Air-conditioning units can help keep a home cool on hot days. They are now becoming increasingly popular in Germany. However, it is still not widely known that the units can also provide heating. This is highlighted by Zukunft Altbau, an information programme funded by the Baden-Württemberg Ministry of the Environment. Air-to-air heat pumps, as they are known technically, cost less to purchase than conventional heat pumps and can be installed relatively easily. They make sense in smaller homes or those used only intermittently. This applies especially to buildings without heating pipes, radiators or underfloor heating that have previously been heated with night-storage heaters, for example. One disadvantage is that they cannot supply the home with hot water for the kitchen and bathroom. Heat distribution must also be planned so that air movement in the room is comfortable and airflow noise does not become too loud. This is a major challenge, particularly in unrenovated older buildings, because of their high heating demand. In such cases, air-to-air heat pumps should generally not be used as the sole heating system.
Air-conditioning units have long been used on a large scale in southern Europe and Scandinavia. Interest among homeowners is growing in Germany as well. Sales figures are rising accordingly: according to room air-conditioning statistics from the German Association of Building Climate Control, sales increased from around 210,000 units in 2018 to around 280,000 in 2024. They are used primarily for cooling.
How an air-to-air heat pump works
In cooling mode, air-to-air heat pumps extract heat from the indoor air and release it into the outside air. This lowers the temperature inside the home. Reversing the process turns the unit into a heater. “Unlike conventional heat pumps, however, air-conditioning systems do not transfer heat via water, heating pipes and radiators. Instead, indoor units heat the room air directly,” explains Frank Hettler of Zukunft Altbau.
In heating mode, the outdoor unit draws in ambient air and extracts heat from it. Electricity is used to raise this heat to a higher temperature, and a refrigerant carries it to the indoor unit. The indoor unit in turn draws in room air, heats it and releases it back into the room. One point is important: there is no exchange of air between indoors and outdoors, so the system does not ventilate the home.
Air-to-air heat pumps are also known as split air-conditioning systems. They come in two forms: single-split systems consist of one outdoor unit and one indoor unit. Multi-split systems, by contrast, connect one outdoor unit to several indoor units that can serve different rooms. The indoor units need an electrical connection and a condensate drain, and are connected to the outdoor unit by a thin refrigerant pipe. Multi-split systems generally have two to five indoor units for each outdoor unit. Depending on the size and geometry of the home and the noise generated by the outdoor unit, two or more outdoor units may therefore be needed to heat the entire home.
Which buildings are suitable for air-conditioning units?
The systems offer several benefits. Air-conditioning systems are usually much cheaper to buy than other types of heat pump, at around 3,000 euros per room. Installation is also comparatively simple and saves costs. The units are useful for people with allergies because the indoor unit is fitted with dust and particle filters. Planning permission is generally not required, although the units must be registered with the grid operator and comply with the noise-control requirements of the German “Technical Instructions on Noise Abatement”.
Air-to-air heat pumps can be used as the sole heating system or to supplement an existing system. As the only heating system, they are suitable for homes used intermittently or infrequently, such as holiday homes, because they can warm rooms quite quickly compared with water-based systems. Smaller homes are also generally well suited, as a single outdoor unit can cover every room. It is usually sufficient to heat living rooms, bedrooms and the kitchen. A unit is not advisable in the bathroom because of the humidity.
Air-conditioning units offer their greatest benefit where there are no water-carrying pipes, radiators or underfloor heating, such as in buildings with night-storage heaters. Retrofitting a water-based heating system, including a conventional heat pump, involves substantial costs in such buildings and can quickly reach the upper five-figure range.
Larger flats or houses are often less suitable for an air-to-air heat pump. Depending on their geometry and room layout, two or even three outdoor units may be required, even with multi-split equipment.
Disadvantages of an air-to-air heat pump
Anyone heating with an air-conditioning unit must find a separate solution for producing hot water, such as an instantaneous water heater, hot-water cylinder or domestic hot-water heat pump. Many homeowners also find an outdoor unit mounted on the wall aesthetically unappealing. In addition, some people find heating the room air less pleasant than a conventional system with radiators. Radiators heat not only the air, but also produce a high proportion of radiant heat, which many people find more comfortable.
There is another consideration: a pure warm-air system such as an air-conditioning unit can create draughts. Some people find this uncomfortable. Heat distribution must therefore be planned carefully to prevent draughts and loud airflow noise. This is particularly difficult in unrenovated existing buildings because they have a high heating demand. In such cases, experts advise against relying exclusively on air-to-air heat pumps.