The heat pumps function very simple, as a refrigerator, only that they are reversed. In a closed circuit, the refrigerant circulates through a evaporator from where a small part of energy is stored in this refrigerant by its transformation from a liquid into a gas. With the help of a spiral compressor, the gaseous refrgerant compresses and heats up. From here the refrigerant continues the circuit and ends up in a condensor where solar energy are transfered to the heating system. In this stage the refrigerant changes from a gas into a liquid and starts the circuit all over again. Heat pumps are usually refered to by using two terms, first refers to the place from where the heat is extracted, and second refers to what needs to be heated.