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What are Heat Pumps

An air source heat pump (ASHP) extracts energy from the outside air, even at the coldest time of the year, to provide heat inside the home and is classed as a renewable system.

An ASHP system will typically provide more than three times the amount of heat for each unit of electricity used to power it. This heat is delivered into the home via new radiators and a hot water system.

The ASHP is an external unit, usually located to the rear or side of the property. They take energy held within the outside air which is run through a compression circuit before releasing this as heat into the home.

The principle of the ASHP has been in use for many years, they actually work in a similar way to a refrigerator but in reverse.


An ASHP can provide an efficient whole house heat solution as an alternative to more traditional heating systems in existing homes.

Lets get really deep into heat pumps

How do heat pumps work?

Although there are different kinds of heat pumps, they all work in the same way. Heat pumps transfer heat from the outside environment into a building through a four-step process:

  • Evaporation

  • Compression

  • Condensation

  • Expansion


This is known as a refrigeration cycle. This heat pump diagram illustrates the process:


Heat pump diagram showing the different stages of the refrigeration cycle



Heat pumps take in heat from the air or ground or sometimes water, transferring it to a heat exchanger that contains a liquid refrigerant. This refrigerant absorbs heat from the outside and evaporates, turning it into a low-pressure, low-temperature gas.



The gas is transferred to an electrically powered compressor that compresses the refrigerant. This compression increases the pressure of the gas, which raises the gas temperature.



The hot gas reaches the heat exchanger, where it’s circulated and transfers its heat to a cold water circuit. This causes the water to heat up as it absorbs heat from the gas. Once the water has reached the desired temperature, usually around 55 degrees, it’s sent to your home’s radiators and underfloor heating to warm your house.

By transferring heat to the water circuit, the refrigerant cools down enough to turn it back into a liquid.



The cooled refrigerant moves through an expansion valve, which lowers the pressure and allows it to absorb more heat energy. From there it’s pumped back into the heat exchanger to repeat the cycle.


There are some differences in how heat pumps work, depending on what type of heat pump you have:

With an air source heat pump

The cold refrigerant starts its journey in the evaporator. It absorbs heat energy from outside air blown across a heat exchanger using fans.

Although the air is cool in the winter, there’s still plenty of energy available because of the large volume of air that passes over the heat exchanger.

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