Last edited 05 Jul 2021

Air source heat pumps

Air source heat pumps (ASHPs) use the external air as a heat source to provide heat for buildings.

They work by running a low-temperature, lower pressure refrigerant fluid in heat exchanger coils through the external air. The fluid ‘absorbs’ heat from the air and boils, even at temperatures below 0° C (although the coefficient of performance (COP) decreases with lower temperature). The gas is then compressed, which further increases its temperature. The gas is passed into heat exchanger coils, where it condenses, releasing its latent heat. The process then repeats.

This is the same process that is used to extract heat from a fridge. It can be used domestically or commercially and some systems can also be used to extract heat from a building to provide cooling.

Air source heat pumps can be air-to-water systems (providing hot water for direct use or to supply radiators or underfloor heating systems), or air-to-air systems (providing hot air, either directly into an internal space, or to be distributed by fans throughout a building).

Air source heat pumps can reduce energy consumption, fuel bills and carbon emissions, particularly where there is no mains gas supply, and so alternative heating systems would consume electricity, liquid petroleum gas (LPG), oil, or coal.

They require the siting of one or more units, similar to an air conditioning units, outside the building. This may require planning permission, although in some areas, some installations may be considered to be ‘permitted developments’. Siting is important, as units can generate noise, and an electricity supply is required.

Air-to-water systems are most effective when providing lower-temperature hot water, over a longer period of time, than might be supplied by a conventional boiler. This lower-temperature hot water is appropriate for underfloor heating, or for large radiators. The coefficient of performance drops at higher sink temperatures (the temperature of the supplied water) required for smaller radiators, or for providing hot water. Higher temperature hot water might be better provided by other low-carbon systems such as solar thermal heating.

Because they provide lower-temperature hot water over long durations, they are generally most suitable for highly-insulated, air-tight buildings.

Air source heat pumps are generally low-maintenance and can be cheaper and easier to install and less disruptive than ground source heat pumps although their efficiency may be lower. Ground source heat pumps use a similar process but absorb heat from the ground, by circulating fluid through buried pipes. See ground source heat pump for more information.

Other benefits include; long life, low maintenance requirements, they are not polluting and unlike conventional heating systems they do not use combustible fuels or require flues. However, care must be taken over the selection of refrigerant, and leaks must be prevented.

Air to water heat pumps are be eligible for payments under the government’s Renewable Heat Incentive (RHI) scheme. They were eligible for funding under the Renewable Heat Premium Payment scheme, but this scheme closed on 31 March 2014 when the domestic Renewable Heat Incentive scheme was introduced. It might also be possible to obtain help with domestic installation costs through the Green Deal, however, there has been very poor uptake of the Green Deal, and it is generally considered that better financial deals are available elsewhere.