Types of heat pumps compared The consumption of resources for heating and hot water production must be reduced in the near future, and heat pumps are one way of doing this. How does a heat pump work? A medium, the refrigerant or working gas, is compressed in a circuit, where it heats up and transfers this heat to another medium, such as the heating circuit. In the other direction, it expands and cools down in the process and can extract heat from the environment. A phase transition of the medium usually plays an important role here, as large amounts of energy can be stored or released. This principle can be used for both heating and cooling. However, the term heat pump is only used for heating systems. One important distinguishing feature is the medium that is used as the heat source, another is the motor, which can be electric or an internal combustion engine. What types of heat pumps are there? Air-to-air heat pump: Here, the exhaust air from the building is used as a heat source and used directly to heat the supply air, e.g. in passive houses. Air-to-water heat pump: Here, the heat from the ambient air is utilised, resulting in a relatively cost-effective system. However, efficiency suffers particularly at low outside temperatures. Salt-to-water heat pump: This system utilises the heat from the ground (geothermal heat) - there are different systems here: On the one hand, possible through deep boreholes and deep probes. Although these are highly efficient, they are also correspondingly complex and expensive and require authorisation. On the other hand, surface collectors are also possible, which are simply inserted into the ground below the frost line. The space requirement must also be taken into account here. Water-to-water heat pump: This system utilises heat from a water reservoir (groundwater). As the temperature of the groundwater is only subject to slight fluctuations, a water-to-water heat pump is highly efficient. Which system is best suited for which application depends on various factors and should be checked by a specialist.