Function of the heat pump explained simply

The heat pump obtains the energy for heating from the environment. For this reason, it is also called environmental heating. But how does a heat pump actually work? The principle is similar to that of a refrigerator - only in reverse. While a refrigerator extracts heat energy from the food, i.e. the interior of the refrigerator, and conducts it to the outside, a heat pump does the opposite. It extracts thermal energy from the environment outside the building and makes it usable for indoor heating. In addition to the indoor or outdoor air, a heat pump is able to tap the thermal energy from the groundwater and the soil.

Structure and operating principle of the heat pump

Regardless of the tapped environmental energy source, the heat pump system consists of three parts:

  • Heat source system: extracts energy from the environment

  • Heat pump: Makes environmental heat usable

  • Heat distribution and storage system: distributes and stores heat in the building

Only in interaction do the components of a heat pump enable the use of environmental energy. The process begins with the heat source system. In geothermal heat pumps, a mixture of water and antifreeze circulates here - the brine, which heats up. Air-to-water heat pumps, on the other hand, draw in outside air via a fan. The brine and outside air then pass to the heat pump itself. In the so-called refrigeration cycle, the pump raises the temperature level before the heat is transferred to the distribution system consisting of panel heating or radiators, or is temporarily stored in a buffer or hot water tank.

Refrigeration cycle process core of the heat pump principle.

Because the temperature of the recovered heat is usually insufficient to heat a building or the hot water, the operation of the heat pump requires a thermodynamic process. The above-mentioned refrigeration cycle process takes place in four steps and is constantly repeated:

At the so-called evaporator, the environmental heat is transferred to a liquid refrigerant, which evaporates already at low temperatures due to its specific properties.

A current-driven compressor, called a compressor, sucks in the refrigerant vapor and compresses it. The increase in pressure also raises the temperature. The principle can be observed not only in the operation of the heat pump. If the opening of a bicycle air pump is kept closed and the air is compressed, the cylinder of the air pump heats up.

At the condenser, the heat from the hot refrigerant vapor is transferred to the heat distribution system. As the refrigerant gives off energy, it cools down.

The so-called expansion valve reduces the pressure of the refrigerant. This causes it to return to its initial state and the heat pump process can start again. The principle can also be observed, for example, in a liquid gas cylinder. If the valve is opened, ice can form on the valve even in summer.

Refrigerant essential for heat pump functionality

A refrigerant is necessary for the heat pump to perform its function. An important feature is its low boiling point, which causes the liquid to change from a liquid to a gaseous state even at low temperatures. Even temperatures of minus 20 degrees Celsius are sufficient for this. This is why the heat pump also works in winter at low outside temperatures. Viessmann heat pumps of the latest generation use the natural refrigerant propane (R290), which is in no way inferior to classic refrigerants in terms of its properties.

Compression requires electrical current

An essential component of the refrigeration circuit is the compressor. This is because without compression, the output temperatures are too low to be able to heat a building to a comfortable temperature – even more so on very cold days with double-digit minus temperatures. In practice, a number of compressors are used, including piston compressors or scroll compressors, which are all electrically driven. The power consumption of the heat pump for compression depends on many factors. These include the heat demand, the compressor technology and, last but not least, the temperature difference between the heat source and the heating system. As a general rule: The higher the temperature differential between the heat source and the flow temperature, the more the compressor has to work.

Compare electricity tariffs for the heat pump

As a rule, the operation of the heat pump is based on electricity. Many energy suppliers offer heat pump tariffs with special conditions for heat pump owners. These can be cheaper than conventional household electricity, but are subject to certain conditions. For example, the power supplier is entitled to disconnect the heat pump from the power supply during so-called blackout periods. In addition, an additional electricity meter is required for the heat pump electricity. Whether such a tariff comes into question and is more favorable should therefore be checked at the outset. However, it should be noted that the eco-balance of a heat pump can be improved if the electricity comes from renewable energies.

Heat supply ensured even in winter

Even at very low outdoor temperatures, the operation of the heat pump is reliable. The heat supply is particularly efficient with water-water and brine-water heat pumps. This is because constant temperatures prevail year-round in the ground and in groundwater. But air-water heat pumps also run at double-digit sub-zero temperatures. In extreme outdoor temperatures, an electric heater provides temporary support - if necessary.

Reversing the function of the heat pump for cooling

The principle of operation of the heat pump is reversible. For this reason, rooms can not only be heated, but also cooled - if the technical requirements are met. A distinction must be made between Natural and Active Cooling. While in the latter the function of the heat pump is actively reversed, it remains switched off in passive or natural cooling.