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Four Heat Loss Surveys, Four Different Answers. How Do I Choose the Right Heat Pump Size?

AnnaB · 2026-06-17T09:27:50Z

We are installing an ASHP at the same time as doing a house renovation/extension. We live in a 1950's detached 3 bed, but will be doing a 2 storey extension and it will become 4 bedroom. There are 5 of us living in the house. I have been researching this for months and feel I have approached 4 reputable companies for quotes. They have all been and measured (where they can, as extension is still only plans) and issued estimated heat loss reports, although one company has not issued the full report as they say we would have that on receipt of a deposit. My problem is that they are vastly different in terms of estimated heat loss, estimated energy requirements and then the recommended heat pump size. The lowest heat loss given is 5.67kW at -1.5degrees and they recommend a Valiant Plus AroTherm 7kw. Another company has also suggested the Arotherm, even though their heat loss estimate is 8.41(also at -1.5 degree).The highest is 9.98kW from a company suggesting we get a Mitsubishi 11kW, however they have calculated this at -3 degrees, which I feel is an over estimate. We have also had a mitsubishi Ecodan 8.5kW form a company who estimated the heat loss at 7.99kW at -2 degrees. I am very confused! Can anyone offer at suggestions on how to choose? We have also had very different estimated heating requirements ranging from 12,344kwh to 18, 579kwh. I am presuming that this is partly due to the different estimated heat losses, but what other information should they be using to calculate this? They have all given the same estimates for hot water requirements, but even so, they seem to have all based this on 4 person occupancy, whereas we are a family of 5. I'd be really grateful for any advice on how to proceed. Many thanks, Anna

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Air Source Heat Pumps (ASHPs)

Last Post by JamesPa 4 months ago

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JamesPa

(@jamespa)

Illustrious Member Moderator

Joined: 4 years ago

Posts: 5561

23/06/2026 10:36 am

Posted by: @bobflux

↑

When mixing occurs in an uncontrolled way, for example in a buffer or low loss header with mismatched flow on both sides, which is pretty much unavoidable

...And If the flow is the same both sides the buffer/llh is doing nothing in which case what value is it?

A buffer can of course be managed to have the same average flow with different instantaneous flow, not so an llh.

4kW peak of solar PV since 2011; EV and a 1930s house which has been partially renovated to improve its efficiency. 7kW Vaillant heat pump.

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AnnaB

(@annab)

New Member Member

Joined: 6 months ago

Posts: 6

Topic starter 24/06/2026 8:54 pm

This is the response I have had to my request for them to explain why the low loss loader is required from the company who have suggested Ecodan with low loss loader...'a low-loss header creates mechanical separation between the heat pump pipework and your central heating pipework (the existing radiators and proposed underfloor heating). The benefit of this is that you can ensure flow rates (the amount of water flowing through the system) can be achieved. Your heat pump, your existing radiators, and your new underfloor heating could all have different flow rates, so a low-loss header can help optimise this.

If you don't need one or there is no benefit, we'd recommend not having one. In that case, we would design out the low-loss header and recommend a volumiser instead. The volumiser isn't mechanically separated, so doesn't incur any losses - which is good! The volumiser increases the water volume in the system to ensure specific flow rates can be met.'

I have also now got the heat loss report from the company who calculated the lowest heat loss and are recommending Valiant Arotherm 7kW, so next I will be looking at this and trying to unpick where the difference comes from....

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JamesPa

(@jamespa)

Illustrious Member Moderator

Joined: 4 years ago

Posts: 5561

24/06/2026 11:15 pm

Posted by: @annab

↑

..'a low-loss header creates mechanical separation between the heat pump pipework and your central heating pipework (the existing radiators and proposed underfloor heating). The benefit of this is that you can ensure flow rates (the amount of water flowing through the system) can be achieved. Your heat pump, your existing radiators, and your new underfloor heating could all have different flow rates, so a low-loss header can help optimise this.

If you don't need one or there is no benefit, we'd recommend not having one. In that case, we would design out the low-loss header and recommend a volumiser instead. The volumiser isn't mechanically separated, so doesn't incur any losses - which is good! The volumiser increases the water volume in the system to ensure specific flow rates can be met.'

Well that could have been generated by AI and says absolutely nothing at all about your property Thus it is almost (but not quite) content free, not quite because it does admit to the possibility of not having an LLH.

TBH I don't necessarily blame installers for doing this, with perhaps a 10:1 hit ratio they have little choice if they wish to remain competitive and simultaneously preserve their sanity and personal relationships.

If you are leaning towards them then tell them so (without committing until you are ready) and seek a property-specific answer. If you aren't leaning towards them then don't waste their time or yours, instead politely thank them and move on.

This post was modified 4 months ago 3 times by JamesPa

4kW peak of solar PV since 2011; EV and a 1930s house which has been partially renovated to improve its efficiency. 7kW Vaillant heat pump.

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