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Comparing air source heat pumps part 2

Posted by John Lightfoot on 25 March 2011 at 9:45 am

In case you haven't had time to read the first half of this two part blog, I will repeat the caveat I started with in Part 1. Before you start selecting the particular air source heat pump you want, make sure that you have taken all practical insulation measures possible to reduce the heat loss from your property.

Minimum operating temperature

Another way of slimming the list down, after going through my recommendations in Part 1, is to make sure the air source heat pump you chose can work at low ambient (outside) temperatures.

This should not be an issue with MCS accredited units, but it is worth checking at what temperatures the heat pump will continue to produce the flow temperature your system requires. It is obviously not much good, if, just when you need heat, if the air source heat pump can no longer produce flow temperatures which are adequate for your application.

Most units continue to work down to -20C, but some will not produce water flow temperatures any where near your design conditions. I would normally recommend ensuring that air source heat pumps can still provide design flow temperatures down to -15C.

Maximum flow temperature

If you intend to use your air source heat pump with radiators, there are models on the market which produce high flow temperatures of 60C+ and still provide reasonable CoPs. It’s worth commenting here though that the lower the flow temperature you can use, by using more emitter surface area (larger radiators) the better your unit's CoP will be and therefore the lower your fuel bill.

However, there are some circumstances due to, for example, lack of wall space, where higher water flow temperatures are not only desirable, but a necessity. In these cases, checking the maximum flow temperatures available from the products you are considering, and their CoP at these conditions, can help you eliminate a lot of products.


As well as the size of unit, from an output point of view, it is worth considering the physical size of the product. Some units are much larger than others for the same output and some are much taller than others, which might be an important consideration if you are positioning the unit under a ground floor window.


You may wish to use a heat pump which can be used with the type of controls that you or your family are used to using. A heat pump can be the most efficient on the market, but only if you control it properly….!

Electrical Supply

When selecting your heat pump make sure the model you select is suitable for single phase electrical supply, not three phase supply, which is only available in less than 1% of the UK’s domestic premises.

Sound levels

Most modern heat pumps are very quiet, but it is worth checking the sound levels between models. This may be the little detail you need to make the choice between your final selection of products.

Have I missed anything?

Let me know if there is anything else you think I have forgotten that should be considered when selecting an air source heat pump.

Happy selecting!

Photo: Heat King

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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