How heat pumps work

Harvesting nature's energy by using a heat pump

Heat transfer

At the heart of a modern air source, ground source or water source heat pump is a refrigeration system. This is essentially a heat transfer system.

How heat pumps work

There are two principle locations in the transfer of heat; the place where heat is absorbed, (the source), and to where it is transferred, (the destination).

The mechanical refrigeration cycle consists of an arrangement of heat exchangers in a loop. One side the loop is set to stay colder than the ambient temperature and contains an exchanger that absorbs heat. This heat is then transferred to the hot side of the loop where it is then used to heat your home and hot water.

In order to absorb and release the heat into and from the refrigerant, the system exploits the ability of the refrigerant fluid to boil from a liquid to a vapour and then to condense back into a liquid. This is a continual process while the compressor is running and circulating the refrigerant.

The cold side of the loop is where heat from the environment is collected. First the refrigerant gas is expanded and then evaporated exploiting two principles:

  1. a gas cools down when it expands (blow on the back of your hand to test it!)
  2. evaporation from a liquid to a gas robs takes heat from the environment (lick the back of your hand and feel it cool as your saliva evaporates)

Similarly, on the hot side, the two opposite principles are exploited:

  1. A gas heats up when it is compressed (so the warmed gas transferred from the cold side is made even hotter). You can feel this at the end of a bicycle pump as you pump up a tyre.
  2. Condensing a gas back into a liquid returns the heat gained during evaporation plus the extra heat gained during compression allowing water to be heated to high temperatures for hot water and heating systems.


Although some heat pumps are configured for heating only, in others it is possible to alter the direction of refrigerant flow within the system so that the system is able to deliver both heating and cooling as desired.