Air & Ground Source Heat Pumps
Heat pumps work in much the same way as a fridge but in reverse. In a fridge the heat is transported from inside to outside, while a heat pump takes heat stored below the underground frost line and transports it via the heating system to the house interior.
Air source heat pumps use the same principle but extract the heat from the air, rather than the ground. As such their installation is much simpler and cheaper but the available heat is not constant.
A heat pump operates most efficiently when the temperature difference between the heat source and the heat demand is smallest. For that reason they are most suited to powering under floor heating systems.