You’ve probably been hearing and seeing lots in the media about heat pumps recently, but very little of it has been explaining what they are or how they work. Well, we’re going to try and explain it fairly simply for you……

There are various types of Air Source Heat Pumps (ASHP for short), available on the market, however, most of the talk going on at the moment, is related to ‘Air-to-Water’ heat pumps.

So, let us start with what is a heat pump and how does it work……

In fact, let us start with your regular old reliable fridge at home……

You know the basics of how these work……you put your food in, magic happens, a few hours later, the food is cool.

SPOILER ALERT……there is no magic inside a fridge!

There is a fairly simple purpose for your fridge……move temperature from one location to another to keep your food cold, it does this by using a transfer medium, a refrigerant.

There are many refrigerants out there, but they all have one thing in common; they are fluids that can boil from a liquid to a gas (vapour) and then condense back to a liquid again. All being well, they are able to repeat this cycle time after time without any reduction in performance.

An overview of the vapor compression cycle:

1. Compressor – Refrigerant leaves compressor as a high pressure, high temperature vapor.
   (This is the heavy noisy bit normally at the bottom of your fridge)
2. Condenser – Refrigerant travels through the condenser coil where the heat is rejected and it changes state. The refrigerant leaves the condenser coil as a high pressure liquid.
   (You know the funny looking fins on the back of the fridge that gets warm)
3. Expansion valve – The refrigerant passes through an expansion valve which causes a drop pressure, which in turn causes a sudden drop in temperature.
   (This bit is hidden within the fridge, so you wont see this bit)
4. Evaporator – the sub-cooled vapour travels through the evaporator coil, whereby the warm air from the room is passed over the coil to absorb the heat.
   (when you touch the back wall of your fridge and it feels nice and cold, that’s your evaporator)
5. The refrigerant leaves the evaporator as a low pressure liquid and the cycle starts again.

Ok, great…..we’ve told you how a fridge works, and now we can now hear you all…….
”what does any of this have to do with heat pumps?”, it will make sense, we promise, anyway back to it…..

So, if we know how a fridge/vapour compression cycle works, what would happen if we was to reverse the way the system worked?

Let’s move the condenser inside the fridge, the evaporator outside of the fridge and swap the compressor and expansion valves around. We would now be absorbing heat from the air within your kitchen, transferring it via a refrigerant and passing it inside the fridge, where we would now have a nice toasty box, and possibly s big puddle of melted chocolate.

All of a sudden, we have a heat pump, well at least the basic principles of one.

The main differences with our example above and the heat pump we’ll use to heat your home are:

• Instead of warming the inside of a box, our condenser shall reject all of its heat into water, which we can then pass around your heating and hot water circuits. This gives us the air-to-water heat pump
• Our evaporator sitting outside your property has a big fan, to draw large volumes of air over the coil, this allows for the maximum amount of heat to be transferred through the refrigerant to the condenser.

So, there you have it, that is how heat pumps work.