In the heat source system, the absorbed environmental heat flows to the evaporator. There it encounters the liquid refrigerant. This refrigerant evaporates even at very low temperatures. This allows the heat pump to absorb heat from the environment even in frosty conditions. The refrigerant absorbs heat from the environment and becomes gaseous.
Function of the heat pump explained simply
Request a free consultationA heat pump, often referred to as environmental heating, draws its energy directly from the environment, whether from the air, the ground, or groundwater. But how exactly does heat pump technology work? The principle is similar to that of a refrigerator—only in reverse. Instead of extracting heat from an interior space and releasing it outside, the heat pump absorbs the ambient heat outside the building and uses it to heat the interior.
Important facts at a glance:
- A heat pump extracts up to three quarters of the energy it needs from the environment (air, ground, or water) and only one quarter from electricity.
- The core components are the evaporator, compressor, condenser, and expansion valve, which work together in a closed circuit.
- Heat pumps function reliably even at very low outdoor temperatures and can be used to cool rooms in addition to heating them.
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
Heat pump principle: refrigeration cycle explained step by step
Because the temperature of the heat extracted is usually not sufficient for heating buildings and producing hot water, a thermodynamic process is necessary for the heat pump to function. The refrigeration cycle mentioned above takes place in four steps and repeats continuously:
The refrigerant, now in gaseous form, enters the compressor. This requires electricity and greatly increases the pressure of the gas. The increase in pressure also raises the temperature of the gas to the level required for heating. The compressor is the central component that uses electrical energy to convert environmental heat into usable heating energy.
Interesting fact: This principle can be observed not only in the functioning of heat pumps. If the opening of a bicycle pump is closed and the air is compressed, the cylinder of the air pump heats up.
The hot, gaseous refrigerant flows into the condenser. There, it releases heat to the heating system by warming the heating water. During this process, the gas cools down and becomes liquid again. The released heat energy is then distributed to the rooms via the heating distribution system (e.g., underfloor heating). Some of it can also be stored in a buffer or hot water tank for later use.
The refrigerant, which is now liquid again but still under pressure, is passed through an expansion valve (also known as a pressure relief valve). This causes the pressure to drop significantly, the refrigerant cools further, and the entire heat pump process begins again.
Differences between energy sources
Although all heat pumps work on the same principle, they differ fundamentally in their choice of energy source:
- Air-to-water heat pump (air heat pump): This most commonly installed type uses a fan to draw in outside air. In the heat pump's evaporator, the heat from the air is transferred to the refrigerant. Installation is inexpensive, but performance also depends on the outside temperature.
- Brine/water heat pump (geothermal heat pump): The brine/water heat pump draws its energy from the ground, where temperatures remain relatively constant throughout the year. A pipe system filled with a water-antifreeze mixture (brine) is laid deep underground. This type of heat pump delivers very high seasonal performance factors (SPF) because the energy source is stable. However, installation is more complex and costly.
You can find detailed information on how they work in our guides to air-to-water heat pumps and brine/water heat pumps.
Refrigerant: Essential for the heat pump to function
A special refrigerant is essential for a heat pump to function. A key feature is its extremely low boiling point. This allows the liquid to turn into a gas even at very low temperatures – sometimes as low as minus 20 °C. This is why a heat pump works reliably even in winter when outside temperatures are low.
Incidentally, the latest generation of Viessmann heat pumps use the natural refrigerant propane (R290), which is in no way inferior to conventional refrigerants in terms of its properties.
Compression and power consumption of the heat pump
The compressor is an essential component of the refrigeration cycle. It plays a crucial role, as the output temperatures of the ambient heat are not sufficient to heat a building without compression. Only through compression can the temperature of the refrigerant be increased to such an extent that efficient heating is possible even on very cold days with double-digit sub-zero temperatures.
Since the compressor is usually electrically driven, electricity is required for it to function. The exact power consumption of the heat pump depends, among other things, on the following factors:
- Heating requirements of the building: How much heat is needed?
- Compressor technology: What type of compressor is used (e.g., piston or scroll compressor)?
- Temperature difference: The greater the difference between the temperature of the heat source (e.g., the outside air) and the desired flow temperature in the heating system, the harder the compressor has to work and the higher the power consumption.
Info: In addition to conventional household electricity tariffs, there are special tariffs for heat pumps. You can read more about these in our guide to heat pump electricity.
Heat pump for heating and cooling
The heat pump functions reliably even at very low outdoor temperatures. Water-to-water and brine-to-water heat pumps are particularly efficient at supplying heat, as the ground and groundwater maintain constant temperatures throughout the year. However, air-to-water heat pumps also operate at temperatures below freezing. Viessmann heat pumps, such as the Vitocal 250-A, remain reliable even at low outdoor temperatures (below -10 °C).
Thanks to their reversible function, heat pumps can also cool in summer
The operating principle of a heat pump is reversible. This means that rooms can not only be heated, but also cooled with a heat pump, provided that the technical requirements are met. There are two different cooling methods:
- Natural cooling: The heat pump remains switched off. The cooler temperature of the environmental heat source (ground, groundwater) is used to cool the rooms via a heat exchanger.
- Active cooling: The function of the heat pump is actively reversed so that the heat is transported from the rooms to the outside. This mode of operation is similar to that of a refrigerator.
You can read about the detailed process of the cooling functions in our guide to natural and active cooling.
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.
FAQ – Frequently asked questions about how heat pumps work
The power consumption of a heat pump depends, among other things, on its efficiency (annual performance factor) and the required heating capacity. Most of the heating energy comes from the environment; the electricity is mainly needed to operate the compressor.
Yes, most modern heat pumps can generate hot water for the household in addition to heating rooms. There are also standalone hot water heat pumps specifically designed for hot water production, which efficiently heat drinking water all year round. These can either use exhaust air from the house (e.g., from the basement or laundry room) or extract heat from the outside air. You can find out more about this special type of heat pump in our guide to hot water heat pumps.
Yes, Viessmann offers heat pumps that reach high flow temperatures (e.g., 70 °C) and can therefore also be used in well-insulated existing buildings with conventional radiators.
You can read more about this in our guide to heat pumps in older buildings.
The combination of a heat pump and a PV system is ideal. The solar power generated during the day can be used directly to operate the heat pump, which further reduces operating costs and increases independence.
A geothermal heat pump uses the heat stored in the ground. There are two main methods used for this:
- Ground probes: These are drilled vertically up to 100 meters deep into the ground. They require little space and are particularly efficient.
- Ground collectors: These are laid horizontally and over a large area at a shallow depth. They are cheaper to install but require a larger garden area.
The most suitable method depends on the conditions of your property. You can find more details in our guides to ground collectors and geothermal probes.
