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Air-water heat pump: On it one should pay attention with the dimensioning

The bigger the heat pump, the better… right? Not quite. When it comes to determining the right size heat pump for your home, there is little margin for error: If it’s sized too large, the heat pump will run too short; if it’s too small, the system will use supplemental heating too often . In either case, the result is an inefficient and costly heating system. To properly size an air-to-water heat pump, you need an experienced and certified heat pump installer with special sizing software. Most inadequately performing heat pumps are due to a lack of knowledge of this software. Since this seems to happen all too often, we’ve put together a quick guide to heat pump sizing to help you find the right size for your home.

Air-to-water heat pump – Intelligent and economical energy production.

Modern heat pumps can draw enough energy for heating even from cold winter air.

The air-to-water heat pump has been called the heat pump of the future because it is robust, simple, space-saving, efficient and environmentally friendly. It uses outside air heated by the sun to produce hot water and warm heating air. Modern systems can draw enough energy for heating even from cold winter air. This heating system also has the advantage of emitting less CO₂ and consuming very little energy. You can find out how much power an air-to-water heat pump needs in this guide to heat pump costs and economics.

Sizing a heat pump

Air-to-water heat pump for new buildings with good insulation

Heat pumps range in size from about 3 kW to 15 kW and are usually rated at the industry standard temperatures, which is 7 °C/35 °C. This means that a 10 kW air-source heat pump will achieve its 10 kW output at 7 degrees outdoor temperature and 35 degrees supply temperature. As a rule of thumb, a well-insulated house requires 1 kilowatt of heat per 25 m² of living space, while the same output will only heat 10 m² in a poorly insulated house.

A heat pump with an output of between 4 kW and 12 kW is normally more than sufficient for most single-family homes in Germany with good insulation. For large, poorly insulated houses, heat pumps with an output of 15 kW and 16 kW are a better choice.

The size of heat pump you need for your home depends mainly on three factors:

  • Design temperature
  • Desired room temperature
  • Flow temperature

Design temperature

Which heat pump for which house

Heat extracted from the outside air is absorbed in the evaporator and transferred to the low-pressure refrigerant. For the refrigerant to start boiling, the ambient temperature must be higher than the refrigerant temperature. The warmer the outside temperature, the better the heat pump will work.

To ensure that the heat pump can meet your home’s heating needs throughout the year, heat pumps are sized based on the coldest air temperatures of the year in your area.

The design temperature for your area is taken from the climate database – the lower the coldest temperature, the larger your heat pump needs to be. For example, the minimum outdoor temperature in Germany is usually between -12 °C and -16 °C. But in Passau, for example, the climate is moderately warm and the outdoor temperature does not drop below -5 °C.

The only drawback is that the heat pump is technically oversized for your heating needs if the outside temperature is above this “worst case” temperature. Another factor that plays a role in accurate sizing is the wind chill effect, which involves the difference between the measured temperature and the perceived temperature, as well as wind speed. It depends on how sheltered or exposed your home is.

Desired room temperature

Air-to-water heat pump Desired room temperature


Desired room temperature is determined by homeowners. As a rule, most people want a warm and comfortable room temperature of 21 °C. However, lower room temperatures require less heating, so a smaller heat pump can be used in your home. The lower the room temperature, the better.

The difference between outdoor and indoor temperatures can be used to determine the total heat loss from a property.

Flow temperature

Air-to-water heat pump - intelligent and economical energy generation

The flow temperature is simply the temperature at which the water must circulate around the radiators in your home to reach the desired room temperature.

This depends on the size of the radiators (and underfloor heating, if any) and the insulation of the house. If the radiators are too small, the flow temperature must be higher to reach the desired room temperature. The better the insulation, the slower the heat can escape to the outside, and the less heating is needed to reach the desired room temperature.

Air-to-water heat pump Outdoor installation