Heat Pumps

Heat Pumps

There is quite a bit of confusion and ignorance over heat pumps so here I will try to give some simplistic explanation as to what they are and what they are good for.

What is a heat pump?

A heat pump is simply a device that transfers heat energy from a “source” to a “sink”.

From my point of view looking at space/comfort heating heat pumps absorb energy from a cold thing and release it to a warmer one (your living or working space) – like a fridge in reverse.

The cold thing can be air, water or the ground – hence air source, water source, ground source.  

A heat pump uses a small amount of external power to accomplish the work of transferring energy from the heat source to the heat sink.

I have most experience with Air Source Heat Pumps (ASHP) – they tend to be the simplest and cheapest – and I am currently working on replacing my old oil boiler with an ASHP.

Heat from the air is absorbed at low temperature into a fluid (refrigerant).

This fluid then passes through a compressor where its temperature is increased, and transfers its higher temperature heat to the heating and hot water circuits of the house.

There are 2 types of ASHP.

Air To Water

An air-to-water system distributes heat via your wet central heating system.

Heat pumps work much more efficiently at a lower temperature than a standard boiler system would.

This makes them more suitable for underfloor heating systems or larger radiators, which give out heat at lower temperatures over longer periods of time.

How an Air Source Heat Pump works

Air To Air

An air-to-air system produces warm air which is circulated by fans to heat your home. They are unlikely to provide you with hot water as well.

It can get heat from the air even when the temperature is as low as -15° C.

Typical outdoor modules for an air source heat pump

Why Bother?  What’s the advantage?

So despite what many people may tell you about electric heating and electric heaters they are actually very efficient – in fact we regard them as 100% efficient so 1kW IN = 1kW OUT.

Doesn’t get better than that right?

Well actually yeah it does.  If you put 1kW of power into a heat pump you can multiply the energy output significantly. 

The ratio between energy IN and energy OUT is described as the Coefficient Of Performance or COP so an electric heater has a COP of 1.

Claims for heat pumps vary wildly and I have seen some frankly ridiculous claims of COP as high as 6 or even 7 but these should be taken with a pinch of salt.

Typically COP for ASHPs will be between 3-4 which means that every 1kW electrical energy input generates between 3 and 4kW useful heat energy in output to the property.


Heat pumps are on the rise and there are some pretty good Gov’t backed schemes on the go just now – one of which I am taking advantage of in my own home to replace oil with an ASHP.

Free Of Charge!

Are there any problems to consider?

Operational issues with heat pumps deliver energy to water are almost always caused by:

  • failings in the initial evaluation
  • poor design
  • inappropriate product specification and/or application
  • method of delivery of energy to the property. 

Rarely is there a problem with the heat pump.

Heat pumps operate most effectively when delivering heated water at the lowest temperature possible which in turns means a higher COP (coefficient of performance).  

So the main things to think about and make sure your specifier / installer has thought about are:

  • Radiator sizing – low water temperature means over-sized radiators or special low water content / high thermal mass units – or underfloor heating;
  • Buffer vessels – very useful to prevent / reduce short cycling of the heat pump (compressors work best flat out for long periods);
  • Electric immersion heaters – many heat pump systems employ these to boost water temperatures – expensive and carbon intensive;
  • Specification – check the heat pump being offered works at the average winter ambient your property “enjoys”
  • Defrost mechanism – a heat pump takes energy from outside air but it can freeze – does it reverse cycle which is good or use an electric element or simply stop working?  
  • Does your specifier / installer even know the answer?
  • Noise from the outdoor unit – should be 50dB or less to ensure no problems with neighbours;
  • Control – does the system operate under weather compensation for example for maximum efficiency;
  • System longevity, warranty etc.

Heat Pumps are worthy for consideration in many applications – however the phrase “buyer beware” has never been more appropriate – speak to an expert and not just an expert salesman.

If you are interested Resource Efficient Scotland has some useful information for business here.