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Common problems with Air Source Heat Pumps

Posted by on 6 October 2010 at 9:35 am

An Air Source Heat Pump is generally a simpler, more straightforward system, than its ground source cousin.

An air source heat pump is best suited to houses that do not have sufficient room for ground collectors, have an outdoor pool or generally have a smaller heating requirement. Well installed systems will happily purr away, offering quiet, unobtrusive and consistent heating and hot water to a property.

This does not mean, however, that the equipment, or the people installing it, are infallible. Here is a list of the challenges faced by air source heat pumps that has resulted in most of the UK’s installed air source heat pumps costing more to run than a gas boiler. 

1. Cheap equipment: This is by far the most frequent problem with poorly installed systems. The vast majority of air source heat pumps are designed for climates radically different to that of the UK. Consequently, the equipment inside the machine is less well equipped to deal with the high levels of water in the British climate. A practical example of this can be seen in any cheap air source heat pump in winter. The water in the air gathers around the heat pump's heat exchanger and freezes. This causes the heat pump to go into a “defrost cycle”, where the machine stops heating the house and uses electricity to defrost the exchanger. The defrost cycle is never discussed or modelled when manufacturers make COP claims about their equipment. The more expensive air source heat pumps feature much larger exchangers to avoid longer defrosting periods even in the dead of winter. This keeps more heating going into the property and keeps everyone (and their wallets) happier. A more expensive ASHP will also be noticeably quieter. There is nothing more down heartening than having an inefficient and noisy machine rattling away in the corner, constantly reminding a home owner of all the money it’s not saving them.

2. Knowing when to use an air source heat pump: Many installers focus only on one make and type of heat pump and sell it as the solution to every problem. This is far from the case. Making the distinction between and air source and a ground source heat pump is an important first step, followed by what type of air source, what make, and what kW rating. While two or three air source heat pumps working together is acceptable, bolting four or more under-powered and inefficient machines together is costly, irresponsible and most infuriatingly, still going on today. 

3. Siting: Knowing where to site an air source heat pump is essential to maximising efficiency and minimising noise generated. Poorly sited machines will cause undue noise, kill plants, bother neighbours and hurt the wallet by consistently recycling their own cooled air and decreasing efficiency. 

4. Lack of installer knowledge: As with ground source heat pumps, air source heat pumps are only as good as the company that installs them. Despite the comparative simplicity of an air source heat pump system, it is not a plug-and-play solution and it still requires experienced and specialised engineers to maximise the system’s potential. Insulating the wrong pipes, fitting the wrong valves and a general misunderstanding of how the system works will only serve to frustrate owners.

Photo by Beverley & Pack

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

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5 comments - read them below or add one

Kathleen Hogan

Kathleen HoganComment left on: 19 June 2015 at 5:05 pm

I have an air source heat pump installed in my 1800 sq foot bungalow.

This was installed by people chosen to do so by the builder.

I had problems in that the stats were incorrectly and then the actuators were the incorrect ones meaning the heat was constantly on.

Following many phone calls i eventually got these issues sorted.  I had problems with noise levels during the winter, but these have dimished now that the weather is warmer.  I am contemplating having my air to water heat pump moved into the garden, to dimish the noise. Can you advise on how this would affect the efficiency of the system.

Many thanks.

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Cathy Debenham

Cathy DebenhamComment left on: 8 December 2014 at 9:35 am

@Petefeet There isn't a definitive answer to this, as it's what suits you and how you live best. However, you might find this blog on a similar theme useful - the bit about the underfloor heating. 

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PetefeetComment left on: 5 December 2014 at 9:51 pm

I have installed ASHP few months ago. What is the most efficient way of running system? I have underfloor heating in bungalow, water heated by immersion. Is 24 hours left running continously cheaper than timed. We also have a woodburner....many thanks Pete.

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Stirling Consulting Limited

Stirling Consulting LimitedComment left on: 24 October 2010 at 10:57 am

Follwoing the Energy Saving Trust report and on-going experience we have the following link which we invite any/all to complete so we can overview the current position:



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Kensa Heat Pumps

Kensa Heat PumpsComment left on: 15 October 2010 at 10:56 am

I think Duncan you are right in the fact that air source do have their place in the market, however I do think you have missed a number of important points regarding air source. 

1) You will need planning permission. They are currently not a permitted development rights (as ground source) and there are certain planning constraints imposed on them. Mainly due to the noise issue and aesthetics. In Scotland, planning requires no more than one unit to be installed within 100m of each neighbouring property.

2) Sizing - If a heat pump is sized for a peak load at 2oC, the actual rated output at -2oC drops. This needs to be taken into account when sizing.

 3) Efficiency - take care what CoP is actually quoted, as a number of manufacturers will quote the CoP at an ambient of 7oC, which results in a high CoP. Just logically thinking about it, the air temperature in winter will be lower and the heat pump has to work harder as the outside air contains less energy. This as well as an output change as described above, will also drop the efficiency of the heat pump.

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