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Key things to consider before installing a heat pump

Posted by John Barker-Brown on 16 April 2013 at 8:52 am

Unlike fossil fuel boilers, heat pumps are unforgiving when it comes to sizing. With a fossil fuel boiler increasing the size to add a safety factor is generally acceptable. The cost difference between a 20kW or 30kW boiler is not large. However, increasing the size of a ground source heat pump can have a marked effect on the cost and cause issues with cycling of the heat pump. Fitting a smaller heat pump also has an effect as the heat pump runs harder, relies on in-built expensive immersion heaters and ultimately could cause the ground to freeze.

Some key points to remember when looking at any heat pump ground source or air source are:-

Sizing a heat pump

Sizing of the heat pump should follow MCS guidelines. This means the heat pump needs to be sized for 100% of the heating load without the use of in-built immersion heaters. Immersion heaters are allowed but only when the heat pump needs to go outside of its design conditions. The more immersion heaters are used the higher the running costs as you are paying for direct electricity. Heat loss calculations should be completed to provide information on the heat pump peak heat load and annual load which will be used to size the ground arrays.

Sizing heat emitters for a heat pump

Heat emitters should be sized for the lowest possible flow temperatures. Any heat pump works more efficiently the lower the required outlet temperature. For example underfloor in screed generally requires a flow temperature of around 35C, where as radiators require 45-50C. Even this 10 to 15C rise in outlet temperature can result in a 25% drop in efficiency. Remember if underfloor is used, a higher flow temperature might still be required if wooden floors or thick carpets/rugs are placed over the underfloor pipes.

Insulation with a heat pump

If the insulation of the property is not as expected then the sizing of the heat pump calculated will be wrong. This might mean that the heat pump (in order to keep the building warm) has to run at a higher flow temperature or run the internal immersion heaters which as mentioned above reduces efficiency. In extreme cases the heat pump at maximum output might not be able to produce enough heat to actually keep the building warm.

Heat pump control strategy and installation

The control strategy and installation needs to be completed to maximise the use of the heat pump and minimise the use of any immersion heaters. DHW production needs to be timed to suit the end user while providing the most efficient use of the heat pump. Again to produce DHW (domestic hot water) the heat pump needs to run at a higher temperature reducing the efficiency of the heat pump.

MCS guidelines

With the introduction of the MCS guidelines for heat pump installations, issues with sizing and installation should now be avoided, leading to installations having the best possible running costs. However with the Government dragging their heels regarding the RHI and the additional costs of complying with the MCS guidelines we are now seeing the development of a two-tier market – MCS approved installations and non-MCS approved installations. This is a dangerous situation and one which could dent customer perception of heat pump technology. 

Photo: Kensa

About the author: John Barker-Brown is special projects manager at British heat pump manufacturer Kensa Engineering.

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

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7 comments - read them below or add one

Tony Stein

Tony SteinComment left on: 11 June 2018 at 3:34 pm

I bought a house around 7 years ago that had had a ground source heat pump installed. It’s worked fairly well, albeit the electricity bills are very high however it has suddenly developed a fault and after a day of trying I can’t find a single engineer to assist in fixing it. All local installers refuse to service a pump they haven’t installed so you’ll only be ok provided the installer remains in business.

i would never recommend anyone install one of these for this reason.

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John Barker-Brown

John Barker-Brown from Kensa Heat PumpsComment left on: 13 May 2013 at 4:20 pm

Hi KP,

It's a bit difficult for me to comment on this as I work for Kensa Engineering, who are the leading ground source manufacturer and supplier in the UK, so whatever I say people will say I'm bias.

However my tips would be to make sure the unit is MCS accredited, has the official list and I don't believe the model you mention is approved. This means that you will not be eligible for any grants such as the RHPP or RHI.

Also try and talk to someone who has one of these units installed and lived with it through the winter.

Make sure the installation is a MCS approved installation as this will provide you with the confidence that the system is fit for purpose, and provides the most efficient performance it can. Again a MCS approved installation is required to gain access to any grants.


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KPComment left on: 13 May 2013 at 4:05 pm


my brother is looking to install a ground source heat pump. Installer wants to go with an Aplha Innotec instead of original Waterkotte or Dimplex. Any ideas on cost of an SWC100H and any info on reliability etc?




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Cathy Debenham

Cathy DebenhamComment left on: 2 May 2013 at 8:16 am

Thank you @dougalj - it's good to hear from those happy with their installations as well as those who aren't.

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dougaljComment left on: 1 May 2013 at 10:12 am

We had a high-temp ASHP (Daikin) installed a year ago, with a 16kW output, to heat our old granite house in Cornwall.  We were advised that the high temperature output (58deg) would be OK with our existing radiators previously heated by oil, so no new pipework or radiators were needed.  We are delighted with the system, which also provides all our domestic hot water.  However, it isn't cheap to run - probably about 80% of the cost of oil - and the capital investment was much greater than a new boiler (c.£10k).  We are very disappointed that the RHI has been repeatedly delayed as this was one of the reasons we chose the technology.  Still, all in all we're pleased, and as we buy all our electricity from Good energy we feel comfortable with our choice.

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John Barker-Brown

John Barker-Brown from Kensa Heat PumpsComment left on: 29 April 2013 at 4:47 pm

Hi Mike,

You’re right if the underfloor hasn’t been designed for a heat pump then the flow temperature will be higher 40-45C.  The design of the heating system is now an important consideration of MCS guidelines, with MCS publishing a heat emitter guide which should be shared with the client advising him on the pipe centres, radiator sizes and floor finishings required to provide the lowest flow temperatures and hence greatest efficiency from the heat pump.

Radiators can and are successfully used with heat pumps and as you say these generally need to be oversized due to the lower flow temperatures from the heat pumps. At a flow temperature of 50C you will roughly need a radiator of about twice the size of a traditional fossil fuel system.

As an aside, it is possible to run a heat pump on off-peak tariffs as opposed to continuously as long as the building is insulated adequately. A number of clients choose to do this to take advantage of the low prices of Off-Peak, in particular there used to be a Economy 10 tariff which was ideal for use with heat pumps. 

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mikem0Comment left on: 29 April 2013 at 2:30 pm


While all of your points are very useful and important considerations when installing a heat pump, I think you are wrong in a couple of the temperatures. Normally underfloor heating has a flow & return temperature of 45 and 40 degrees C, flow temperatures of 35 degrees are possible but need special designs (closer pipe spacing). Wooden floors without voids fitted onto to a solid floor have minimal effect on underfloor heating carpeted floors can be a problem if very insulating. UFH flow temperature is usually only a problem when fitted into timber suspended floors (normal UK 1st floor construction) when a plated system, there the flow temperature need to ne circa 60 degrees C. Conventional radiators systems have a flow temperature of 80 degrees C but low temp radiators can run as low as 55 degrees but have to be much bigger.

UFH gives the best results with heat pumps when installed in a solid concrete floor running at 35 degrees C and running almost constantly during the heating period.

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