I thought I would start my series of blogs on Air Source Heat Pumps with an explanation of how a heat pump works and how in particular how a Air Source Heat Pump works. If you want to look at the diagram alongside the text, double click on it to enlarge it.

To start, just imagine a low pressure gas inside a pipe, shown as point A on the diagram. (At this stage it doesn’t matter what the gas is made of). You may recall from your school days that the pressure, volume and temperature of a gas are interactive, in other words you cannot alter one without affecting the other two. By compressing the gas (by reducing it’s volume) we increase its temperature.

So after putting the gas through a compressor we now have a hot gas (point B on the diagram). We then pass this gas through a heat exchanger where the heat from the gas is given up to the water from the heating circuit, (Point C on the diagram) in other words heating the water and cooling the gas. When the gas cools it turns to a liquid, just as steam from your kettle turns from a gas (steam) back to a liquid (water) when it cools.

Still with me? Good! We now have a warm liquid (Point D on the diagram) which we pass through a device called a expansion device which forces the liquid to expand. The pressure, volume and temperature relationship kicks in again and by expanding the liquid the temperature of the liquid drops (Point D on the diagram). The more we expand the liquid the more the temperature drops.

Now comes the last part of the journey, where we absorb heat from a totally renewable source, the outside air, we pass this cold liquid through a coil which has outside (sometimes referred to as “ambient”) air passing over it. The ambient air warms up the coil, which in turn warms up the liquid, (which can have boiling points as low as -43C) to above its boiling point so by the time the refrigerant has left the coil it has transformed or should I say boiled off into a low temperature, low pressure gas (Point E on the diagram) from where it is sucked into a compressor and the journey begins again.

So a nice little story, but what has it got us?

Well, with outside temperatures of around 0C modern heat pumps will provide around 3kW of heat for every kW of electricity consumed, 2kW absorbed from the renewable source, outside air, and a further 1kW converted from the mechanical heat produced by the compressor.

About the author: John Lightfoot is director at Thermal Energy Ventures Ltd.

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