Air at any temperature above absolute zero contains some energy. An air source heat pump transfers some of this energy as heat from one place to another, for example between the outside and inside of a building. This can provide space heating and hot water. A single system can be designed to transfer heat in either direction, to heat or cool the interior of the building in winter and summer respectively. For simplicity, the description below focuses on use for interior heating.
The technology is similar to a refrigerator or freezer or air conditioning unit: the different effect is due to the physical location of the different system components. Just as the pipes on the back of a refrigerator become warm as the interior cools, so an ASHP warms the inside of a building whilst cooling the outside air.
The main components of an air source heat pump are:
- An outdoor evaporator heat exchanger coil, which extracts heat from ambient air
- One or more indoor condenser heat exchanger coils, which transfer the heat into the indoor air, or an indoor heating system such as water-filled radiators or underfloor circuits and a domestic hot water tank.
Air source heat pumps can provide fairly low cost space heating. A high efficiency heat pump can provide up to four times as much heat as an electric resistance heater using the same amount of electricity.[2] The lifetime cost of an air source heat pump will be affected by the price of electricity compared to gas (where available). Burning gas or oil will emit carbon dioxide and also nitrogen dioxide, which can be harmful to health. An air source heat pump issues no carbon dioxide, nitrogen oxide or any other kind of gas. It uses a small amount of electricity to transfer a large amount of heat: the electricity may be from a renewable source, or it may be generated from power stations which burn fossil fuel or nuclear energy.
A "standard" domestic air source heat pump can extract useful heat down to about 5 °C (41 °F).[3] At colder outdoor temperatures the heat pump is less efficient; it could be switched off and the premises heated using only supplemental heat (or emergency heat) if the supplemental heating system is large enough. There are specially designed heat pumps that, while giving up some performance in cooling mode, will provide useful heat extraction to even lower outdoor temperatures.
In some weather conditions condensation will form and then freeze onto the coils of the heat exchanger of the outdoor unit, reducing air flow through the coils. To clear this the unit operates a defrost cycle, switching to cooling mode for a few minutes, heating the coils until the ice melts. Air to Water heat pumps use heat from the circulating water for this purpose, which results in a small and probably undetectable drop in water temperature;[4] For air-to-air systems heat is either taken from the air in the building or using an electrical heater.[5] Some air-to-air systems simply stop the operation of the fans of both units and switch to cooling mode, so that the outdoor unit returns to being the condenser such that it heats up and defrosts.
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