Investment in an air-to-water heat pump is supported by the Federal Subsidy for Efficient Buildings (BEG). The exact amount of the subsidy depends on various technical requirements. Information on current conditions can be found in the corresponding subsidy overviews.
How the air source heat pump works
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Important facts at a glance:
- An air-to-water heat pump extracts heat from the ambient air even at low temperatures (down to -20 °C) to heat buildings and produce hot water.
- The cycle is based on a refrigerant that evaporates even at low temperatures and is brought to the required heating level by a compressor.
- Thanks to optimized components, modern devices are very efficient (COP of up to 4.0) and operate very quietly.
- Air-to-water heat pumps can also be used to cool rooms and can further increase efficiency and cost-effectiveness by using their own solar power.
- They are ideal for modernization and are more cost-effective than brine/water heat pumps, as no earthworks are required.
An air-to-water heat pump uses the free energy from the ambient air to supply an entire building with heat. But how exactly does this process work, even when it is very cold outside? In this guide, we explain the operating principle in detail.
The refrigeration cycle process as part of the mode of operation
An air-to-water heat pump is capable of extracting heat from the ambient air even at low temperatures of down to minus 20 degrees Celsius. This is because the refrigerant used evaporates even at this temperature. The entire process of air-to-water heat pumps is based on a refrigerant cycle, which can be divided into four basic steps:
- Evaporation: A fan actively draws in outside air and directs it to an evaporator. A special refrigerant circulates in this heat exchanger. Since this refrigerant changes its physical state even at low temperatures, it absorbs the heat from the outside air and begins to evaporate.
- Compression: The resulting refrigerant vapor still has a relatively low temperature. An electrically powered compressor increases the pressure of the vapor, which also causes its temperature to rise significantly.
- Condensing: The now hot vapor flows to a condenser. Here, it releases the absorbed and increased heat to the heating system, causing the refrigerant to condense again and become liquid. This heat can now be used to heat rooms or for hot water production.
- Relaxing: Before the cycle can start again, the cooled, liquid refrigerant flows through an expansion valve. There, the pressure and temperature drop to the initial level, and the cycle repeats.
Air source heat pump
Air source heat pumps utilise free energy from the ambient air for heating. They are ideal for modernisation projects, and allow for flexible siting.
Heat from the air
[1] Vitocal air source heat pump [2] DHW cylinder [3] Heating water buffer cylinder
Heat from the air (with outdoor unit)
[1] Vitocal split air source heat pump [2] DHW cylinder [3] Heating water buffer cylinder [4] Outdoor unit
Economy and efficiency: Optimal components for economical operation
The economical operation of an air-to-water heat pump depends largely on its efficiency.
- COP value (coefficient of performance): This measure describes the ratio of useful heat generated to the drive energy used. Modern Viessmann heat pumps achieve a COP of up to 4.0 at an outside temperature of 2 °C and a flow temperature of 35 °C. This means that 1 kWh of electricity generates 4 kWh of heating energy. You can read more about the COP value and other efficiency parameters in our guide “Heat pump efficiency.”
- Optimally coordinated components: High efficiency is achieved through the interaction of several components. These include an energy-saving high-efficiency pump, a speed-controlled fan, a vibration-mounted compressor, and an electronic expansion valve.
Tip: Combine your heat pump with your own photovoltaic system. This allows you to use the solar power you generate yourself directly to operate your heat pump, thereby significantly reducing your electricity costs. Some energy suppliers now also offer special heat pump tariffs.
Flexibility: Cooling in summer and easy installation
In addition to their main task of heating, air-to-water heat pumps can also be used to cool rooms, as they have a reversible operation mode. This ensures a pleasant indoor climate even in the warm summer months. Detailed information on this can be found in the guide “Natural and Active Cooling.”
The installation of an air-to-water heat pump is particularly flexible and also cheaper than brine/water heat pumps.
- No earthworks: Since the heat pump extracts energy from the air, there is no need for complex and expensive earthworks for ground probes or surface collectors.
- Flexible installation: The units can be installed either inside or outside the house. Outdoor installation is particularly space-saving.
FAQ: Frequently asked questions
An air-to-water heat pump extracts heat from the air and transfers it to the central heating water, which then flows through radiators or panel heaters. An air-to-air heat pump, on the other hand, transfers the absorbed heat directly to the room air and is therefore often used to heat and cool individual rooms.
Yes, an air-to-water heat pump can extract heat from the outside air even at temperatures as low as -20 °C. This is possible thanks to a special refrigerant that evaporates even at very low temperatures.
Air-to-water heat pumps are ideal for modernization projects, but are also an efficient solution for new buildings. The Vitocal 250-A and 252-A models from Viessmann, for example, are optimized for modernization projects, while the Vitocal 200-A is well suited for new buildings.
