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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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JamesPa

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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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