Heating pipe for preheating a heat transfer medium

Abstract

Heating pipe (1) for preheating a heat transfer medium of a central heating system or for preheating drinking water, wherein the heating pipe (1) comprises a line (2) for transporting the heat transfer medium or the drinking water, a heating resistance (4) in thermal contact with the heat transfer medium or drinking water carried in the line (2), a heat-insulating sheath (3) which encloses the line (2) and the heating resistance (4), a safety temperature limiter for interrupting the preheating by means of the heating pipe (1) at a first and / or second temperature, wherein the heating pipe (1) has at least one sensor device (8) which detects at least one value of an external quantity, wherein a heating power of the heating resistance is controllable depending on the external quantity.

Description

[0001] The invention relates to a heating pipe for preheating a heat transfer medium in a central heating system or for preheating drinking water. The heating pipe comprises a conduit for transporting the heat transfer medium or drinking water, as well as a heating resistor wound around the conduit.

[0002] Central heating systems are predominantly used to heat buildings. Central heating systems are characterized by having a single central heating source and supplying generated heat to several rooms or a building via a heat transfer medium. Hot water heating systems use heated water as the heat transfer medium. Common heating sources are boilers or combi boilers. The latter are heating devices that serve both to heat the heating medium and to heat the drinking water.

[0003] The heated heat transfer medium is piped into the rooms or building and distributed there via a system of pipes, the so-called heating circuit. The heat transfer medium heated at the central heating point is then piped from the heating point via a flow line of the heating circuit to radiators or heaters in the building. These radiators or heaters act as heat exchangers and transfer some of the thermal energy transported by the heat transfer medium to the surroundings of the rooms to be heated. The heat transfer medium is piped back to the central heating point via the return line of the heating circuit. The temperature of the heat transfer medium arriving at the heating point via the return line is lower than the temperature of the heat transfer medium heated by the heating point and piped via the supply line. The temperature difference is usually around 5°C.

[0004] To ensure consistent heating of the building, the returning heat transfer medium must be heated by the heating point to the desired temperature for the supply line. Heating the returning heat transfer medium requires more energy, the greater the temperature difference between the heat transfer medium in the supply and return lines.

[0005] Therefore, it may be desirable to keep the aforementioned temperature difference as small as possible. One passive measure is to insulate the heating system's pipes wherever heat transfer is not desired. Ideally, however, this measure can only maintain the temperature of the heat transfer fluid after it has passed through the radiators.

[0006] Energy is also required for the heating of drinking water, which is typically also provided by the heating point that heats drinking water stored in a buffer tank.

[0007] The object of the present invention is to regulate the temperature of a heat carrier in the flow and return of a heating system or in a drinking water pipe of a building in an energy-efficient manner.

[0008] This problem is solved by the subject matter of the independent patent claim. Advantageous embodiments and further developments are set forth in the subclaims.

[0009] According to one aspect of the invention, a heating pipe for preheating a heat carrier of a central heating system or for preheating drinking water is specified, wherein the heating pipe has a line for transporting the heat carrier or the drinking water, a heating resistor in thermal contact with the heat carrier or drinking water carried in the line, and a heat-insulating sleeve which encloses the line and the heating resistor.

[0010] The heating tube has a safety temperature limiter for interrupting the preheating by means of the heating tube at a first and / or second temperature in order to ensure that neither the heated medium becomes too hot nor the heating wire is heated too much.

[0011] The heating tube further comprises at least one sensor device that detects at least one value of an external variable. The heating output of the heating resistor can be controlled depending on the external variable. For this purpose, the heating tube comprises, for example, a controller configured to control the heating output of the heating resistor depending on the external variable. The controller can be spatially separated from the heating tube and integrated, for example, into a smart home control system or a central heating system.

[0012] The heating pipe has the advantage of being able to support central heating or domestic hot water production. This is particularly useful when electrical energy is available at reasonable prices to operate the heating pipe, for example, when a solar system provides more power than a building consumes. This can optimize the building's energy supply and reduce energy costs.

[0013] The sensor device serves to detect at least one value of an external variable. This external variable is preferably characteristic of the power provided by a solar power system. The external variable is particularly preferably characteristic of an excess of power generated by a solar power system. The external variable can thus preferably be used to determine the power produced by a solar power system, and particularly preferably the power produced by a solar power system, or the power that remains unused.

[0014] The sensor device can, in particular, be a brightness switch that can be used to determine the brightness of daylight as an external variable. The brightness of daylight can be used as a measure of the power provided by a solar system.

[0015] Alternatively, the sensor device can be designed as a sensor for detecting the electrical power or current provided by a solar power system, thus directly measuring the power produced by a solar power system. This makes it possible to determine how much surplus power from the solar power system is fed into the public power grid. It is thus conceivable that the presence of a surplus of electricity produced by the solar power system could be detected based on the external value.

[0016] For example, a current sensor can measure the electrical current fed into the public power grid. This current feed-in results from the excess electrical power produced by the solar power system but not consumed. It is conceivable that the current could be measured at the point where it is to be fed into the public grid. Preferably, the current is measured in the cable with galvanic isolation.

[0017] Electricity meters can also be used as current sensors, preferably electronic meters and especially smart meters. The latter can send and receive digital data, so it's conceivable that they could communicate wirelessly with the control system.

[0018] The advantage of the current sensor is that an effective excess of current can be detected and the heating tube can be activated or deactivated accordingly.

[0019] The sensor device may also be another sensor for detecting a quantity that characterizes the electrical power produced by a solar system.

[0020] The line serves, on the one hand, to transport the heat transfer medium or drinking water through the heating pipe. The heat transfer medium or drinking water flows into the line at a first end of the heating pipe, then flows through the pipe and out of the pipe again at a second end of the heating pipe. On the other hand, the line serves as a heat exchanger. For this purpose, the line absorbs the thermal energy from the heating resistor and transfers it to the heat transfer medium or drinking water in order to heat the latter. The line is therefore advantageously a pipe or hose made of copper or aluminum. However, other materials, preferably metals, are also conceivable. The line is preferably a straight pipe. This has the advantage that the heating pipe can be easily inserted into the piping system of the heating circuit or into the drinking water line, with a section of the piping system being replaced by the heating pipe.However, it is also conceivable that the line and thus the heating pipe has a pipe bend or turns.

[0021] According to one embodiment, the heating tube is straight, i.e., cylindrically shaped, and the line and the casing are arranged concentrically or slightly eccentrically. The heating tube has a front end at each of its two ends, through which the line passes, forming a connection for a pipe of a piping system.

[0022] Thus, an inlet and an outlet for the heat transfer medium and drinking water, respectively, are formed on two opposite ends of a cylindrical body. This design has the advantage that the heating pipe can be easily replaced with an existing pipe section, making it particularly suitable for retrofitting. Connection to the existing pipe can be made using standard connectors.

[0023] The central heating system is preferably a hot water heating system, meaning that the heat transfer medium is preferably water. However, other heat transfer media, such as thermal oils, are also conceivable, so other heating types are also suitable.

[0024] The heating resistor converts electrical energy into thermal energy. In a very simple embodiment, the heating resistor is a heating conductor, i.e., a metal wire. However, other heating resistor designs are also conceivable, such as tubular heaters, in which the heating resistor is coiled, corrugated, or has a meandering shape. The thermal energy generated by the heating resistor is transferred to the pipe and from there to the heat transfer medium or drinking water.

[0025] Alternatively, in addition to the heating resistor, an induction heater could be used to heat the conductor, allowing the conductor to transfer the thermal energy to the heat transfer medium. It is also conceivable that a bare wire heating element could be used instead of the heating resistor. The heat transfer medium flows directly around this element to heat it. Therefore, the bare wire heating element is located inside the cable. However, other concepts for converting electrical energy into thermal energy are also conceivable.

[0026] The pipe and heating element are enclosed in a thermally insulating sleeve. The sleeve can be wrapped around the pipe and heating element, foamed, or applied in another way. The sleeve reduces the loss of thermal energy to the environment, ensuring that the thermal energy generated by the heating element is transferred primarily to the pipe and thus to the heat transfer medium or drinking water. The sleeve can be made of natural and / or synthetic insulating materials.

[0027] For example, the insulation material could be a polyurethane foam (PU foam). The casing should preferably be highly fire-resistant or at least fire-retardant.

[0028] The heat output converted by the heating resistor can be adjusted via a phase-angle control. The phase-angle control does not have to be in direct contact with the other components of the heating tube, but can also be mounted externally and preferably connected to these, in particular to the control device and the heating resistor, via cables. The phase-angle control can also be embedded in the casing of the heating tube. The phase-angle control is operated with alternating current, with the amplitude of the input voltage preferably being in a range of 200 V to 250 V and preferably being 230 V. The effective voltage at which the heating resistor is operated is adjusted by pulse width modulation. For this purpose, the current flow within the phase-angle control is preferably controlled by a triac.

[0029] The phase control can be controlled by a control device taking into account the first temperature, the second temperature, and the value of the external variable. The control device does not need to be in direct contact with the remaining components of the heating tube, but can be mounted externally, preferably connected via cable to the first and second temperature sensors, the sensor device, and the phase control. However, the control device can also be embedded in the casing of the heating tube. It is also conceivable for the control device to communicate wirelessly, in particular with the sensor device.

[0030] A phase-angle control is very powerful, but also complex to implement. Alternatively, the heating pipe can be equipped with a power supply via a switchable relay or a switchable socket. This type of smart power supply can be used to switch the heating pipe on and off.

[0031] For example, the heating pipe can be powered by the home's internal electrical grid, which supplies electricity from a solar power system. This has the advantage that the heating pipe can be powered by the electricity produced by the solar power system without having to be directly connected to it. In particular, the heating pipe can be integrated into a smart home system.

[0032] According to one embodiment, the heating pipe is integrated into the central heating circuit, preferably in the return line of the central heating system, particularly preferably directly upstream of the heating point. It is conceivable that an existing central heating system could be expanded, i.e., converted, to include the heating pipe.

[0033] This involves removing part of the existing return pipe and replacing it with the heating pipe. It's also conceivable that the heating pipe could be installed during the installation of a new central heating system.

[0034] The heating pipe can also be integrated into the hot water system, preferably in the cold water inlet to the hot water system, and especially preferably in the cold water inlet immediately before the water heater. It is conceivable that an existing hot water system could be expanded, i.e., converted, to include the heating pipe. In this case, part of the existing cold water inlet is removed and replaced with the heating pipe. It is also conceivable that the heating pipe could be installed during the installation of a new hot water system.

[0035] In one embodiment, the heating resistor at least partially encloses the cable. The heating resistor encloses the cable with several turns, with the heating resistor and the cable being in direct contact with each other.

[0036] Due to the direct contact between the cable and the heating resistor, the thermal energy is transferred from the heating resistor to the cable in the best possible way. To achieve the largest possible contact area and provide the greatest possible heating resistance, the heating resistor is wound around the cable as often as possible. The heating resistor preferably encloses the conductor so that only a small area at the ends of the conductor is not covered by the heating resistor. Sufficient space should preferably be provided for the fastening device and / or the sheath.

[0037] In another embodiment, the heating resistor consists of a wire made of an alloy that exhibits a nearly constant electrical resistivity over wide temperature ranges, for example, the alloy of 55% copper, 44% nickel, and 1% manganese known by the brand name Constantan. The diameter of the wire is preferably approximately 0.5 mm.

[0038] However, other well-known heating conductor alloys such as Kanthal are also conceivable for the heating resistor.

[0039] In another embodiment, the wire of the heating resistor has insulation made of perfluoroalkoxy polymers (PFA). The diameter of the heating resistor is thus preferably 0.8 mm. PFAs have the advantageous property of being thermoplastically processable, allowing the PFA insulation to be applied to the wire by extrusion. PFAs are also high-temperature resistant and flame-retardant.

[0040] It is also conceivable that the insulation consists of polytetrafluoroethylene (PTFE) or other suitable insulators.

[0041] In a further embodiment, the heating tube is radially constructed such that the line is arranged inside, the heating resistor is arranged outside the line, and the two temperature sensors are arranged outside the heating resistor. Furthermore, the casing is arranged outside the line and at least partially outside the heating resistor and outside the two temperature sensors. The casing completely encloses the heating resistor and the two temperature sensors and at least partially encloses the line. The casing has at least a first layer, which is intended and intended for thermal insulation.

[0042] Preferably, the phase control and the control device are embedded in the casing and particularly preferably in the first layer of the casing.

[0043] Preferably, the casing has a second layer on the outside, which is intended and intended to protect the first layer or the heating pipe from external influences, such as moisture, chemicals or mechanical influences.

[0044] Preferably, the casing and / or the line have radial passages through which connections and cables for the heating resistor, the flow sensor, the temperature sensors, the safety temperature limiter, the phase control and / or the control device are guided.

[0045] In a further embodiment, the heating pipe has a flow sensor for measuring the flow of the heat transfer medium or drinking water through the pipe.

[0046] The flow sensor measures the flow of the heat transfer medium or drinking water transported through the pipe. Various types of flow sensors can be used, such as turbine flow meters. This allows it to be detected whether the heat transfer medium or drinking water is flowing through the heating pipe. The heat output of the heating resistor can be regulated depending on the measurement result of the flow sensor. It is conceivable that at least two states are provided for the heating wire. For example, the heating wire can be deactivated in a first state and emit full heat output in a second state. However, other states are also conceivable, whereby the heat output can assume between 0% and 100% of the maximum heat output, depending on the flow value.

[0047] According to one embodiment, the heating tube has a safety temperature limiter (STB). The safety temperature limiter serves to interrupt preheating by means of the heating tube if the STB detects a first temperature of more than 75°C and / or a second temperature of, for example, more than 120°C or more than 130°C.

[0048] This prevents damage to the heating pipe itself, the destruction of individual components of the heating pipe, or damage to the central heating system. For example, the heating pipe can only be reactivated by manually switching on the safety limiter, or it is automatically activated when the temperature falls below the threshold again.

[0049] According to one embodiment, the safety temperature limiter comprises at least one first bimetallic strip and at least one second bimetallic strip, which are connected in a circuit of the heating resistor in such a way that the circuit is interrupted when a first temperature acting on the first bimetallic strip exceeds a predetermined first limit value and / or when a second temperature acting on the second bimetallic strip exceeds a predetermined second limit value.

[0050] According to one embodiment, the safety temperature limiter comprises only a bimetallic strip connected to the heating resistor's circuit in such a way that the circuit is interrupted when a temperature acting on the bimetallic strip exceeds a predetermined limit. This embodiment is particularly simple, but requires additional sensors or devices if a second temperature is also to be monitored.

[0051] The embodiments comprising at least one bimetallic strip have the advantage of being particularly simple and robust. If a temperature limit is exceeded, the respective bimetallic strip bends so far that contact with the power supply of the heating resistor is interrupted. The electrical circuit for the heating resistor is thus interrupted. As soon as the bimetallic strip has cooled sufficiently, contact is re-established without the need for intervention.

[0052] According to a further embodiment, the safety temperature limiter comprises a first temperature sensor for measuring a first temperature of the heating resistor and a second temperature sensor for measuring a second temperature of the heating resistor, wherein the first temperature sensor is arranged upstream of the second temperature sensor.

[0053] This design has the advantage that the sensor data allows for a variety of control options. In particular, temperature limit values ​​can also be changed via software.

[0054] The first temperature sensor is designed to measure a first temperature of the heating resistor. The second temperature sensor is designed to measure a second temperature of the heating resistor. Any suitable temperature sensors can be used for this purpose. These can be designed as bimetallic thermometers, for example. They can also comprise NTC thermistors or PTC thermistors.

[0055] According to one embodiment, the first temperature sensor and the second temperature sensor are mounted externally on the heating resistor and spaced apart from each other along the longitudinal axis of the heating tube. Preferably, the first temperature sensor is mounted closer to the first end of the heating tube, and the second temperature sensor closer to the second end of the heating tube. In other words, the first temperature sensor is arranged upstream of the second temperature sensor. In this way, the temperature of the heating resistor can be monitored using the two temperature sensors. This serves to protect the heating resistor. The heat output of the heating resistor can be regulated based on the two values ​​determined by the temperature sensors.

[0056] It is also conceivable that a safety temperature monitor (STW) with a temperature threshold lower than that of the STW is connected upstream of the STL. The heating tube is also deactivated if the STL's temperature threshold is exceeded, but is reactivated if the temperature falls below the STL's threshold.

[0057] The heating pipe therefore offers a way to feed excess energy from a solar power system into the heating circuit of a central heating system or a hot water system. For this purpose, the heating pipe is integrated into the heating circuit of the central heating or hot water system. The two temperature sensors and the sensor device determine specific values, which are transmitted to the control device. Based on the transmitted values, the control device adjusts the heating resistance or the heat output of the heating resistance.

[0058] Preferably, the operating range of the heating tube is between 28°C and 150°C at a temperature of the heating wire, measured at the first and / or second temperature sensor. Preferably, the heating tube is deactivated at a temperature of the heating wire, measured at the first and / or second temperature sensor, of 110°C.

[0059] Further advantages, objects, and features of the present invention will be explained in the following description of the accompanying figures. Similar components may have the same reference numerals in the various embodiments.

[0060] The figures show: Figure 1 is a perspective view of the heating tube with partial section, Figure 2 is a schematic view of an embodiment of the heating tube, and Figure 3 is a schematic view of an alternative embodiment of the heating tube.

[0061] In Figure 1An embodiment of the heating tube 1 is shown in partial section. In this embodiment, the heating tube 1 extends along the axis A. The heating tube has a line 2, which also extends along the axis A. The line 2 is designed as a tube having a circular cross-section. The longitudinal axis of the line 2 in this embodiment is congruent with the axis A. The line 2 has a first end 11 on one side and a second end 12 on the opposite side of the line 2 along the longitudinal axis.

[0062] Line 2 has an opening at each of its first end 11 and its second end 12. For example, a heat transfer medium can flow through these openings into line 2 at the first end 11 and out of line 2 at the second end 12. The flow direction within line 2 is predominantly in direction 16, with this direction 16 running parallel to axis A.

[0063] Viewed radially, the heating resistor 4 is arranged outside the line 2, wherein the heating resistor 4 is in contact with the line 2 and is wound around it with a plurality of turns. The heating resistor 4 thus extends in the direction of the axis A over a region B1 of the line 2, whereby the region B1 does not extend to the ends 11 and 12.

[0064] Viewed radially, a sheath 3 is also arranged outside the line 2. The sheath 3 extends in the direction of the axis A over an area B2, wherein the area B2 is larger than the area B1 in the illustrated embodiment. Furthermore, viewed radially, the sheath 3 is also arranged outside the heating resistor 4 in the area B1, wherein the sheath 3 is in contact with the heating resistor 4 within the area B1. Outside the area B2, the sheath 3 is in contact with the line 2.

[0065] The casing 3 has a first layer 14 and a second layer 15. The second layer 15 forms the outer side of the casing 3, so that the second layer 15 forms the two end faces and the outer surface of the casing 3. The first layer 14 is located inside the casing 3, i.e., enclosed by the second layer 15 with respect to the end faces and the outer surface. The inner surface of the casing 3 forms the first layer 14.

[0066] At its ends 11, 12, the heating pipe 1 has connections (not shown in detail) for integrating the heating pipe 1 into a water pipe. These connections are standard connectors.

[0067] Figure 2 shows a sectional view of the Figure 1 shown heating pipe 1.

[0068] The longitudinal axes of the cable 2 and the sheath 3 continue to coincide with the axis A. The heating resistor 4 is wound around the cable 2 with several turns.

[0069] At the first end 11 and at the second end 12, fastening devices 13 are provided around the line 2, which are designed as connecting pieces for integrating the heating pipe 1 into a piping system. These fastening devices 13 protrude beyond the line 2 at the second end 12 along the axis A and in the direction 16, and at the first end 11 along the axis A and opposite the direction 16. The fastening devices 13 are flush with the casing 3 at the second end 12 along the axis A and opposite the direction 16, and at the first end 11 along the axis A and in the direction 16, so that the fastening devices 13 lie outside the area B2. The casing 3 and the fastening devices 13 touch each other with their end faces.

[0070] The heating tube 1 has a first temperature sensor 6 inside the line 2 and a second temperature sensor 7 arranged downstream thereof, wherein the temperature sensors 6 and 7 are spaced apart from one another and the first temperature sensor 6 is preferably located at the first end 11 and the second temperature sensor 7 is preferably located at the second end 12. The first temperature sensor 6 and the second temperature sensor 7 are each located within the area B1 and within the area B2. The two temperature sensors 6 and 7 are each connected to the control device 10 via separate cables. The cables run from the line 2 and through the casing 3 essentially in a radial direction.

[0071] The flow sensor 5 is located inside the line 2. It is connected to the control device 10 via a cable. The cable runs radially from the line 2 and through the casing 3. Alternatively, a wireless connection can be provided.

[0072] The control device 10 is located decentrally relative to the line 2, the casing 3, and the heating resistor 4. Thus, the control device 10 is arranged externally to these components of the heating pipe 1. The control device 10 is connected by a cable to the temperature sensor 6, the temperature sensor 7, the flow sensor 5, and the sensor device 8, and receives data from these components. Furthermore, the control device 10 is connected to the phase control 9 via a cable.

[0073] The phase control 9 is located decentrally relative to the line 2, the sheath 3, and the heating resistor 4. Thus, the phase control 9 is arranged externally to these components of the heating tube 1. The phase control 9 is connected to the control device 10 via a cable and to the heating resistor 4 via another cable.

[0074] The sensor device 8 is located decentrally relative to the line 2, the casing 3, and the heating resistor 4. Thus, the sensor device 8 is arranged externally to these components of the heating tube 1. In this embodiment, the sensor device 8 is connected to the control device by a cable.

[0075] Figure 3 shows a schematic representation of an alternative embodiment of the heating tube 1 in a sectional view.

[0076] This differs from the one in Figure 2shown in that it does not provide a phase control 9 and no temperature sensors. Instead, the heating tube 1 has a first bimetallic strip 21 and a second bimetallic strip 22 for temperature limitation both in the heated medium and in the heating resistor. The bimetallic strips 21, 22 are connected to the power supply 20 of the heating resistor 4. If the temperatures of the medium to be heated and the heating resistor 4 are below predetermined limits, the power supply circuit is closed via the bimetallic strips 21, 22. This situation is in Figure 3 If at least one of the temperatures rises above the limit value, the corresponding bimetallic strip 21, 22 bends out of the Figure 3 shown position and loses contact with the heating resistor 4. The circuit of the power supply 20 is thus interrupted until the temperature falls below the limit again.

[0077] To control the heating output, a wirelessly controllable socket 18 is provided, which is located outside the heating tube 1 and into which a power supply cable 20 can be plugged. The control device 10 controls the socket 18 to provide power to the heating tube 1 when a surplus of solar power is being produced and to not provide power when no surplus of solar power is being produced. List of reference symbols

[0078] 1 Heating pipe 2 Pipe 3 Sheath 4 Heating resistor 5 Flow sensor 6 First temperature sensor 7 Second temperature sensor 8 Sensor device 9 Phase control 10 Control device 11 First end of the heating pipe 12 Second end of the heating pipe 13 Fastening device 14 First layer / insulation material 15 Second layer 16 Flow direction of the heat transfer medium 18 Wirelessly controllable socket 20 Power supply 21 First bimetallic strip 22 Second bimetallic strip ALongitudinal axis of the heating pipe B1 First area B2 Second area

Claims

1. Heating pipe (1) for preheating a heat transfer medium of a central heating system or for preheating drinking water, wherein the heating pipe (1) has a line (2) for transporting the heat transfer medium or the drinking water, a heating resistor (4) in thermal contact with the heat transfer medium or drinking water carried in the line (2), a heat-insulating sleeve (3) which encloses the line (2) and the heating resistor (4), a safety temperature limiter for interrupting the preheating by means of the heating pipe (1) at a first and / or second temperature, wherein the heating pipe (1) has at least one sensor device (8) which detects at least one value of an external variable, wherein a heating output of the heating resistor can be controlled as a function of the external variable.

2. Heating tube (1) according to claim 1, wherein the sensor device (8) is a brightness switch with which the brightness of daylight can be determined as an external variable.

3. Heating pipe (1) according to claim 1, wherein the sensor device (8) is a sensor for detecting an electrical power provided by a solar system.

4. Heating pipe (1) according to one of the preceding claims, wherein the heating pipe is substantially cylindrical in shape, wherein the heating pipe has at each of its two ends an end face through which the line (2) passes and each forms a connection for a pipeline of a line system.

5. Heating pipe (1) according to one of the preceding claims, wherein the heating resistor (4) at least partially encloses the line (2), wherein the heating resistor (4) encloses the line (2) with several turns and the heating resistor (4) and the line (2) are in direct contact with each other.

6. Heating tube (1) according to one of the preceding claims, wherein the heating resistor (4) consists of a constantan wire.

7. Heating tube (1) according to the preceding claim, wherein the constantan wire has an insulation made of extruded PFA.

8. Heating pipe (1) according to one of the preceding claims, wherein the safety temperature limiter comprises a first bimetallic strip (21) and a second bimetallic strip (22) which are connected to a power supply (20) of the heating resistor (4), so that the power supply (20) is interrupted when a first temperature acting on the first bimetallic strip (21) exceeds a predetermined first limit value and / or when a second temperature acting on the second bimetallic strip (22) exceeds a predetermined second limit value.

9. Heating pipe (1) according to one of the preceding claims, wherein the safety temperature limiter has a first temperature sensor (6) for measuring a first temperature of the heating resistor (4) and a second temperature sensor (6) for measuring a second temperature of the heating resistor (4), wherein the first temperature sensor (6) is arranged upstream of the second temperature sensor (7).

10. Heating pipe (1) according to one of the preceding claims, wherein the heating pipe (1) has a flow sensor (5) for measuring the flow of the heat transfer medium or the drinking water through the line (2).

11. Heating tube (1) according to one of the preceding claims, wherein the safety temperature limiter is designed to interrupt the preheating by means of the heating tube (1) at a first limit value of 75°C and / or a second limit value of 120°C.

12. A system comprising - a device for heating a heat transfer medium of a central heating system or drinking water; - a photovoltaic system for providing electrical energy from sunlight; - at least one heating pipe according to one of claims 1 to 11, wherein the heating pipe is connected into a piping system of the system that carries the heat transfer medium or drinking water, wherein the heating pipe is operable with electricity provided by the photovoltaic system.

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