Heat pumps: information about operating principles, energy sources and subsidies

The outstanding design of the STIEBEL ELTRON heat pump

What is a heat pump? That's easy to explain.

Heat pumps convert energy stored in the air, water or beneath the ground into heating energy. And not only that! Combined with a DHW cylinder, this method of heating also safeguards the domestic hot water supply. The three natural elements mentioned above contain practically limitless supplies of energy.

STIEBEL ELTRON offers highly efficient heat pumps for heating, domestic hot water heating and cooling that are suitable for for every type of home and installation space – new build or modernisation project, indoor or outdoor installation, every requirement is catered for. Engineered in Germany.

How does a heat pump work? The principle is simple: the heat pump draws energy contained in the ground, air or water and transfers it to the heating system in your home.

Operating principles

A heat pump converts environmental energy into heat for your home. First, the heat energy present in water, the ground, outdoor air or extract air is transferred to a refrigerant via a heat exchanger (evaporator). With the aid of a compressor, the refrigerant boosts this energy to a higher temperature level, making it suitable, via another heat exchanger (condenser), for the heating system or DHW heating.

Depending on the energy source, up to five parts environmental energy can be converted to heating energy for one part of electrical energy used. The environmental energy in the form of a temperature of -20 °C to +35 °C (air) must be brought to a temperature level suitable for heating (DHW). During this process, the water is "pumped" from the low temperature level to the required temperature.

Did you know: the use of renewables is set to overtake fossil fuels! With the right energy management, you can efficiently generate and store energy for your home.

Operating modes

Mono mode
Here, the heat pump is the heat generator in the building. This operating mode is suitable for all low temperature heating systems up to 55 °C flow temperature.

Mono energetic
In this operating mode, the electric heat pump is backed up by an electric booster heater. This is required, particularly with air|water heat pumps, to provide sufficient heating output at lower outside temperatures. During the longest runtimes in a temperature range down to approx. -7 °C, the heat pump functions as the sole heat generator. In terms of investment and efficiency, it can make sense to use the booster heater as backup during the few hours when the temperature falls below this value.

Dual mode – alternative
Down to a fixed outside temperature (e.g. 0 °C), the heat pump delivers all of the heating energy. When the temperature falls below this value, the heat pump switches off and the second heat generator takes over the heating operation.

Dual mode – parallel
Down to a certain outside temperature, the heat pump alone delivers the required heating energy. A second heat generator starts at lower temperatures. When the outside temperature falls below the second limit temperature, the heat pump switches off and the second heat generator takes over the entire heating operation. Unlike in dual mode alternative operation, the heat pump provides a much higher proportion of the total annual output.

Where does a heat pump draw its energy from?

Heat pump with air as heat source

Heat source: air

Air heated by the sun is universally available. Air is at its coldest when the most heating energy is needed, such as in winter. However, heat pumps can still extract enough energy for heating at outdoor air temperatures as low as -20 °C. Air|water heat pumps offer the particular advantage of being easy to install, as no extensive ground work or well drilling is required, as in the case of brine heat pumps, for example.


Air|water heat pumps
Heat pump with groundwater as heat source

Heat source: water

Groundwater stores solar energy very well. Even on the coldest days in winter, temperatures of +7 °C to +12 °C are maintained. The steady temperature level of this heat source enables the heat pump to achieve an almost constant coefficient of performance all year round. Although groundwater of suitable quality is not universally available in sufficient quantities, it is worthwhile using groundwater when possible. Utilising this heat source requires the drilling of a delivery well and a return well. In Germany, the use of groundwater usually requires the approval of the relevant authority: generally the local water board. Your local water board will advise you about the possibility of utilising these waterways.

Water|water heat pumps
Heat pump with geothermal collector (left) or geothermal probe (right) as heat source

Heat source: ground

Below a depth of approx. 1.20 m to 1.50 m, the ground remains sufficiently warm in Central Europe to allow economical heat pump operation even on cold winter days. Brine|water heat pumps, therefore, have no output fluctuations even at temperatures below freezing. They absorb the heat from the ground through pipes laid underground, through which environmentally friendly brine circulates. The brine, which cannot freeze, conducts the absorbed heat to the heat pump's evaporator. There are two types of brine|water heat pump, which differ in their installation method: the required pipework can be inserted into boreholes drilled deep into the ground, or laid horizontally. Geothermal probes, which are inserted into vertical boreholes, have the advantage in that they do not take up a lot of space. The horizontally laid geothermal collectors, on the other hand, need about two to three times as much ground surface area as the living area to be heated. If your property is large enough, you can use a brine|water heat pump to take advantage of the almost inexhaustible supply of energy available to you from the ground. These heat pumps are, moreover, virtually maintenance-free.

Brine|water heat pumps

Heat pump comparison

There are many types of heat pump – each works in a different way and makes use of a different energy source. This comparison provides information about energy recovery methods, installation space requirements and other details relating to each type of heat pump. This will help you choose the right heat pump to meet your requirements.

Heat source Outdoor air Ground: geothermal probe Ground: geothermal collector Groundwater

Type of installation

Indoor or outdoor installation

Indoor installation

Indoor installation

Indoor installation

Suitable for

Building with no garden or a small garden

Building with a small plot of land

Building with a large, treeless plot of land

Building with a large garden that is not within a water protection zone

Efficiency at extreme outside temperatures?

Low output fluctuations

No output fluctuations

No output fluctuations

No output fluctuations

Is approval required?


Yes, apply for approval

The water board must be notified

Yes, approval from local water board required.

Maintenance effort


Practically maintenance-free

Practically maintenance-free


Seasonal performance factor (SPF)

More than 3

More than 4

More than 4

More than 5

Installation effort/initial outlay

Low – no drilling or groundwork required.

Moderate – vertical geothermal probes are required.

High – pipework must be laid horizontally in the garden.

Moderate – two boreholes for delivery well and return well in the garden.


The air|water heat pump is your best choice if space is limited. The costs of both heat pump and installation are also low.
Air|water heat pumps are very versatile: given their low cost, they offer very high efficiency levels.

The groundwater situation is unsuitable? Then the brine|water heat pump is your best choice! Installation costs and effort differ with each version. The heat pump's efficiency is very high here. The heat source can also be used beyond the heat pump's service life.

The groundwater situation is unsuitable? Then the brine|water heat pump is your best choice! Installation costs and effort differ with each version. The heat pump's efficiency is very high here.
The heat source can also be used beyond the heat pump's service life.

The initial outlay is comparatively high, but this is offset by an very good efficiency during operation. Length of service life depends on the quality of the groundwater, but is generally comparable with that of a ground source heat pump. The conditions required for this type of heat pump are not always available, however.

From modernisation of an older building to a new build: the right heat pumps for your home

A heat pump is highly efficient and the heat it supplies to your home is CO2 neutral. This makes it fit for the future, as well as an excellent choice today. But which heat pump systems are right for which type of building?

Essentially, a heat pump is an option no matter what type of building. From highly energy efficient new builds through to modernised, or even unmodernised, detached houses – STIEBEL ELTRON offers the right heat pump solutions for buildings of almost any shape and size.

Discover more
The new WPL 19/24 as an indoor installation: the new inverter heat pump from STIEBEL ELTRON for modernisation projects.

What is the difference between the heat pumps for indoor installation and those for outdoor installation?

Air|water heat pumps basically differ in terms of where they are installed – indoor air heat pumps inside the building and outdoor heat pumps outside. Outdoor installation is usually the cheaper solution. This is particularly popular in new builds. In highly built-up residential areas where space is at a premium, indoor installation has a clear advantage due to the low external sound emissions. Furthermore, no space is needed outside of the building. This means the look of the building is not affected either – which makes it ideal for listed buildings, for example.

Brine|water heat pumps and water|water heat pumps are always installed indoors.

Heat pumps for new build

  • Outstanding efficiency
  • Automatic ventilation is recommended
  • Compact appliances can be used

From PlusEnergy and passive houses through to low energy houses – those who build a new house today should look to play in the premier energy league. A highly efficient heat pump is practically a given for this. Complete systems with integral ventilation offer outstanding convenience, but a separate ventilation unit or heat pump can also make sense.

Savings potential when modernising older buildings

When you opt for environmentally friendly and sustainable energy technology, you contribute to the responsible use of natural resources. There is also a lot of potential for savings when modernising older buildings. Independence from finite resources is increasingly important here as well. After all, independence means freedom. This is exactly what you achieve in energy terms with modern heat pumps. Invest once during your modernisation project to enjoy long term benefits. Your personal energy transition also adds value to your property, with the side-effect of conserving fossil fuels. Take a look at our example of a "Detached house from 1983".


Products from STIEBEL ELTRON are recipients of internationally recognised awards for their outstanding design, such as the Red Dot Award and the iF Design Award. The design work and R&D for all of our products is carried out in Germany to ensure "Made in Germany" quality. The STIEBEL ELTRON Group's largest production facility is located in Holzminden, Lower Saxony. We manufacture in various locations around the world for different markets. In doing so, we can draw on more than 40 years of experience in the development and production of heat pumps.

Our products also carry the "ErP" energy label, which rates each product's energy efficiency.

The independent EHPA quality label also confirms the excellent quality of our products. The European Heat Pump Association (EHPA) represents the interests of the European heat pump industry. Its aim is to further promote heat pump technology in the European market for residential, commercial and industrial applications.

STIEBEL ELTRON is also committed to working with associations both at home and abroad. This includes the Bundesindustrieverband Deutschland Haus-, Energie- und Umwelttechnik e.V. [German association for the building services, energy and environment industry – BDH] and the Bundesverband Erneuerbare Energie e.V. [German Renewable Energy Federation – BEE]. Find out more about our activities.

Steps to the right heat pump

  1. Building inspection
  2. Calculate the heat load
  3. Select the heat pump functions
  4. Identify the type of heat pump
  5. Find an installation location
  6. Obtain your personal quotation
  7. Service and promise of guarantee

Domestic hot water heat pumps

If you need an affordable solution for your domestic hot water: for DHW heating from renewables, the DHW heat pump offers a quick and convenient solution. DHW heat pumps are affordable to buy and easy to install. They are outstandingly efficient, as the heat is recovered from the ambient air at the installation site.