Heat pump

In cold weather a heat pump can move heat from the cool outdoors to warm a house; the pump may also be designed to move heat from the house to the warmer outdoors in warm weather.

As they transfer, rather than generate, heat, they are more energy-efficient than other ways of heating a home.

When in heating mode, a refrigerant at the warmer temperature is compressed, becoming hot. Its thermal energy can be transferred to the cooler space.

After being returned to the warmer space the refrigerant is decompressed — evaporated. It has delivered some of its thermal energy, so returns colder than the environment, and can again take up energy from the air or the ground in the warm space, and repeat the cycle.

Air source heat pumps are the most common models, while other types include ground source heat pumps, water source heat pumps and exhaust air heat pumps. Large-scale heat pumps are also used in district heating systems.

The efficiency of a heat pump is expressed as a coefficient of performance (COP), or seasonal coefficient of performance (SCOP). The higher the number, the more efficient a heat pump is.

When used for space heating, heat pumps are typically much more energy-efficient than electric resistance and other heaters.

Because of their high efficiency and the increasing share of fossil-free sources in electrical grids, heat pumps can play a key role in climate change mitigation.

Consuming 1 kWh of electricity, they can transfer 3 to 6 kWh of thermal energy into a building.

The carbon footprint of heat pumps depends on how electricity is generated, but they usually reduce emissions in mild climates.Heat pumps could satisfy over 80% of global space and water heating needs with a lower carbon footprint than gas-fired condensing boilers: however, in 2021 they only met 10%