All heat pumps use the principle of Boyles law –€“ that the temperature and vapour pressure of a substance are directly and proportionately related at a constant volume.

Boyles Law:
PV/T = constant

With most heat pumps the overall heat pump itself has a fixed volume hence as pressure is changed the temperature changes proportionately and vice versa.

The Vapour Compression Cycle

By far the most common form of heat pump uses a vapour compression cycle and hence has some form of compressor installed in the circuit which is most usually powered by electricity but can be driven by a gas internal combustion engine.

Gas Absorption Heat Pumps


These use the principle of applying heat –€“ most usually direct gas fired –€“ to create temperature and thus pressure differentials in a mixed compound substance such as ammonia/water or lithium bromide/water.


All heat pumps have the basic need for an evaporator and condenser which perform the function of drawing heat in from a source and expelling it (respectively).

They also need a form of circulator which is a compressor in vapour compression cycle or a ‘solution pump’ in an absorption process.