An air source heat pump (ASHP) is a system which transfers heat from outside to inside a building, or vice versa. Under the principles of vapor compression refrigeration, an ASHP uses a refrigerant system involving a compressor and a condenser to absorb heat at one place and release it at another. They can be used as a space heater or cooler, and are sometimes called “reverse-cycle air conditioners”.
In domestic heating use, an ASHP absorbs heat from outside air and releases it inside the building, as hot air, hot water-filled radiators, underfloor heating and/or domestic hot water supply. The same system can often do the reverse in summer, cooling the inside of the house. When correctly specified, an ASHP can offer a full central heating solution and domestic hot water up to 80 °C.
ASHPs are often paired with auxiliary or emergency heat systems to provide backup heat when outside temperatures are too low for the pump to work efficiently, or in the event the pump malfunctions. Since ASHPs have high capital costs, and efficiency drops as temperature decreases, it is generally not cost-effective to size a system for the coldest possible temperature scenario, even if an ASHP could meet the entire heat requirement at the coldest temperatures expected.
Propane, natural gas, or oil furnaces can provide this supplementary heat. All-electric heat pump systems have an electric furnace or electric resistance heat, or strip heat, which typically consists of rows of electric coils that heat up. A fan blows over the heated coils and circulates warm air throughout the home. This serves as an adequate heating source, but as temperatures go down, electricity costs rise. Electrical service outages pose the same threat as to central forced-air systems and pump-based boilers, but woodstoves and non-electric fireplace inserts can mitigate this risk.
From experience we know that ASHPs have caused some difficult issues – not the least of which is finding a quality engineer in the UK who is able to diagnose when there is a problem and how to go about fixing it!