Air source heat pumps use a compressor, expansion valve, refrigerant, and electric heat exchangers to transfer heat in and out of a building to provide space heating and cooling. ASHPs use the outdoor air as a reservoir for extracting heat (to provide space heating) and rejecting heat (to provide space cooling). ASHPs can achieve high operating efficiencies because they move heat rather than generating it. Furthermore, when supplied by renewable electricity, they provide an emissions-free heating source.
Although ASHPs have traditionally been used in warmer climates, new cold-climate heat pumps can provide useful heating in temperatures as low as -15°F; however, at such temperatures, both capacity and efficiency are significantly reduced. This means that in most typical buildings, ASHPs require use of a backup heating source (e.g. electric resistance baseboards or the existing heating system). ASHPs are installed either as ductless systems where one or more indoor units are typically mounted on walls, or as ducted systems that use the existing forced air duct system to distribute heat throughout the building.
Ground source heat pumps (often referred to as geothermal heat pumps) use an indoor heat pump unit and a heat exchanging ground “loop” buried underground (or underwater) to transfer heat in and out of a building. GSHPs take advantage of relatively stable subsurface and/or groundwater temperatures, which remain more consistent than highly-variable air temperatures.
GSHPs can extract more heat with typically greater efficiency than ASHPs in colder weather, typically enabling GSHPs to provide sufficient heating to serve the whole seasonal building load. However, due to the drilling requirements and ground loop components, the installed cost of GSHPs are significantly higher than ASHPs on a per-ton basis. Similar to ASHPs, GSHPs can provide an emissions-free heating source when supplied by renewable electricity.