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 air source heat pump 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 air source heat pump 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 air source heat pump can offer a full central heating solution and domestic hot water up to 80 °C.
Air source heat pumps systems consist of four major elements that allow the refrigerant to pass from the liquid state to the gas: a compressor, a condenser, an expansion valve and an evaporator. When the refrigerant passes through the heating system, the high temperature (usually 100 degrees or more) transforms it into vapor or gas while the energy produces heat. The gas then goes through the compressor that increases its temperature, and then through the expansion valve that makes the hot air enter the building. Next, the hot air passes in a condenser that turns the gas into liquid again. The heat produced by the energy in the evaporation phase passes through the heat exchanger again to restart the cycle and it is used to make the radiators work, for underfloor heating (air-to-air system) or for domestic hot water.
There are main advantages related to this technology both on environmental and economic sides. First of all, air source heat pumps don't have an environmental impact as significant as the heat they use for the process is extracted either by air, water or ground and it is continuously regenerated although they still make use of electricity in the process. On the financial side, it can allow expenses savings with respect to electricity-powered heatings, it is supported by the State through the Renewable Heat Incentive and householders can reduce carbon emissions by cutting on harmful fuels. Furthermore, this technology does not need frequent maintenance but it usually works smoothly after the installation and it is cheaper to install than ground source pumps as it does not need any kind of excavation site. However, it could be less efficient than the ground pump and its performance can be negatively affected by low temperatures and it usually needs a long time and bigger surfaces to heat the interiors.