster emergency control signal of the thermostat compared for example to the thermostat 13 of the heating means 3. The control signal of the thermostat 13 illustratively comprises a command to adjust or stop the operation of the heating means 3, which will prevent the overheating and damage of the heater 1 while allowing for normal heating of the liquid during normal operation without unwanted disruption of operation due to unwanted triggering of such an emergency (control) signal. After resolution of the leakage, the hydraulic trap 30 and thereby the heater 1 can be simply reset.

[0155] As an example, to illustrate the principle of the hydraulic trap 30 further: a predetermined setpoint temperature can be predefined fitting the parameters of the respective heater 1 regarding the hollow element dimensions and regarding the thermostat parameters, in particular the setpoint of the thermostat 13 of the hydraulic trap 30. The thermostat setpoint can be set to trigger, in other words generate the control signal of the thermostat 13, such as for example a command for an emergency stop of the heater 1 and / or the heat pump system if the detected temperature reaches or exceeds for example a temperature selected from the range of degrees comprised in 80 °C to 120°C, in particular 80°C to 115 °C, in particular 85°C to 115 °C. The degrees can be chosen in said range per degree, thus for example 80°C or 81 °C until 120°C, or for example in tenth of a degree steps between 80°C and 100°C, or for example in half degree steps, such as 80 °C, 85°C and so on until 100°C or in 5 degree steps, such as 80 °C, 85°C continued until to 120 °C, all steps in the example sequences are included and each temperature within the sequences can be chosen as appropriate for the respective heater. The setpoint temperature can include a tolerance, for example ± 6°C. For example, 85 °C with a tolerance of ± 6°C is chosen. At the higher end, for example also a setpoint temperature of 115 °C can be chosen. The response time of the thermostat of the hydraulic trap depends on this selection and is typically between 7 and 15 seconds. Fast thermostats can have a response time of 3 to 5 seconds. The formula for determining the setpoint can be formulated as performance of the heater 1 and taking into consideration the temperature of the water T initially Tini and needs to fulfill T plastics < T melting as long as the heating time t is < time to reach the maximum allowed heater temperature tmax in dry conditions. For example, in case of a 9 kW heater: initial Temperature 30°C, the tmax would be < 15 seconds. So, for the example 9 kW heater, Tini 30°C, Tplastics < T melting if tmax 15 seconds. At the time of reaching the predetermined setpoint temperature, the temperature of the heating means 3 is under the 400 °C of the dry heating curve of Fig. 9a for typical heating means for heaters according to the invention due to the cooling effect of the retained water in the system and / or in case the heater is not heating at full capacity. For example, if the heater is heating at only 3kW the time to reach the max heater temperature under dry conditions is for example < 15 to 30 seconds. So, the system remains intact in case of a leakage leading to a too low water volume in the tank of the heater.

[0156] So, the setpoint can for example be predefined based on the performance of the heater (Pheater in kW), the initial temperature (Tini), the time (tmax) at which the maximum allowed surface temperature of the heater (Tmaxheater) in dry conditions is reached to ensure that the temperature (Tplastics) of the tank wall is remains below the melting temperature (Tmelting) of the tank wall, in case the tank wall is made of (thermo-)plastics. In addition the influence of the geometry of the heating means 3 on heat transfer by radiation and natural convection can be taken into account. This allows configuring the setpoint (Tthermostat) such that the performance of the heater 1 under normal conditions is not affected taking the parameters of the respective heater 1 and heat pump system (not shown) into consideration and ensuring the safety of the plastic components of the heater 1.

[0157] In other words, the hydraulic trap 30 allows that the heater 1 remains sufficiently immersed in any leakage scenario, such as liquid leakage of the fittings and / or flask and / or hydraulic circuit. In particular, the level of immersion can be implemented in a predefined way by way of constructively simple dimension choices at the design and assembly stage, taking into account the performance of the heater 1. The hydraulic trap 30 also allows that the thermostat 13 of the hydraulic trap 30 can be reset - as simple as pushing a button - after the leak situation has been resolved.

[0158] In other words, the hydraulic trap 30 retains sufficient water in the heater 1 at the liquid level 28 of the hydraulic trap 30 that in case of the leakage 22 in the heat pump system (not shown) which leads to low water volume in the heater 1 so that the heating means 3 are stopped in such a short response time that the heating means 3 will not heat beyond the melting point of the (thermoplastic tank wall 16 . In other words, the hydraulic trap 30 allows for sending a stop signal in a very short response time based on the temperature of the liquid in the heater 1 and allows that during normal operation and with sufficient liquid in the system, the heating means 3 is not shut off erroneously.

[0159] Thus, the heater 1 according to the invention has a reduced risk of melting, electrical hazards and fires, in particular in case of being made of thermoplastic material, while allowing for flexibility in design. The heater 1 can be designed in a constructively simple way to predefined prevent exposing parts of the heating element to air in case of low water volume due to leakage. This prevents “dry fire” when the heating element operates without being fully submerged in water leading to damage and possibly even burnout of the (back-up) heater. In addition, the heater 1 has a profile comprising conformity of the heater 1 with the thermostat 13 requirements of NF EN 60335-2-40 having a thermostat 13 which allows manual reset. The heater 1 has a safety design which allows selectively for upflow or downflow operation of the (backup) heater 1 . The heater 1 further has a safety design that allows for a plastic housing, in other words the tank wall 16. The heater 1 can further be operated with a glycol addition where the housing, in other words tank wall 16, is kept intact and is still fully functional even in case of a leakage whilst reducing or preferably preventing unwanted tripping of the safety functionality of the hydraulic trap 30 during normal operation.

[0160] Fig. 9a shows a diagram of the surface temperature of a heating means 3 which could be used in any standard heater with a plastic housing and also in any embodiment of the heater 1 (as shown in the Figs, for example Fig. 1a and 8) in dry condition over time. The heating means is a U-shaped heating means which is suitable and can be used as a heating means 3 in the heater 1 according to the invention. The heating means 3 shown in Fig. 9a is monitored for the heat increase in dry condition. Dry condition means that the entire heating means 3 is exposed to air. Figure 9a shows how long for example the heater 1 as shown in Fig. 1a or any standard heater with a plastic housing can operate in dry condition before reaching the melting temperature of the tank wall 16 made from (thermo-)plastic material. The melting temperature of such a tank wall 16 is usually around 280 °C. The heating means 3 has an inhomogeneous surface temperature distribution. The heating means has a large surface area of heating elements 32 of the heating means 3 which has a very high temperature and an area of the mounting flange 34 comprising the electrical connections which has a low temperature. Fig. 9a shows the surface temperature development of the heating elements 32 as temperature curve 36. As can be seen in Fig. 9, the temperature curve 36 of the surface temperature of the heating elements 32 and also the average temperature curve 38 rise at a pace of about 250 °C each 10 seconds. Very high temperatures far above 500 °C are reached within 20 seconds. The temperature curve 40 of the mounting flange 34 shows a very different temperature profile where the temperature after 60 seconds is still under 100 °C. The temperature curve 38 shows the temperature average of the heating means 3. The melting temperature (about 280 °C) of the tank wall 16 of the heater 1 is reached in under 15 seconds. Fig. 9a also shows that if the heating system (not shown) comprising the heater 1 would rely on a thermostat determining the surface temperature of the heating means 3, which triggers after 45 seconds, the average temperature curve 38 is already just below 1000 °C. Fig. 9a shows the need of a hydraulic trap 30 of the heater 1 according to the invention (as shown in any embodiment, in particular Fig. 1a and Fig. 8), retaining sufficient fluid, such as water, to cover the heating means 3 and determining the temperature of said sub-volume of liquid through thermostat 13 and to trigger an emergency stop signal, when the water starts to boil (at normal pressure) or even below the boiling point (at normal pressure and in particular at a high pressure of for example 3 bara), which according to the graph of Fig. 9a is under 10 seconds in dry conditions. The trigger temperature for the thermostat 13 can for example be as low as > 80°C without disrupting normal operation.

[0161] Fig. 9a further elucidates why the thermostat (not shown) of a heating means 3 itself is too slow the consequence of relying on the thermostat of the heating means 3 alone is shown also in Fig. 9b. The heating means 3 operates at very high temperatures and the control thermostat (not shown) of the heating means 3 determines the surface temperature of the heating means 3. This thermostat easily has to allow for up to 700 °C surface temperature as normal operation depending on the performance of the heater 1. Thus, such a thermostat (not shown) has to wait until the thermostat reaches very high temperatures up to around 1000 °C before an emergency stop can be triggered by such a thermostat (not shown). By that time the tank wall 16 will be severely damaged. If this thermostat were stopped at the temperature at which water can have a temperature of 80°C during normal operation, the thermostat of the heating means 3 would be at high risk of disrupting normal operation on a regular basis.

[0162] Fig. 9b shows a heater 48 without a hydraulic trap 30 with a standard heating means 3 in dry conditions between 0 seconds and 60 seconds with the thermostat controlling the heating means (not shown) triggered at 45 seconds. The heater 48 does not comprise a hydraulic trap 30. The heater 48 is built the same as a standard stainless steel heater with a thermoplastic housing instead of the standard steel housing. The heater 48 has a heating power from 0 to 9 kW heating power. The heater 48 comprises a tank, heating means, thermal insulation, electrical connections, a flange and inlet and outlet ports (not shown). The heating means are controlled by a dedicated thermostat (not shown) measuring the surface temperature of the heating means as shown in Fig. 9a. Fig. 9b shows the heater 48 in dry conditions at 0 seconds Os. The heater 48 is functional and intact. However, the heating means are exposed to air due to a leak which caused the water to be siphoned out of the tank of the heater 48. Siphoning refers to the process where liquid flows from one container to another, driven by gravity, through a tube or hose. Illustratively, this can for example occur when the end of the tube inside the tank is submerged, and the other end is for example placed at a lower elevation, allowing the liquid to flow out due to the difference in pressure. In case of the heater 48, the siphoning occurs due to pressure differences in the heating system due to the leak and the heating occurring in the heater 48 as explained in this application.

[0163] The heating means surface temperature develops over time following the heating curve of Fig. 9a. within 15 seconds, the heating means surface temperature has reached nearly 400 °C. The melting temperature of the plastic material usually used for the housing, in other words the tank wall, of such heaters 48 typically has a melting temperature around 280°C. Thus. Within 15 seconds, the heating means has reached a surface temperature far above the melting temperature of the housing, in other words the tank wall, of the heater 48. By 30 seconds, the surface temperature has risen further to around 700 °C and keeps rising. The heating means surface temperature rises by about 250°C every 10 seconds. The thermostat typically controlling the heating means is not responding to this rise in temperature, yet. The thermostat is triggered 15 seconds later, at 45 seconds 45s. This stops the heating operation of the heating means. At this point, the surface temperature of the heating means has reached nearly 1000 °C. The temperature still rises after the heating means is triggered and after 60 seconds 60s. The temperatures within the heater 48 are such, that the heating means is severely and permanently damaged after 60 seconds from the moment the heater ran dry, and the heating means was exposed to air. Fig. 9b thus shows the dry fire caused in a heater 48 with a plastic housing and no hydraulic trap 30, if the heater 48 relies only the thermostat of the heating means.

[0164] Fig. 10 shows an embodiment of the method according to the invention. Fig. 10 shows the steps for operating a heat pump system (not shown) comprising a heater 1 according to the invention, wherein the heat pump system comprises a control unit (not shown) and / or a remote control unit and wherein a thermostat 13 of the heater 1 is configured to communicate with the control unit and / or the remote control unit. The thermostat 13 may have its own control unit (not shown).

[0165] The control unit can comprise a printed circuit board or be a printed circuit board (PCB). The control unit can further comprise a microprocessor or processor, which can be described as the brain of the control unit which processes data and executes control algorithms. The control unit further can comprise sensors and input devices like temperature sensors, such as the thermostat 13, buttons or switches which input data into the control unit. The control unit can further comprise actuators and output devices, thus components such as relays, motors, or LEDs which the control unit activates to perform specific tasks. The control unit can further comprise communication interfaces, such as modules for wired communication (for example ethernet, or RS232) or wireless communication (for example Wi-Fi or Bluetooth). The control unit, in particular the PCB, further can comprise conductive pathways connecting the various components, enabling the flow of electrical signals. The control unit can further comprise components to regulate and distribute power to the different parts of the control unit. For example, voltage regulators can ensure a stable power supply for the control unit, such as to the microcontroller and other components. The microcontroller on the control unit can further run software code which governs the operation of the control unit. The software can comprise logic for data processing, decision making, and control actions. The method comprises the steps of: monitoring in step S101 a temperature of a liquid in the heater 1 via the thermostat 13. Generating in step S103 a control signal 103 of the thermostat 13 based on the value derived from the thermostat 13 in case the monitored temperature reaches a predetermined setpoint, for example falls outside of a predefined temperature range or reaches or surpasses a predefined temperature value. The setpoint can also be a temperature development over time, so illustratively, the speed with which the temperature rises.

[0166] In a step S105 the control signal 103 of the thermostat 13 is transmitted to the control unit of the heat pump system. Additionally or alternatively, the control signal 103 of the thermostat 13 can be transmitted in step S105 to a remote control unit (not shown). The control unit and / or the remote control unit receive in a step S107 the control signal 103 of the thermostat 13. In a step S109 the control unit and / or the remote control unit generate a further control signal 109 of the control unit and / or the remote control unit to activate an adjustment or shutoff mechanism via the control unit and / or the remote control unit in response to the control signal 103 of the thermostat 13 for adjusting or stopping the operation of the heater 1 and / or of the heat pump system. The further control signal 109 can illustratively be called a thermal adjustment command of the control unit and / or the remote control unit.

[0167] The setpoint can for example be predefined based on the performance of the heater 1 (Pheater in kW), the initial temperature (Tini), the time (tmax) at which the maximum allowed surface temperature of the heater (Tmaxheater) in dry conditions is reached to ensure that the temperature (Tplastics) of the tank wall is remains below the melting temperature (Tmelting) of the tank wall, in case the tank wall is made of (thermo-)plastics. This allows configuring the setpoint (Tthermostat) such that the heater’s performance under normal conditions is not affected taking the parameters of the respective heater and heat pump system into consideration and ensuring the safety of the plastic components of the heater. In addition the influence of the geometry of the heating means on heat transfer by radiation and natural convection can be taken into account.

[0168] For the heater 1 of Fig. 1 a, the setpoint can be based on Pheater is 9kW, Tini is 30°C, Tplastics < Tmelting is fulfilled if the max heating time under dry conditions is < 15 seconds which leads to a setpoint value for example of 85 °C which can include a tolerance of ± 6 °C, taking into consideration also the geometry of the heating means. If partial heater capacity is taken into account, then if there is only a partial heater capacity 3kW initially, tmax can be between 15 to 30 seconds.

[0169] The method can further comprise a step of checking the heat pump system and / or a step of resetting the thermostat 13 prior to restarting the heater 1 and / or the heat pump system.

[0170] The method according to the invention can optionally be integrated with smart home systems for automated alerts and control, further enhancing safety and usability. Thus, the method further can comprise an integration step to transmit the further control signal of the control unit and / or the remote control unit to a smart home system.

[0171] The method can optionally allow for the ability to remotely stop or adjust the thermostat 13 and / or the heat pump system to ensure the safety of the heater and / or the heat pump system, in particular in case of a leakage.

[0172] The method according to the invention can further comprise a step of generating and executing a diagnostic tool, and / or generating and executing reports, alerts, and / or notifications relating to the leakage and the heat development, in particular the status of the heater, in the heat pump system.

[0173] The method can further comprise a step of generating a report, log and / or securing data for analysis, decision making and / or improvements to the heater and / or heat pump system. The method according to the invention further can optionally allow users to monitor and / or control the heater 1 from a distance, enhancing convenience and peace of mind. Thus the method optionally comprises a step of monitoring and / or remote controlling the heater 1 and / or the heat pump system.

[0174] The method can optionally further comprise monitoring additional sensors of the heat pump system, in particular mass flow sensors, volume flow sensors, additional temperature sensor and / or pressure sensors.

[0175] Reference signs

[0176] (back-up) heater tank heating means tank end thermal insulation first end section electrical connection flange second end section first (inlet) port second (outlet) port a, 12b secondary water flow ports thermostat inner tube (hollow element) a opening of inner tube b end of inner tube outer tube (hollow element) a opening of outer tube b end of inner tube tank wall tank bottom guiding feature liquid flow direction leakage liquid level full tank during normal operation liquid level siphon effect liquid level hydraulic trap hydraulic trap heating elements of heating means 3 mounting flange comprising electrical connections 36 surface temperature development of the heating elements 32 as temperature curve 36

[0177] 38 The temperature curve 38 shows the temperature average of the heating means 3

[0178] 40 temperature curve 40 of the mounting flange 34

[0179] 42 temperature curve temperature values at 15 seconds

[0180] 44 temperature curve temperature values at 30 seconds

[0181] 46 temperature curve temperature values at 45 seconds

[0182] 48 heater without a hydraulic trap 30

[0183] I longitudinal direction t / r transverse / radial direction ig guiding level

[0184] Ir retaining level

Claims

Claims1. A heater (1) for connection to a liquid circuit and configured to heat liquid, in particular as a back-up heater in addition to a heat pump unit, the heater (1) comprising a tank (2) for receiving liquid and having a tank wall (16) extending basically in a longitudinal direction from a tank end (4) and defining a liquid tank volume, the longitudinal direction (I) basically running opposite or parallel to gravity, when the heater (1) is installed on site, at least a first port (10) and a second port (11) for passage of liquid of the liquid circuit into and out of the tank (2), respectively, or out of and into the tank (2), respectively, at least one heating means (3) having at least one heating element (3a) inside the tank (2) for heating liquid inside the tank (2), and at least two hollow elements (14, 15) provided inside the tank (2) and extending in the longitudinal direction (I), the at least two hollow elements (14, 15) each defining a liquid sub-volume and for substantially separating a respective liquid sub-volume from the remaining liquid in the tank along the longitudinal direction (I), wherein the at least two hollow elements (14, 15) are in fluid communication with the remaining liquid in the tank at an opening (14a, 15a) of the respective hollow element (14, 15), wherein the heater (1) is configured such that one of the at least two hollow elements (14, 15) guides liquid along at least a part of the respective hollow element for flow of liquid along the heating element of the at least one heating means (3) in the longitudinal direction (I), and the other one of the at least two hollow elements (15, 14) retains liquid in the tank (2), such that at least a part of the heating means remains immersed in liquid in the tank in case of shortage of liquid in the tank wherein the heating means (3) is, at least partially along the longitudinal direction (I), located between the first hollow element (14, 15) and the second hollow element (15, 14), seen in the transverse direction (t), andwherein the heater (1) further comprises a thermostat (13) inside the tank (2), in particular configured to measure the temperature of the liquid inside the tank (2) or of steam generated by the heating means (3).

2. Heater (1) of claim 1 , wherein the first hollow element (14, 15) and the second hollow element (15, 14) at least partially overlap each other in the longitudinal direction (I) and / or are distanced from each other in a transverse direction (t) perpendicular to the longitudinal direction (I), and / or optionally extend in parallel and / or are tubular and / or are concentrically arranged.

3. Heater (1) of claim 1 or 2, wherein the first hollow element (14, 15) and second hollow element (14, 15) are configured to retain liquid in the tank (2) at a position higher than the lowest point of a heating surface of the heating means (3) wherein the first hollow element (14, 15) is in the retaining configuration in a first orientation and the second hollow element (14, 15) is in the retention position in the second, inverted orientation.

4. Heater (1) of any of the preceding claims, wherein the first port (10) is located in a first end section (6) of the tank (2) and the second port (11) is located in a second end section (9) of the tank (2) substantially opposite to the first end section (6) and lower than the first end section (6), when seen in the longitudinal direction, when the heater (1) is installed on site, and the first hollow element (14) extends from the second port (11) and / or the tank bottom (17) in the longitudinal direction (I) to a retaining level (Ir) at which the opening (14a) is located, when viewed in the longitudinal direction (I), when the heater is installed on site, wherein the heater (1) is configured such that the outside of the first hollow element (14) and an outer counterpart retain liquid in the tank (2) and prevent liquid from draining off the lower port of the first and second ports (11).

5. Heater (1) of claim 4, wherein the at least one heating means (3) is at least partially located below the retaining level (Ir) and, seen in the transverse direction (t), between the first hollow element (14) and the outer counterpart,optionally the tank wall (16), such that the heater (1) is configured to at least partially keep the heating means (3) immersed in liquid, in case of shortage of liquid in the tank.

6. Heater (1) of claims 3 and 5 and optionally of any of the remaining preceding claims, wherein the opening (15a) of the second hollow element (15) defines a guiding level (Ig), wherein the second hollow element (15) is at least partially, seen in the longitudinal direction (I), located between the first hollow element (14) and the tank wall (16), seen in the transverse direction (t), and the heater (1) is configured to guide flow of liquid in the tank (2) from the first port (10) along at least a part of the second hollow element (15) to the guiding level (Ig), then at least to the retaining level (Ir) preferably along at least a part of the at least one heating means (3) and along at least a part of the first hollow element (14) and the second hollow element (15), and then inside the first hollow element (14) to the second port (11) and / or from the second port (11) along at least a part of the second hollow element (15) to the retaining level (Ir), then at least to the guiding level (Ig) preferably along at least a part of the at least one heating means (3) and along at least a part of the first hollow element (14) and the second hollow element (15), and then inside the first hollow element (14) to the first port (10).

7. Heater (1) of any of the preceding claims 1 to 3, wherein the first port (10) is located in a first end section (6) of the tank (2) and the second port (11) is located in a second end section (9) of the tank (2) substantially opposite to the first end section (6), seen in the longitudinal direction (I), and lower than the first end section (6), when viewed in the longitudinal direction, when the heater (1) is installed on site, and the first hollow element (15) extends from the tank end (4) in the longitudinal direction (I) and has the opening (15a), which defines a retaining level (Ir), when viewed in the longitudinal direction, when the heater (1) is installed on site,wherein the heater (1) is configured to retain liquid in the tank (2) at the retaining level (Ir) inside the first hollow element (15), if the liquid circuit is interrupted and liquid drains off the lower one of the first and second ports (11).

8. Heater (1) of claim 7, wherein the heating means (3) is at least partially located below the retaining level (Ir) and, seen in the transverse direction (t), inside the first hollow element (15), such that the heater (1) is configured to at least partially keep the at least one heating means (3) immersed in liquid, in case of shortage of liquid in the tank.

9. Heater (1) of claims 3 and 8 and optionally of any of the preceding claims 2 and 7, wherein the opening (14a) of the second hollow element (14) represents a guiding level (Ig), when viewed in the longitudinal direction, when the heater (1) is installed on site, wherein the second hollow element (14) is at least partially, seen in the longitudinal direction (I), located inside the first hollow element (15), seen in the transverse direction (t), and the heater (1) is configured to flow liquid in the tank (2) from the first port (10) along at least a part of the second hollow element (14) to the guiding level (Ig), then at least to the retaining level (Ir) preferably along at least a part of the at least one heating means (3) and then outside the first hollow element (15) to second port (11) and / or from the second port (11) along at least a part of the first hollow element (15) to the retaining level (Ir), then at least to the guiding level (Ig) preferably along at least a part of the at least one heating means (3) and then along the second hollow element (14) to the first port (10).

10. Heater (1) of any of the preceding claims, wherein the thermostat (13) inside the tank (2)is located within at least one of the at least one or two hollow elements (14, 15).

11. Heater (1) according to any of the preceding claims, wherein the thermostat (13) is located at the tank end (4), when the tank (2) is installed on site.

12. Heater (1) of any of the preceding claims, wherein an end (14b, 15b) of the at least one or two hollow elements (14, 15) opposite to the opening (14a, 15a) in the longitudinal direction, abuts an end (4) of the heater (1), and is optionally sealed to the basis (4).

13. Heater (1) of any of the preceding claims, wherein the tank (2), preferably excluding the flange (8), is made of plastics, in particular (thermo-) plastics, preferably PPS, and / or the at least one or two hollow elements is / are made of metal, in particular stainless steel or copper.

14. Heater (1) of any of the preceding claims, wherein the at least one heating means (3) is a resistive heating means and / or has helicoidal or U-shaped heating elements.

15. Heater (1) of any of the preceding claims, wherein one of the first and second ports (10, 11) is located at the tank wall, and the other one of the first and second ports (11 , 10) is located at a tank end.

16. Heat pump system comprising a heater (1) of any of the preceding claims, wherein the heat pump system comprises a heat pump unit for at least heating of liquid of the liquid circuit and the heater (1) is configured to function as a back-up heater to alternatively or additionally heat the liquid of the liquid circuit.

17. Method of operating a heat pump system according to claim 16 comprising a heater (1) according to any one of claims 1 to 15, wherein the heat pump system comprises a control unit and / or a remote control unit and wherein a thermostat (13) of the heater (1) is configured to communicate with the control unit and / or the remote control unit, the method comprising the steps of:monitoring (S101) a temperature of a liquid in the heater (1) via the thermostat (13), generating (S103) a control signal (103) of the thermostat (13) in case the monitored temperature reaches a predetermined setpoint, transmitting (S105) the control signal (103) of the thermostat (13) to the control unit and / or the remote control unit, receiving (S107) the control signal (103) of the thermostat (13) at the control unit and / or the remote control unit, and generating (S109) a further control signal (109) of the control unit and / or the remote control unit to activate an adjustment or shutoff mechanism via the control unit and / or the remote control unit in response to the control signal (103) of the thermostat (13) for adjusting or stopping the operation of the heater (1) and / or of the heat pump system.

18. Method according to claim 17, further comprising a step of checking the heat pump system and / or the step of resetting the thermostat (13) prior to restarting the heater (1) and / or the heat pump system.

19. Method according to claim 17 or 18, further comprising the steps of: a.) checking the heat pump system and / or a step of resetting the thermostat 13 prior to restarting the heater 1 and / or the heat pump system; and / or b.) transmitting the further control signal (109) of the control unit and / or the remote control unit to a smart home system; and / or c.) generating and executing a diagnostic tool, and / or generating and executing reports, alerts, and / or notifications relating to the leakage and the heat development, in particular the status of the heater, in the heat pump system; and / or d.) generating a report, log and / or securing data for analysis, decision making and / or improvements to the heater and / or heat pump system, and / ore.) monitoring and / or remote controlling the heater 1 and / orthe heat pump system; and / or f.) monitoring additional sensors of the heat pump system, in particular mass flow sensors, volume flow sensors, additional temperature sensor and / or pressure sensors.

20. Data processing device comprising means for carrying out the method of at least one of the claims 17 to 19.

21. A computer program product comprising instructions to cause the heat pump system of claim 16 to execute the steps of the method of any of the preceding claims 17 to 19.

22. A computer readable data carrier having stored thereon the computer program product according to claim 21 .

23. A data carrier signal carrying the computer program product according to claim 21.

24. Use of a heater (1) according to any one of claims 1 to 15 in a heat pump system, in particular according to claim 16, and / or in a method according to any one of claims 17 to 19 and / or in controlling the safety of a heat pump system, in particular of a heater (1) within the heat pump system via a control unit of the heat pump system locally and / or remotely, in particular via a local control unit and / or via a remote control unit.

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