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A fridge in reverse: the air source heat pump

Posted by Anna Carlini on 11 January 2017 at 12:10 pm

What is an air source heat pump?

Air source heat pumps allow us to generate hot water or warm air, by making the most of the free heat in the air outside our house. But how exactly do they work? This informative video produced by YouGen member, Nu-Heat, explains the process the air source heat pump uses to generate hot water for use in the home.

The air source heat pump uses the same technology that cools our fridges, just on a somewhat larger scale. Whereas a fridge removes energy from a well-insulated box and dumps it on the outside, a heat pump takes the energy in the outside air and uses it for heating.

In both cases heat is being extracted from a potentially very cold environment - the outside air or the interior of our fridge. So the heat pump doesn’t need warm air to work. Outside temperatures can be as low as -15° C [1]. The air is cold, but it still contains energy which can be converted into heat.

Heat can’t flow from a cold environment to a warmer one, at least not without putting more energy into the system. But by exploiting the behaviour of gases under pressure we can get around this annoying aspect of thermodynamics and concentrate the energy that is in the cold outside air.


How it works:

  • Fans draw air from outside the building into a device called the heat exchanger where it flows over an antifreeze mix in a closed loop.
  • The heat pump forces this heat exchange fluid through a compressor. Compressing a gas causes its temperature to increase, so the air gets hotter the more it is squashed into a confined space. More information on this process can be found on this page about the workings of a fridge [2].
  • Electricity is needed to operate the fan and compressor but the cost will be low relative to the value of the warmth the heat pump produces.
  • Heat always moves from a warmer medium to a cooler one, so once the air becomes warmer than its surroundings it will begin to lose heat through conduction. We can use this heat flow from the hot air to warm up other things, like air or cold water.
  • A heat pump typically generates water of 50oC. This can then be stored in an internal cylinder from which it is fed to baths, showers and underfloor heating.

So that’s how an air source heat pump can warm your house, even if the weather outside isn’t particularly warm to start with.

Video: Nu-Heat

Photo: SuperHomes


  1. Energy Saving Trust
  2. Explain that Stuff

By Anna Carlini and Alex Barrett

More information about Heat Pumps on YouGen.

Find a Heat Pump installer

Need help with any Jargon?

If you have a question about anything in the above blog, please ask it in the comments section below.

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3 comments - read them below or add one

Gordon Glass

Gordon Glass from Comment left on: 10 March 2017 at 12:49 pm

@ Richy - It would be worth googling on fridge/freezer and fan - there are some helpful images which put the use of concealed fans in context.

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Andy from Dorset

Andy from DorsetComment left on: 1 February 2017 at 5:39 pm

It seems to me you have missed out the crucial role played by the refrigerant. Cold air is not brought in from outside, far from it. A refrigerant is used to bnring energy into the house from outside and through the compression / expansion cycle can bring in this heat energy at a lower price than straight forward heating.

The compressed refrigerant flows outside and is allowed to expand through a small nozzle in the outside heat exchange unit, the box with the fans. When gases expand they become very cold and thus are able to absorb heat from their warmer curroundings, ie, the air outside which is blown passed a heat exchanger. The then warmer refrigerant comes inside and is compressed. It gets hot when compressed and thus can give off heat at this point. We take this heat for water heating etc and thus cool the refrigerant. The refirgerant then passes outside to expand again and gains warmth/energy by absorbing heat as it expands.

Hope this helps.

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Richy_WhiteComment left on: 1 February 2017 at 7:08 am

If a heat pump is a fridge in reverse why do these units you are showing have or need a big moving part like the fan? They are the weak point in most systems and require maintenance. Never owned a fridge or freezer that has had a fan! Curious.

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