Hi, @matwin  

There are a couple of other things, I think, to add into the mix as well.

In your previous posts in the intro thread, @jamespa rightly said that a small oversizing is not the big issue gross oversizing would be. It's also the case that some brands/ranges of heat pumps modulate better than others. When we got our heat pump installed, the company we used originally spec'd a 7kW Vaillant but, as a result of supply chain issues, gave us the option of an 8.5kW Mitsubishi that was in stock. Knowing what I know now, I suspect I would have chosen to wait but we decided on the Mitsi unit so we could press ahead and in fact it has been fine; reliable and economical. My guess is that this is at least in part because Mitsubishi kit is recognised as able to modulate down quite well, thereby mitigating the short cycling issues we might have seen if we'd gone for another brand.

The big question, of course, is whether a jump from 7.4kW to 9.2kW is a moderate or a gross oversizing. My (heavily underlined) AMATEUR feeling is that it may well still be within reasonable bounds depending on the brand. I would, however, happily defer to anyone else with any kind of reasonable claim to an expert opinion.

As for air changes per hour, I found an article that gives some information on the MCS standard on which the calculation is based. It seems using air changes per hour is the bread and butter of the 2003 standard whereas the 2017 standard requires more ventilation data per room. It also says that, from 2nd June 2025, the 2003 model will no longer be permitted. Given that's less than two weeks away and your potential installers should be up to speed with the new calculation tools and measurements, I wonder if it may be worth asking them to demonstrate their answer won't change under the new MCS standards. Either you'll get confirmation or a new answer.

As I said, I'm no expert so take all that with a pinch of salt but it's a start.

@majordennisbloodnok thanks. I didn't know about the MCS change

@matwin I'm going through a similar quotation journey currently (& hopefully nearing the end of it now).  One relevant piece of information I had from one provider on sizing was that Vaillant (and I assume other manufacturers will have similar rules) insist on there being 120% coverage for their pumps, so this has to be factored into the solution proposed by anyone quoting one of their systems to ensure that the warranty is effective.  It was a relevant issue for the quote in question, as the calculation based on a 7kW heat pump maxed out at 102%, meaning that the proposal had to use the next model up, in this case a 10kW unit.

Consequently, modest over-sizing is likely to be present in most designs, if all manufacturers apply similar criteria.  The issue, as previously noted, is to avoid gross oversizing.

Also, based on my experience so far of seeing the results of heat loss surveys, the potential for error from ACH assumptions is no different to the potential for error in material U-value assumptions (in my case, I've had the same polycarbonate conservatory roof assessed with U values of 0.4 and 5.11 in two separate surveys).  Ultimately, the heat loss surveys, even at a room by room level, are a calculation, and are only as good as the data used within them.  Consequently, there will be an (unknown) margin of error within them that needs to be allowed for.  In my case I've sense checked the calculations I received and did my own sensitivities on them based on where I felt I could reasonably question the data.  This gave me a best/worst case range for the heat loss figure that I could use to assess the various proposals, rather than being wedded to a single number.  I've found it helpful doing so, particularly as our property appears to be right within the 8-10kWh assessed heat loss range that falls in the gap of many manufacturers heat pump sizing ranges.

It's still only a sense check, at the end of the day, but it's enabling me to have sensible discussions with the providers and I'm finding that their responses and attitudes to such are a much more relevant indicator as to my overall assessment of them.

@sheriff-fatman thanks and good luck with your installation