A milestone of heating technology

Heat pumps have their origin in the 19th century. Nevertheless, due to the industrial revolution and numerous other historical events, they have been overlooked for a very long time. It was not until the 1970s, due to very high petroleum prices, that further development of the heat pump was given attention for the residential sector. By the end of the 1990s, heat pumps as we know them today were ready for mass production. In 2020, a milestone was reached in Germany – the 1,000,000th heat pump was installed.

The topics at a glance

What is a heat pump?

A heat pump is a modern, climate-friendly and efficient heating system that is becoming increasingly popular. Basically, it is nothing more than a refrigerator, only one that operates in reverse and has additional functions. Heat pumps use free thermal energy stored in the environment to efficiently and cost-effectively heat buildings and provide hot water. Unlike conventional heating systems, combustion does not take place in a heat pump. Instead, a refrigerant is brought to a higher temperature level by using over three quarters environmental energy and one quarter electrical energy. Even at sub-zero temperatures, there is still enough thermal energy in the environment for this heat pump circuit/refrigeration circuit to work. Many heat pumps can also provide cost-effective, climate-friendly cooling for a building in the summer.

Luft/Wasser-Wärmepumpe Jersey

Step by step - how does a heat pump work?

Just like the first heat pumps in the 19th century, modern heat pumps continue to use the four main components: an evaporator, a compressor, a condenser and an expansion valve.

All heat pumps have a so-called refrigeration circuit, i.e. an evaporator in which there is a liquid refrigerant. This is heated with the help of environmental heat and thus, even at very low temperatures, becomes gaseous and begins to evaporate. In the next step, the gaseous refrigerant enters a compressor. This compresses the gaseous refrigerant, which raises its temperature. For this process, heat pumps require electrical energy (electricity) – the lower the temperature difference between the heat source and the refrigerant, the less they need.

The heat generated is then transferred to the heating circuit via a heat exchanger. This causes the temperature of the refrigerant to drop again and it slowly liquefies, but remains under high pressure. To stabilize it again, it is passed through an expansion valve. After that, the process starts again and so the heat pump cycle runs over and over again.

Overview of heat sources

Luft/Wasser Wärmepumpen

Air-source-water heat pumps use thermal energy from the outside air and can either be placed inside the building (air-to-water heat pumps for indoor installation) or placed outside, on house walls, in the garden or on roof surfaces (air-to-water heat pumps for outdoor installation). Even at sub-zero temperatures, air-to-water heat pumps still operate very efficiently.

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Sole/Wasser Wärmepumpen

Ground-source heat pumps use thermal energy from the ground and are installed inside the building. Usually, geothermal heat pumps are operated by deep boreholes (ground probes) or ground collectors. In addition, brine-to-water heat pumps can be operated with so-called ground baskets or ring trench collectors. In the pipes to the heat pump there is a water/glycol mixture, also called brine liquid, which supplies the necessary thermal energy to the heat pump. Due to the constant temperatures of approx. 8 -12° C in the ground, the operation of ground source heat pumps is very efficient and cost-effective.

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Advantages and disadvantages of a heat pump



Independence from fossil fuels

Higher planning costs, as in some cases
subject to approval ( ground-source heat pumps)

Low maintenance operation

High electricity costs due to incorrect

Increase in value of the property

Noise emission in case of incorrect
Positioning (air-source-water heat pumps
for outdoor installation)

High efficiency = low heating costs


CO2 free operation