Heat pumps in older buildings: size, flow temperature and radiators

Whether an older house is suitable for an air-to-water heat pump depends less on the year of construction than on one figure: the flow temperature – in other words, how hot the heating water needs to be to heat all the rooms. The lower it is, the less electricity the heat pump requires. This page explains how to work this out for your house and what you need to bear in mind regarding radiators and their size.

A heat pump transfers heat from the outside air to the temperature of the heating water. The higher the temperature it has to raise the water to, the more electricity it requires to do so.

• Rule of thumb: According to the Consumer Advice Centre, a building is suitable for a heat pump if the flow temperature remains below 55 °C throughout the year wherever possible (as of March 2026). The German Energy Agency (dena) also recommends a maximum of 55 °C.
• Every degree counts: according to dena, every degree reduction in flow temperature reduces the heat pump’s electricity consumption by 1.5 to 2 per cent (as of May 2026).
• An example from the product data sheet: For the 10 kW TCL air-to-water heat pump in the range, the product data sheet specifies, in accordance with EU Regulation 811/2013, a space heating energy efficiency of 199 per cent at 35 °C and 138 per cent at 55 °C. This means that, to produce the same amount of heat, the unit requires around 44 per cent more electricity at 55 °C than at 35 °C.

Many heat pumps can also achieve higher flow temperatures; according to product data, models in the range can reach up to 65 or 80 °C. However, ‘possible’ does not mean ‘economical’: for efficient operation, what matters is how low the temperature can be, not how high it is permitted to be.

You can test whether your existing radiators can operate at a lower temperature using your current heating system. The Consumer Advice Centre’s energy advisory service describes the test as follows:

1. On a cold winter’s day, limit the flow temperature of your existing heating system to 55 °C. You can find out how to do this on your boiler in its instruction manual, or ask your heating engineer.
2. Set all thermostats on the radiators to level 3.
3. Check that all rooms are warming up.

Here’s how to interpret the result, according to the Consumer Advice Centre:

• All rooms will be warm: a heat pump is a viable option.
• Some rooms remain cold: it is likely that simply replacing the radiators there with larger ones will suffice.
• All rooms remain too cold: without additional insulation, economical operation is unlikely to be achieved.

You can get more precise results using the Federal Environment Agency’s ‘Heat Pump Building Check’ tool. The tool calculates, room by room, how much heat a room needs on a cold day and what the radiator can deliver using a heat pump; according to the Federal Environment Agency, this takes 15 to 45 minutes.

It is not usually necessary to convert an entire house to underfloor heating.

• Often, replacing just a few is enough: according to the Consumer Advice Centre, simply replacing individual radiators that are too small is often enough to lower the flow temperature sufficiently.
• The weakest room is the determining factor: according to DENa, the radiator with the lowest capacity sets the temperature for the entire system. Replacing this particular radiator offers the greatest return on investment in relation to the cost.
• Larger or different radiators: Modern flat radiators give off more heat for the same size, and switching from Type 22 to the thicker Type 33 can save 13 to 17 per cent on electricity, according to dena. There are also low-temperature or heat pump radiators.
• Well-designed radiators are sufficient: in the Fraunhofer ISE field test, adequately sized radiators achieved, on average, similarly low temperatures to underfloor heating systems.

The conversion includes hydraulic balancing: afterwards, each radiator receives as much hot water as its room requires. According to the Federal Environment Agency, this is a prerequisite for low heating temperatures and efficient operation. It is carried out by a heating engineer.

The appropriate output is determined by the heating load of the house, not by the living area alone. The heating load is calculated room by room in accordance with DIN EN 12831 by a qualified professional; the Federal Environment Agency expressly advises against making a simple estimate.

• Not too large: According to the Consumer Advice Centre, a heat pump that is oversized is also inefficient in operation. In the Fraunhofer ISE field test, the units were often larger than the actual heat consumption would have required, and consequently switched on and off more frequently.
• For reference: co2online quotes a figure of up to approximately 0.05 kW per square metre of living space for an older building that has been retrofitted with insulation; for 130 m², this amounts to around 6.5 kW, excluding hot water. This does not replace the actual calculation.
• Consumption of the old heating system: A method set out in Supplement 2 to DIN EN 12831 estimates the heating load based on previous gas or oil consumption; for this purpose, the consumption data must be provided to the specialist.

Air-to-water heat pumps are available in the 8, 10, 12 and 16 kW model classes.

Between 2021 and 2024, Fraunhofer ISE carried out measurements in 77 detached, semi-detached and terraced houses, 83 per cent of which were built before 1995, with the older ones having been largely partially refurbished. The 61 air-to-water heat pumps achieved an average annual coefficient of performance of 3.4, with the lowest being 2.6 and the highest 4.9 (press release, November 2025). This means that, on average, one kilowatt-hour of electricity was converted into 3.4 kilowatt-hours of heat. The researchers found no correlation between the year of construction and efficiency. According to the Consumer Advice Centre, a heat pump is considered efficient if it has an annual coefficient of performance of 3 or higher.

A monoblock heat pump is located entirely outdoors; mainly the water pipes and electrical connections lead into the house. Noise level is a key factor when choosing the installation site: according to the data sheet, the units in the range have a sound power level of 60 dB at 8 kW up to 68 dB at 16 kW; information on how distance and walls affect the noise level can be found under ‘Heat Pump Noise Level: Values and Installation’. The relevant authority will advise you on the noise limits applicable at the property boundary and at neighbours’ windows; a specialist contractor can assist with planning the installation site.

A heat pump requires electricity, particularly in winter, when a photovoltaic system generates little power. However, during the transitional period and for domestic hot water in summer, the system’s own solar power can cover part of the demand. Information on how to plan a system can be found under ‘PV system’. If you only want to heat individual rooms, you can also do so with a split air conditioner; details of its performance in this regard can be found under ‘Heating with an air conditioner’. The differences between the two types are explained under ‘Air-to-air or air-to-water heat pump?’.