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7.5kW Heat Loss, But Quoted a 10kW Midea. No Re-Pipe, No Buffer Tank. Does This Add Up?

Penval · 2026-04-08T16:39:03Z

We are at the stage of getting quotes for an ASHP retrofit. I recently posted about potential issues with microbore pipes and a huge thank you to everyone who gave their views it was very helpful and I am, to some extent, wiser than I was when I posted the initial question. We have just had our second quote and it's very different to the first one. Quote one proposes a complete re-pipe the reasons for which I now understand. The second one is proposing a high output ASHP specifically a 10 Kw Midea. The heat loss calculation for the house (which is a self build to 2006 regs but has an uninsulated floating floor downstairs) is 7.5 KW, we have a 250 Ltr cylinder and 3 bathrooms. The only reservation seemed to be the recovery time for the cylinder which is currently 20 mins but would be closer to an hour. Proposal has no buffer tank as part of the design. I would add that despite the floor downstairs it's a warm house which used 1600 litres of oil last year, in the past if it's a very cold winter it usually uses 1800 litres. Obviously the cost is way less without the re-pipe but we're not comparing like with like - we still have one quote to come. I don't want people to make a decision for us but I would like a few thoughts on what I'm looking for here. All comments gratefully received, many thanks in anticipation.

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Air Source Heat Pumps (ASHPs)

Last Post by LeJamaisContent 3 weeks ago

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LeJamaisContent

(@lejamaiscontent)

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12/04/2026 9:31 pm

Oh, how I love these old scientists!
I'm in with your 5 kW -- thanks, that helped a lot.
Overall, I enjoy this conversation very much.

I do, while any outsider invariably will think 'Oh, it's t h a t typical, asking five self-appointed experts and you get six opinions at least!' Which again is true and indeed any first real-expert answer will be 'That depends on ....' with something following no layman can and wants to cope with .... sorry.
When any expert answer turns out to be inaccurate, defence will be 'This is because ....' with something following .... etc.

In this thread the 'because' is, because there is no real data set, we rely on estimations, and the one which OldScientist put in seems me the best available by now.
To get closer, we would need to go the full mile about square meters, wall build, window types, thermal bridges, radiator wattage, and last not least flow resistance, some of which is difficult to see without a dowsing rod, crystal ball and a pair of radar glasses.
When in doubt, we need to estimate, guess, predict and forecast, and while doing so, try to cover uncertainties by 'laying to the safe side', with the safe side being something different for anyone involved. And when uncertainties add, whole-number factors will result sooner or later, to one 'safe' side or another.
For an installer, 'safe' mostly will be a big heat source, for me here, it's an hydraulic system which can cope with the heat offered.

So I assume that even an experienced engineer on-site would have trouble to supply a conclusive answer to the question given in the headline, without the possibility of seeing into walls and listening to water flow.

Admittedly, some measuring can be done, but when a pro will be paid for, 't will come out as total write-off -- far too expensive.
So it's left to you, Penval, the house owner, to own this problem and to run trials, mimicking heat pump operation, make notes of what the system will show, and discuss here or elsewhere.
Or to rely on your installer.
Or to pick one of our many opinions.
I will be highly interested about any outcome, as, I am sure, many others here -- keep us updated!

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