It’s a sweltering summer afternoon. You step indoors, switch on the air conditioner, and within minutes the air begins to cool. What the unit actually does is move heat, not make cold. In cooling mode it removes heat from inside and releases it outdoors via a refrigerant loop. Most modern air conditioners can run in reverse in winter, extracting heat from outside air and delivering it indoors.
Heat pumps and reversible air conditioners (air-to-air heat pumps) are spreading rapidly. In 2024 some 2.3 million new heat pumps were installed in Europe, about 40% of them air-to-air systems. Today roughly 10% of building space worldwide is heated with AC systems and other heat pumps; the International Energy Agency (IEA) forecasts that share will rise to about 20% by 2030 and over 40% by 2050.
These devices primarily move environmental heat from air, ground or water to the building, making them highly efficient and, when powered by low-carbon electricity, far more climate-friendly than fossil-fuel heating. Aside from outdoor air, usable heat sources include warm exhaust from buildings, factories and data centers, plus rivers, groundwater, wastewater and soil.
Efficiency has improved substantially. In temperate climates like Germany, modern systems can produce more than five kilowatt hours (kWh) of heat per kWh of electricity on average over a year, which makes heating roughly a third cheaper than using gas. In warmer regions, performance is even better; in southern Europe good air conditioners often deliver more than six kWh of heat per kWh of electricity annually. In Spain and Portugal, that can translate to about two-thirds lower heating costs compared with gas, helped by relatively low electricity prices (around €0.25/kWh in some places).
Specialized heat pumps can operate in very cold conditions, down to about −30 °C (−22 °F). In Nordic countries a large share of buildings is heated this way: around 60% in Norway, 40% in Finland and 25% in Sweden. Norway has restricted fossil-fuel heating since 2020 as part of emissions-reduction efforts.
Heating with AC systems usually costs less than oil, gas or electric resistance heating. Unlike coal or wood stoves, which emit soot and nitrogen oxides, heat pumps produce no local combustion pollution and are especially low-carbon when supplied by renewables. Different system types vary: air-to-water heat pumps heat water for radiators or underfloor circuits and incur additional losses, making them somewhat less efficient than air-to-air units that warm room air directly.
Air-conditioning heating also offers practical and construction advantages. New builds may avoid water-based heating systems entirely, removing the need for pipes and radiators and cutting both upfront construction and long-term energy costs—particularly in well-insulated buildings suited to air heating. Existing buildings can replace or supplement fossil-fuel systems with AC-based heating to reduce oil and gas consumption.
Because AC heating works with warm air, rooms can heat up within minutes, reducing the need for continuous operation. The direct airflow can feel uncomfortable in very cold rooms, but modern units often include occupant sensors and variable airflow to mitigate drafts.
Installation and siting matter. Typical split systems have an outdoor compressor that can serve multiple indoor units—often up to five. Outdoor units are frequently placed on street-facing facades, but can be located on rear walls, in backyards or on roofs for better appearance. Compact through-wall or portable units exist too; they require only two wall openings for air exchange. Choosing the right technology and sizing the system for a building requires careful planning. Resources such as manuals, energy labels, consultants, specialist installers and experienced neighbours can help compare options and costs, including against full heat-pump systems that also provide cooling.
Demand for cooling is driving sales: the IEA reported more than 140 million air conditioners sold worldwide in 2024. In Germany a simple residential unit can cost between €1,000 and €2,000, making them an accessible option for many households aiming to cut both bills and emissions.
This article was originally written in German.