Air source heat pumps (ASHPs) absorb heat from the outside air. This heat can then be used to heat radiators, underfloor heating systems, or warm air convectors and hot water in your home.
Different from a ground source heat pump, an air source heat pump extracts heat from the outside air in the same way that a fridge extracts heat from its inside. It can get heat from the air even when the temperature is as low as -15° C. Heat pumps have some impact on the environment as they need electricity to run, but the heat they extract from the ground, air, or water is constantly being renewed naturally.
The Heat Pump Cycle
As can be seen from the diagram above, the heat pump cylcle is relatively simple and has had successful and trouble free use in fridges, freezers and air conditioning equipment for decades.
Essentially the energy in the outside air is absorbed by the refrigerant gas and then, via a compressor, the hot gas transfers heat energy to the circulating water to be used to heat the building and provide hot water in the cylinder. A rough rule of thumb is that for every kW of electical energy used by the heat pump, roughly 4 kW of heat are made available to the building, meaning that the surrounding air is providing 3 free kW of energy.
So while Heat Pumps are not a truly renewable technology (as they need electrical energy to run) they to take advantage of the free energy in the air around us.
If installed correctly in a well insulated building then they will be cheaper to run than oil, LPG or other forms of electric heating.