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Comparing air source heat pumps part 1

Posted by John Lightfoot on 28 February 2011 at 11:31 am

Before you start thinking about which particular air source heat pump you want, make sure that you have taken all the practical insulation measures to reduce the heat loss from your property.

Let’s slim the choice down quickly!

The first thing I would suggest you can do is eradicate any heat pumps that are not certified under the Microgeneration Certification Scheme, or MCS as it has now become known.

Government and industry has worked hard over the past few years to develop a certification scheme with exacting standards to ensure that consumers get a product that meets high quality and performance standards.

So it’s MCS accredited what’s next?

When comparing the performance of heat pumps I  recommend you compare the performance in terms of heat output, normally quoted in kW, and efficiency, normally quoted in terms of Coefficient of Performance (or CoP). While I am sure you will be able to relate to heat output in kW, you may not be conversant with CoP. If you are, skip the next paragraph…!

Coefficient of Performance

CoP basically compares the energy in, in terms of kW of electricity, to the energy out in terms of kW of heat. So if, with a flow temperature of 35C, the air source heat pump produces 3kW of heat when the ambient (outside) condition is say -1C,  and it consumes 1kW of electricity its CoP, at that condition, will be its output (3kW) divided by its electrical input (1kW), which in this example equals a CoP of 3.  If it was only producing 2.1kW at this condition it’s CoP at this condition would be 2.1! So far, so good?

Conditions to compare at

When comparing duty and CoP I would advise you to compare them at the design condition for your geographical area and application. By your area I mean the recommended design condition for your particular geographical position in the UK, or indeed the world.

In Scotland for example a common design condition is -5C, in Northern England -3C, in the South East of England -1C. By your application I am referring to the type of heat emitters you are using, under floor systems can be designed typically using flow temperatures between 35C and 40C, whilst radiator systems can use water design flow temperatures of 50C+.

So it is important when comparing heat pumps for your property to do so at the design flow temperature and design ambient condition. The headline outputs and CoP quoted by manufacturers are normally based on a European Standard of an outside condition of 7oC and a flow temperature of 35oC. While one has to accept this is probably a good average condition for Europe as a whole, it is not much use when selecting equipment for the UK.

You will consume most of your energy when the temperatures are hovering around zero and below, not when they are 7oC and above, so it is important to select at these conditions. Personally I would be suspicious if a manufacturer / supplier could not provide me with this information.

Some less scrupulous manufacturers for example, although none to my knowledge listed under MCS, quote their headline heat outputs and CoPs at 20C ambient conditions. This is not much good to you given you are unlikely to require much heating when the outside temperature is 20C!

This subject will take rather more than the normal 500 words or so I would normally use to in my blog, so I have split this subject in two halves with the second half coming shortly. In the next installment I will cover other issues to be considered in your selection process such as: minimum operating temperatures; maximum flow temperatures; size; controls; electrical supply; and sound levels

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.

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