Retrofit kit for retrofitting an existing, fuel-fired heating system with a heat pump, as well as heating systems with an existing, fuel-fired heating system and a heat pump.

Abstract

Retrofit kit (10) for retrofitting an existing fuel-operated, in particular fossil fuel-operated, heating system (3) with a heat pump (9), wherein the retrofit kit (10) comprises at least one outdoor unit (11) of a heat pump (9) designed as an air-to-air heat pump (9) and an indoor unit (12) with a heat exchanger, characterized in that the heat exchanger (13) is designed as a plate heat exchanger, that the retrofit kit (10) further comprises a temperature sensor (19) for measuring a return temperature and a flow sensor (22) for detecting a flow through the heating system (3), and that the heat pump (9) is designed as an inverter device.

Description

The present invention relates to a retrofit kit for retrofitting an existing fuel-fired heating system with a heat pump, and to a heating system comprising a fuel-fired heating system and a heat pump, wherein the heat pump is connected in a return line of the heating system and in series with a burner of the heating system. The heat pump comprises an outdoor unit of an air-to-air heat pump and an indoor unit with a heat exchanger. Various systems for heating buildings are known in the state of the art. In Germany, only a small proportion of buildings are supplied with district heating. Another portion is heated with individual heating systems such as night storage heaters, individual gas stoves, or individual wood and pellet stoves. However, the vast majority of buildings are heated by central heating systems, most of which still use fossil fuels such as gas and oil. Heat pumps are also increasingly being used. These utilize a high degree of renewable energy and can be operated particularly energy-efficiently in well-insulated new buildings, as they can be heated with low flow temperatures. However, heat pumps are only conditionally suitable for uninsulated older buildings, which require higher flow temperatures. Extensive renovation work is often necessary to operate heat pumps economically in older buildings.Otherwise, lower efficiency must be accepted, which can make operating a heat pump uneconomical. To reduce the energy consumption of heating systems and increase the share of renewable energies, integrating additional heat generators into heating systems has become common practice. For example, solar thermal collectors are frequently used either to heat domestic hot water using solar energy or to support the heating system with solar energy. However, these are weather-dependent and, depending on their size, can only make a relatively small contribution to covering the total energy demand. Hybrid heating systems have also become popular, combining fuel-based heating systems (e.g., oil, gas, or pellets) with heat pumps. These hybrid systems are typically designed so that the heat pump is the primary energy source, with the fuel-based system only used to cover peak loads. This makes the entire heating system technically complex, requiring significant installation effort and therefore comparatively expensive. Due to the complex control systems required, these hybrid heating systems are not suitable for retrofitting existing heating systems. German patent DE 30 49 132 C2 describes an existing oil-fired heating system in which a heat pump is connected in series with the boiler's return line. The heat pump's flow is connected to the boiler's return line via a charging pump. The heat pump is an air-to-water heat pump. The heat pump's electrical circuitry can also be configured to preheat the heating system's return line. The arrangement shown in DE 30 49 132 C2 is also suitable for retrofitting an existing heating system, but it is still relatively expensive. The object of the present invention is to propose a retrofit kit and a corresponding heating system which enable a simple and cost-effective integration of a heat pump into an existing heating system. The problem is solved by a retrofit kit for retrofitting an existing, fuel-operated heating system with a heat pump, as well as a corresponding heating system with the features of the independent claims. To retrofit an existing fuel-operated heating system, especially one powered by a fossil fuel, with a heat pump, the heat pump is connected in a return line of the existing heating system and in series with a burner of the heating system. A retrofit kit is proposed for upgrading an existing fuel-operated heating system, particularly one powered by fossil fuels, with a heat pump. The kit comprises at least one outdoor unit of an air-to-air heat pump and one indoor unit with a heat exchanger. The heat exchanger is a plate heat exchanger. The retrofit kit also includes a temperature sensor for measuring the return temperature and a flow sensor for detecting flow through the heating system. The heat pump is an inverter unit. For retrofitting, an outdoor unit of the air-to-air heat pump is placed on the exterior of the building to be heated, and refrigerant lines are routed from the outdoor unit through an exterior wall into the building. The refrigerant lines are connected to the first ports of the heat exchanger for a primary refrigerant, and the second ports of the heat exchanger for a secondary refrigerant are connected to the return line of the existing heating system. A flow sensor is activated to detect flow in the heating system. A temperature sensor is also installed in the return line, upstream of a return inlet or near the return inlet of the plate heat exchanger, to measure the return temperature. A compressor of the heat pump is switched on and off based on signals from both the flow sensor and the temperature sensor. Unlike the prior art, which uses expensive air-to-water heat pumps in hybrid heating systems, the present invention utilizes an outdoor unit from a standard air conditioner (air-to-air heat pump). Such air conditioners are manufactured in very large quantities and can therefore be purchased at very affordable prices. The outdoor unit of the standard air conditioner is supplemented with an equally standard and cost-effective heat exchanger, in the form of a plate heat exchanger, as the indoor unit, which raises the return temperature of the existing heating system. This allows the indoor unit to be easily integrated into the existing heating system. The air conditioning system, originally designed as an air-to-air heat pump, is thus converted into an air-to-water heat pump. Since the plate heat exchanger can simply be installed in the return line, the installation effort for the heat pump is minimal. The proposed retrofit kit therefore allows a heat pump to be integrated into an existing heating system for a cost of less than €5,000. This significantly increases the share of renewable energy in the total heating energy demand and significantly improves the efficiency of the entire heating system. Extensive renovation work is not required. In contrast, the installation of conventional air-to-water heat pumps, even in hybrid systems, incurs costs of between €25,000 and €30,000, which can increase further due to necessary renovations. The aforementioned advantages can also be achieved with a heating system comprising a fuel-operated, particularly fossil fuel-operated, heating system and a heat pump connected to the heating system's return line and in series with the heating system's burner. Such a heating system is also used in this study. The heat pump consists of an outdoor unit of an air-to-air heat pump designed for installation on the exterior of a building to be heated, and an indoor unit with a heat exchanger. Refrigerant lines run from the outdoor unit to the indoor unit and are connected to the first ports of the heat exchanger for a primary refrigerant. The heat exchanger is a plate heat exchanger. Second ports of the heat exchanger for a secondary refrigerant are connected to the heating system's return line.Furthermore, a temperature sensor for measuring the return temperature is installed in the return line, upstream of or near the return inlet of the plate heat exchanger. A flow sensor is also installed to detect flow in the heating system. The heat pump's compressor can be switched on and off depending on a signal from the temperature sensor and a signal from the flow sensor. The plate heat exchanger enables particularly efficient energy transfer between the refrigerant in the outdoor unit and the water in the existing heating system. Furthermore, such heat exchangers are inexpensive to purchase. Following a further advantageous development, the heat pump in the retrofit kit and the heating system is advantageously filled with the refrigerant R290. Preferably, the heat pump is already pre-filled, which further simplifies installation. The natural refrigerant R290 has a very low global warming potential and contains no harmful chemicals such as fluorinated and chlorinated hydrocarbons. A particular advantage of the retrofit kit and the heating system is that the refrigerant charge is no more than 150 g. This significantly reduces or even eliminates the risk of fire and explosion, even in the event of a complete release of the refrigerant inside the building. According to the invention, a temperature sensor, in particular a thermocouple, is installed in the return line to measure the return temperature. Similarly, a temperature sensor, in particular a thermocouple, is installed in the return line of the heating system to measure the return temperature. This allows the compressor of the heat pump to be switched on and off depending on a signal from the temperature sensor. A separate control unit for the heat pump is therefore unnecessary. Instead, it is sufficient for the heat pump to be switched on and off based on the return temperature when there is flow through the heating system. This further contributes to the cost-effective integration of the heat pump into the existing heating system. The intended design is to install the temperature sensor upstream of the return inlet of the plate heat exchanger. This ensures the temperature is measured at a point where it has already cooled down as much as possible. However, it would also be conceivable to position the temperature sensor at the return inlet of the plate heat exchanger. According to the invention, a temperature threshold for the return temperature can also be set, and the compressor of the heat pump can be switched on as soon as the temperature falls below this threshold. The heat pump is thus only activated when the return temperature has cooled down sufficiently to require its activation. Preferably, the temperature threshold is 55 °C. To control the heat pump with minimal effort, switching it on and off, it is conceivable to tap into a pump signal from the existing heating pump. This pump signal is then used to activate the compressor or a compressor control unit. The heat pump's compressor is thus switched on, for example, when the pump signal is present and the temperature threshold is simultaneously undershot. According to the invention, a flow sensor is used to detect flow in the heating system. The compressor is then switched on and off depending on a signal from the flow sensor, or can be switched on and off in the heating system. The compressor of the heat pump is thus switched on, for example, when flow is detected and the temperature threshold is simultaneously undershot. Preferably, the heat pump compressor is also switched off if no flow is detected by the flow sensor or if the flow rate falls below a minimum value. Thus, the heat pump is advantageously operated only when heat can be drawn off. Preferably, the flow sensor is connected in the return line. This line has to be cut anyway to integrate the heat pump into the heating system, so the flow sensor can also be easily installed this way. The aforementioned compressor control via the flow sensor is particularly advantageous because it is very simple in design and does not require a separate control system for the heat pump. Instead, the heating system, now expanded to include the heat pump, can use the existing heating system control system. According to the invention, the heat pump is also designed as an inverter unit. The compressor speed of the heat pump can be controlled depending on the measured return temperature and / or the outside temperature. This allows the heat pump to be operated particularly economically. The retrofit kit is designed to include a flow sensor for detecting flow through the heating system. The heating system is also designed to include a flow sensor for detecting flow, particularly in the return line, and the compressor can be switched on and off depending on a signal from the flow sensor. It is also advantageous if the heat pump has a heating capacity of no more than 4.5 kW, preferably less than 4.2 kW, and most preferably less than 3.7 kW. Almost all commercially available air conditioning units cover this capacity range, making the heat pump inexpensive to purchase. Particularly during the transitional seasons, even a heat pump with this capacity can make a significant contribution to covering the overall energy demand. Further advantages of the invention are described in the following exemplary embodiments. Figure 1 shows a schematic, partially cutaway view of a building with a heating system comprising a conventional, fuel-fired heater and a heat pump; Figure 2 shows a schematic, partially cutaway view of a building with a heating system comprising a conventional, fuel-fired heater and a heat pump according to an alternative embodiment; Figure 3 shows a schematic view of a retrofit kit comprising an outdoor unit and an indoor unit of a heat pump; Figure 4 shows a schematic, partially cutaway view of a heat pump as part of a heating system comprising an outdoor unit and an indoor unit in the installed state; Figure 5 shows a schematic, partially cutaway view of a heat pump as part of a heating system in the installed state according to a non-inventional embodiment.Fig. 6 shows a schematic, partially cutaway representation of a heat pump as part of a heating system in the fully installed state according to an embodiment according to the invention, and Fig. 7 shows a schematic, partially cutaway representation of a heat pump as part of a heating system in the installed state according to a further alternative embodiment. In the following description of the exemplary embodiments, identical features, or features that are at least comparable in their design and / or function, are designated with the same reference numerals. Furthermore, these features are only explained in detail upon their first mention, while subsequent exemplary embodiments focus solely on the differences compared to those already described. For the sake of clarity, often only one or a few identical components or features are labeled. Fig. 1 shows a building 2 with a heating system 1, which includes a fuel-fired heating system 3 and a heat pump 9. The fuel-fired heating system 3 can be, for example, an oil or gas heating system, and in particular a heating system 3 powered by a fossil fuel. The present invention can be used to particular advantage in such heating systems 3 to increase the share of renewable energy in the total energy supply. However, the heating system 3 can also be a biomass-fired heating system 3 or any other fuel-fired heating system 3. The heating system 3 can be an existing heating system 3 which is retrofitted with a heat pump 9 according to the invention using a retrofit kit 10 (see Fig. 3). However, the heating system 1 can also be installed as a complete new system. The heating system 3 comprises, as is known, a burner 4 which supplies several radiators 6 with hot water via at least one flow line 5. Furthermore, the heating system 3 comprises at least one return line 7 which returns the cooled water to the burner 4, where it is then reheated. According to the illustration in Fig. 1, the components of the flow lines 5 are shown with solid lines and the components of the return lines 7 with dotted lines. Such heating systems 3 are installed in many existing buildings 2 and can often only be replaced by heating systems 3 based on renewable energies with considerable effort. Supplementing existing heating systems with a hybrid system is also frequently very expensive and requires significant installation work. According to the invention, the heating system 3 is now supplemented by a heat pump 9, which is based on a conventional air conditioner. This air conditioner is used as a second heat generator within the heating system 1 and can be easily connected in series with the burner 4 in the return line 7 of the heating system 3. The air conditioner / heat pump 9 is therefore very easy to install and requires only minimal control effort. Because the existing heating system 3 can continue to be used in its entirety and such commercially available air conditioners, especially in the range of up to 3.7 kW output, are very inexpensive, the heat pump 9 can be integrated very cost-effectively. For the installation of the heat pump 9, the outdoor unit 11 of an air-to-air heat pump 9, i.e., an air conditioner, is installed on an exterior surface 14 of the building or outside the building 2, for example, in the area of ​​an exterior wall 15 of the building 2 to be heated. The indoor unit 12 of the heat pump 9, i.e., the air conditioner, is then formed not by a fan unit, but by a heat exchanger 13 designed as a plate heat exchanger (see Fig. 3). The outdoor unit 11 is connected to the indoor unit 12, located inside the building 2, by refrigerant lines 16 that penetrate an exterior wall 15 of the building 2. The refrigerant lines 16 are shown here by dashed lines, with a (warm) refrigerant supply line indicated by short dashed lines and a (cooled) refrigerant return line by long dashed lines.The indoor unit 12, through which the warm refrigerant flows, is thus able to increase the return temperature in the return line 7 before the burner 4. This reduces the load on the fuel-fired heating system 3, as the burner 4 has to start less frequently and / or achieve a smaller temperature difference. A particular advantage is that, with the help of a very cost-effective, commercially available air conditioner, the building 2 can potentially even be supplied solely by the heat pump 9 during the transitional seasons. Since such commercially available air conditioners typically have a low heating output of approximately 3.5 kW to 3.7 kW, very favorable operating conditions can also be achieved. This allows the heat pump 9 to operate at full load for large parts of the heating season, resulting in a very high COP and avoiding unnecessarily high, wear-inducing switching cycles. The CO2 emissions of the entire heating system 1 can thus be significantly reduced.The indoor unit 12 converts the heat pump 9, originally designed as an air-to-air heat pump 9, into an air-to-water heat pump – but at significantly lower costs than installing a conventional air-to-water heat pump. Nevertheless, a renewable energy share of 50% of the total energy can be achieved with very low material and installation costs, for example, only around €3000. Operating costs can also be kept low, as the high COP of 4 and above results in minimal electricity consumption. Fig. 2 shows a building 2 with a heating system 1 according to a further, slightly modified embodiment. As already described in relation to Fig. 1, the heating system includes a fuel-fired heating system 3 and a heat pump 9, which is based on a conventional air conditioning system or air-to-air heat pump 9. In contrast to the embodiment of Fig. 1, the heating system 3 also has a storage tank 8. The storage tank 8 can be designed, for example, as a domestic hot water storage tank or as a buffer storage tank. This allows the heat pump 9 to also support the burner 4, for example, during hot water preparation in the summer months. The integration of a buffer storage tank can be particularly advantageous if the heat pump 9 can be operated with electricity from a photovoltaic system. Otherwise, the heating system 3 corresponds to Fig. 1. The heat pump 9 with the indoor unit 12 and the outdoor unit 11 is explained below with reference to Fig. 3 and Fig. 4. Figure 3 shows a retrofit kit 10 for retrofitting an existing fuel-operated heating system 3. The retrofit kit 10 comprises an outdoor unit 11, which corresponds to the outdoor unit 11 of a conventional air conditioner or air-to-air heat pump 9. The outdoor unit 11 includes an evaporator 26, which is formed by the condenser of the original air-to-air heat pump 9, a fan 25, a compressor 21, and a pressure control valve 24. A short dashed line indicates the refrigerant supply 27, which supplies the compressed and heated refrigerant to the indoor unit 12. A long dashed line indicates the refrigerant return 28, which returns the cooled refrigerant to the evaporator. The retrofit kit 10 also includes an indoor unit 12, which contains a heat exchanger 13.The heat exchanger 13 is designed as a plate heat exchanger and includes first connections 17 for a first medium, here the refrigerant, and second connections 18 for a second medium, in this case the heating water. Besides an outdoor unit 11 of a standard air conditioner, only a plate heat exchanger as an indoor unit 12 is required for the retrofit kit 10. This allows the heat pump 9 to be implemented very cost-effectively. Furthermore, the retrofit kit 10 includes a temperature sensor 19 for measuring the return temperature and a flow sensor 22 for detecting flow through the heating system 3. These are shown in Fig. 6. Fig. 4 shows the heat pump 9 in its assembled state, integrated into the heating system 1. The outdoor unit 11 was installed on an exterior wall 14 of the building 2, and the indoor unit 12 with the heat exchanger 13 was placed inside the building 2, specifically in a boiler room, in front of the burner 4 (not shown). Refrigerant lines 16 were routed through a wall penetration (not labeled) in the exterior wall 15 of the building, connecting the outdoor unit 11 to the indoor unit 12. These lines connect the refrigerant supply 27 and the refrigerant return 28 to the first connections 17 of the plate heat exchanger of the indoor unit 12. As can also be seen, the second connections 18 of the heat exchanger 13 are connected to the return line 7. The return line 7, coming from the radiators 6 and shown here as a dotted line, is connected to the return inlet 20 of the heat exchanger. The plate heat exchanger enables efficient heat exchange between the refrigerants and the returning heating water, thereby raising the return temperature. As already mentioned, heat transfer can be particularly efficient using the plate heat exchanger. The other, second connection 18 of the heat exchanger 13, on the other hand, is connected to the part of the return line 7 leading to the burner 4 (not shown here), which is shown here in solid lines to symbolize that the return temperature of the heating water has now already been raised. The refrigerant lines 16 can also be part of a retrofit kit 10 or added separately. Fig. 5 shows another embodiment of a heat pump 9, not according to the invention, in its assembled state or integrated into the heating system 1. The heating system 1 or the retrofit kit 10 includes a temperature sensor 19 for controlling the heat pump 9. The compressor 21 of the heat pump 9 can thus be switched on and off according to temperature. For example, the heat pump 9 or the compressor 21 could always start up when the return temperature reported by the temperature sensor 19, which is represented here by a dotted line to the compressor 21, is below 55 °C. A temperature threshold for the return temperature can be set. If the heat pump 9 is designed as an inverter unit, the return temperature reported by the temperature sensor 19 could also be advantageously used to control the speed of the compressor 21 according to the measured return temperature. To ensure that the compressor 21 is only switched on when heat is being drawn via the heat exchanger 13, a pump signal 23 from the circulation pump of the heating system 3 is also applied to the compressor 21, or to the circuit or control of the compressor 21, as shown in the present illustration. The compressor 21 is thus only switched on when the circulation pump of the heating system 3 is also in operation and the measured return temperature is below the set temperature threshold. As soon as the measured return temperature exceeds the set temperature threshold, the compressor 21 is switched off again. The same applies if the circulation pump no longer outputs a pump signal 23. Otherwise, the design shown in Fig. 5 corresponds to that described previously in Fig. 4. Of course, an inverter unit could also be used as the heat pump 9 in this design. Fig. 6 shows an embodiment of a heat pump 9 according to the invention in its fully assembled state, i.e., integrated into the heating system 1. In contrast to the embodiment of Fig. 5, the pump signal 23 of the circulation pump of the heating system 3 is not applied to the compressor. Instead, a flow sensor 22 is additionally installed in the return line 7 or, optionally, in the return inlet 20 of the heat exchanger 13, which is designed as a plate heat exchanger. In this case, the compressor 21 is switched on and off depending on a signal from the flow sensor 22. If the return temperature measured by the temperature sensor 19 is below the fixed or preferably preset temperature threshold and a flow through the flow sensor 22 is simultaneously registered, the compressor 21 is put into operation.Compressor 21 is switched off when no flow is detected by the flow sensor 22 or when the flow rate falls below a minimum value, which can also be adjustable. Similarly, as in this example, compressor 21 is also switched off when the measured return temperature is above the fixed or set temperature threshold. Compressor 21 is therefore only activated when heat can be dissipated. Otherwise, the design in Fig. 6 corresponds to that in Fig. 4. Finally, Fig. 7 shows another embodiment of a heat pump 9 in its assembled state, integrated into the heating system 1. In contrast to the embodiment shown in Fig. 6, the heat pump 9, the heating system 1, and, if applicable, the retrofit kit 10, also have an additional temperature sensor 19 for the outside temperature. This sensor is mounted on an exterior surface 14 of the building 2 and additionally transmits the outside temperature to a control unit for the compressor 21. This allows the compressor 21 to be controlled as a function of the outside temperature. For example, such a temperature sensor 19 could be advantageously used in conjunction with the inverter unit to control the speed of the compressor 21 as a function of the outside temperature. Otherwise, the heat pump 9 corresponds to the embodiment shown in Fig. 6. The main advantage of the invention lies in its cost-effective design through the use of a standard air conditioning unit and the resulting very rapid amortization compared to conventional air-to-water heat pumps, which are used in the prior art to supplement fossil fuel heating systems. Furthermore, unlike when using conventional air conditioning units as air-to-air heat pumps, no unpleasant air currents occur in the building being heated. In addition, the heat pump according to the present invention can be installed very cost-effectively because, by adding the plate heat exchanger, it can continue to utilize the existing heating system and its control system. The present invention is not limited to the embodiments shown and described. Modifications within the scope of the claims are possible, as is a combination of the features, even if these are shown and described in different embodiments. Reference symbol list 1 Heating system 2 Building 3 Heating unit 4 Burner 5 Flow pipe 6 Radiator 7 Return pipe 8 Storage tank 9 Heat pump 10 Retrofit kit 11 Outdoor unit 12 Indoor unit 13 Heat exchanger 14 Exterior 15 Exterior wall 16 Refrigerant line 17 First connection 18 Second connection 19 Temperature sensor 20 Return inlet 21 Compressor 22 Flow sensor 23 Pump signal 24 Pressure control valve 25 Fan 26 Evaporator 27 Refrigerant supply 28 Refrigerant return QUOTES INCLUDED IN THE DESCRIPTION This list of documents cited by the applicant was automatically generated and is included solely for the reader's convenience. The list is not part of the German patent or utility model application. The DPMA accepts no liability for any errors or omissions. Cited patent literature DE 30 49 132 C2

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Claims

Retrofit kit (10) for retrofitting an existing fuel-operated, in particular fossil fuel-operated, heating system (3) with a heat pump (9), wherein the retrofit kit (10) comprises at least one outdoor unit (11) of a heat pump (9) designed as an air-to-air heat pump (9) and an indoor unit (12) with a heat exchanger, characterized in that the heat exchanger (13) is designed as a plate heat exchanger, that the retrofit kit (10) further comprises a temperature sensor (19) for measuring a return temperature and a flow sensor (22) for detecting a flow through the heating system (3), and that the heat pump (9) is designed as an inverter device. Retrofit kit (10) according to the preceding claim, characterized in that the heat pump (9) is pre-filled with the refrigerant R290, wherein preferably the refrigerant charge is at most 150 g. Retrofit kit (10) according to one of the preceding claims, characterized in that the heat pump (9) has a heating capacity of a maximum of 4.5 kW, preferably of less than 4.2 kW and particularly preferably of less than 3.7 kW. Retrofit kit (10) according to one of the preceding claims, characterized in that the temperature sensor (19) is designed as a thermocouple. Heating system (1) comprising a fuel-operated, in particular fossil fuel-operated, heating system (3) and a heat pump (9), wherein the heat pump (9) is connected in a return line (7) of the heating system (3) and in series with a burner (4) of the heating system (3), wherein the heat pump (9) comprises an outdoor unit (11) of a heat pump (9) designed as an air-to-air heat pump (9) for arrangement outside a building (2) to be heated, and an indoor unit (12) with a heat exchanger (13), and wherein refrigerant lines (16) are led from the outdoor unit (11) to the indoor unit (12) and are connected to first connections (17) of the heat exchanger (13) for a first medium, characterized in that the heat exchanger (13) is designed as a plate heat exchanger, and that second connections (18) of the plate heat exchanger for a second medium are connected to the return line (7) of the heating system (3). heating system (3) are switched on,that a temperature sensor (19) for measuring the return temperature is connected in the return line (7) in the direction of flow upstream of a return inlet (20) of the plate heat exchanger or in the area of ​​the return inlet (20) of the plate heat exchanger, that a flow sensor (22) for detecting a flow in the heating system (3) is connected, that a compressor (21) of the heat pump (9) can be switched on and off depending on a signal from the temperature sensor (19) and depending on a signal from the flow sensor (22), and that the heat pump (9) is designed as an inverter unit, wherein a compressor speed of the heat pump (9) can be controlled depending on the measured return temperature and / or depending on an outside temperature, and that a temperature threshold for the return temperature can be set and the compressor (21) of the heat pump (9) can be switched on by falling below the temperature threshold. Heating system (1) according to the preceding claim, characterized in that the heat pump (9) is filled with the refrigerant R290, wherein preferably the amount of refrigerant is at most 150 g. Heating system (1) according to one of the preceding claims 5-6, characterized in that the heat pump (9) has a heating capacity of a maximum of 4.5 kW, preferably of less than 4.2 kW and particularly preferably of less than 3.7 kW. Heating system (1) according to one of the preceding claims 5 - 7, characterized in that the temperature sensor (19) is designed as a thermocouple. Heating system (1) according to one of the preceding claims 5 - 8, characterized in that the flow sensor (22) is connected in the return line (7).

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