Air-to-air, air-to-water and ground-source heat pumps
Heat pumps use a liquid refrigerant that absorbs heat from the ground (ground sourced) or the air (air-to-water or air-to-air), which is then compressed, increasing the pressure and turning the liquid into a gas state. The warm gas then condenses, releasing the trapped heat that can be used to heat water or room space. Heat can be squeezed out of the air at temperatures of up to –20°c. The process is very cost efficient.
Flow temperatures achieved from the pump are not like those of a gas or oil boiler, which means radiators have to be slightly oversized to compensate.
Heat pumps working in combination with underfloor heating make good bedfellows. Air-to-air heat pumps can work in reverse in the scorching hot summer months by extracting the heat from a building and expelling it to atmosphere.