We found this at St Fagans museum in Cardiff. It is like a fridge in reverse. They use a liquid that evaporates easily at normal air temperature. Then an electrically powered compressor squeezes the gas. This does work on the gas which by the First Law of Thermodynamics raises the temperature of the gas. The hot gas passes thermal energy into the heating system of the building via a heat exchanger. It isn't really free energy since an electric pump is needed to run it. The question is whether it is more efficient and less polluting than other forms of electrical heating.
