Air conditioners don’t create cold so much as move heat. In cooling mode they extract heat from interior air and dump it outside via a refrigerant loop. Most modern units can reverse that process in winter: they take heat from outdoor air and deliver it indoors. These reversible systems are commonly called heat pumps or air-to-air heat pumps.
Heat pumps are becoming widespread. In 2024 about 2.3 million new heat pumps were installed across Europe, roughly 40% of them air-to-air. Today about 10% of global building floor area is heated by air conditioners and other heat pumps; the International Energy Agency expects that share to rise to roughly 20% by 2030 and over 40% by 2050.
Rather than burning fuel, heat pumps move environmental heat from sources such as outdoor air, the ground, surface water or waste heat streams (for example warm exhaust from buildings, factories or data centers). Because they transfer heat instead of producing it by combustion, they can be far more efficient. When supplied with low‑carbon electricity, they also cut CO2 emissions compared with fossil-fuel heating.
Performance has improved significantly: in temperate climates like Germany, modern systems often deliver more than five kilowatt-hours of heat for every kilowatt-hour of electricity used on an annual average — a seasonal coefficient of performance (COP) above 5. That level of efficiency typically makes heating about one third cheaper than using gas. In warmer regions, air-source systems can perform even better; in southern Europe many units average over six kWh of heat per kWh of electricity, which—combined with relatively low retail electricity prices in some places (around €0.25/kWh)—can cut heating bills by roughly two thirds compared with gas.
Specialized heat pumps are designed to work in very cold conditions (down to roughly −30 °C). Nordic countries already rely heavily on them: about 60% of buildings in Norway, 40% in Finland and 25% in Sweden use heat-pump technology for space heating. Norway has also limited fossil-fuel based heating since 2020 as part of emissions reduction policies.
Compared with oil, gas or electric-resistance heating, heat pumps usually cost less to run. They avoid on-site combustion, so there’s no soot or nitrogen‑oxide emissions in the home, and their carbon footprint falls as the electricity supply decarbonizes. Different system types have different trade-offs: air-to-water heat pumps heat water for radiators or underfloor systems and can incur additional distribution losses, while air-to-air systems heat room air directly and are often slightly more efficient for this use.
There are practical and construction benefits too. In many new builds a well‑insulated design combined with air-based heating can eliminate water‑based radiators and pipework, lowering upfront construction costs and future maintenance. Existing buildings can replace or supplement oil and gas systems with AC-based heating to reduce fuel use. Because heat pumps warm the air directly, rooms can reach comfortable temperatures in minutes, reducing the need for continuous operation. Direct airflow can feel drafty in very cold rooms, but newer units offer features such as variable airflow and occupancy sensors to limit discomfort.
Proper selection, sizing and siting are important. Typical split systems place the compressor outside and can serve several indoor units—often up to five. Outdoor units can be mounted on street-facing facades, rear walls, roofs or placed in gardens, depending on aesthetics and local rules. Compact through-wall units and portable options also exist; these require only two wall openings for air exchange. To choose the appropriate technology and size the system correctly, consult manuals, energy labels, qualified installers or experienced neighbours and compare costs with full heat-pump systems that also provide cooling.
Demand for cooling continues to drive sales: more than 140 million air conditioners were sold worldwide in 2024. In markets such as Germany a basic residential air-to-air unit typically costs between €1,000 and €2,000, making this a relatively accessible option for households looking to cut both energy bills and emissions.
This article was originally written in German.