Other than heat loss, radiator sizing, ASHP sizing, what else goes into a design and how important is it?

petch · 2026-03-04T22:12:51Z

We're looking at replacing a boiler in an early 2000s detached house in the UK. After getting multiple heat loss surveys done (varying from 8.5kW to 11.5kW heat loss) I've created a spreadsheet to compare the individual heat loss per room across each survey. I've removed anomalies and averaged them to get a better idea of what radiators I actually need. To me, that seems simple enough. However a lot of people talk about the importance of design, so I've got to ask what else goes into a design that I'm not really thinking of? So far all the surveyors have looked at the existing pipework (mix of 22mm and 15mm, but no clue how far the 22mm goes before it turns into 15mm) and essentially shrugged and said it's fine, or even said it's great/ideal. During their visit they only seem to care about heat loss (which has never been anything more than educated guesswork) and radiator sizing and then picking an ASHP that has enough output. What else should a good design be considering? What questions should I ask and what should I look out for? The only other thing I have a basic understanding of is system volume, but again, every installer seemingly goes off gut feeling for this (or worse, just always do/don't install a buffer/volumiser). Even this topic seems to have wildly different opinions where you look, this forum I read you should have 15-20L of system volume per kW of output on the ASHP, but if I look at a manual for a 12kW ASHP it says a minimum of 16L and a recommendation of 64L, which is just over 5L per kW - significantly less than what you might read online. Basically, I don't want to end up with a bodge job. But I also don't want to pay thousands more for an expertly crafted design that's never going to recoup the extra cost incurred instead of just throwing in some bigger radiators.

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JamesPa

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06/03/2026 1:02 pm

Posted by: @old_scientist

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We overestimated our heat loss (mainly due to overly high ACH figure used in older MCS calculations). Besides our heat pump being oversized, it has mainly resulted in allowing lower flow temps which of course is no bad thing.

Of course you can get the lower flow temps without it being oversized by just putting in bigger radiators!

Posted by: @old_scientist

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@petch just wondering how your averaged heat loss survey data compares to actual usage data. Is your existing boiler gas, and if so do you have access to half hour data (or daily data) for gas usage from a smart meter?

100% agree with this, but perhaps I am a bit paranoid about oversizing.

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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