Heat pumps may be used to heat buildings or water. In essence a heat pump works like a refrigerator or an air-conditioner - but in reverse!
A heat pump extracts the heat energy in the outside air by forcing it through an evaporator. In the evaporator the heat energy is absorbed by refrigerant. A compressor pressurises the refrigerant which raise its temperature. The hot refrigerant may then be passed through a heat exchanger to heat air inside a building or water inside a storage tank.