How heat pumps work

Heat pumps use the same technology as is used in a refrigerator, exploiting the temperature difference between the refrigerant gas and its surroundings. 

Both rely on three key facts:

1. Heat will move down a ‘temperature gradient’ from a higher temperature to a lower temperature.
2. When a liquid changes to a gas, heat is extracted from the surrounding to provide the energy for this change (e.g. boiling water to steam).
3. Refrigerant gases have a boiling point well below 0°C. For example the R410A refrigerant used in Toshiba heat pumps has a boiling point of -48.5°C.

In a refrigerator the refrigerant gas extracts heat from the air inside the refrigerator, thus cooling the air. The extract heat is released into the room through the heat exchanger on the back of the unit.

A heat pump works in the same way, except that heat is removed from the air, the ground or a body of water, so that heat pumps are known as air-source, ground-source or water-source.

In all of these cases, the temperature of the heat source (air, ground, water) is higher than the -48.5°C boiling point of the refrigerant, even in a very cold winter.

So the refrigerant is circulated round the system under pressure to keep it in liquid form until it enters the evaporator. At this point, the pressure is released and the refrigerant turns to gas, extracting heat from its surroundings.

The refrigerant is then re-pressurised and converted back to a liquid in a condenser. At this point the heat that was captured from the air, ground or water is released into the system and can be used to heat water. This is effectively ‘free heating’ because the only energy that has been consumed is that needed to run the heat pump compressor to circulate the refrigerant.

The amount of heat energy that can be extracted is proportional to the temperature difference between the heat source and the refrigerant. This means air source heat pumps are more efficient in the summer but can still yield reasonable amounts of heat in winter.

The temperature of soil and water tend to fluctuate less than air temperature through the year, so ground-source and water-source heat pumps have higher seasonal efficiencies than air-source heat pumps.