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

Posted by: @matwin

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1. can you get the MCS £7,500 grant if you dont use their ACHs?
2. is there an easy way to measure your house's ACH?
3. what are the calculatable downsides of installing a heat pump that is 25% too big for your house?

1. I certainly did, I found two installers who would accept evidence of actual heat loss and factor it into their calculations in a way which meant that they used ach of <1 not the standard ones. 

The bus requirement is that the heat pump is capable of heating the whole house.  It does not specify ach values. 

That said it is also a requirement to use MCS, their rules for calculating heat loss appear recently to have changed and now just reference a bs standard.  I don't know exactly what that says and it may affect what installers now do.  There are certainly installers out there that don't trust the standard ach values.

2 you can get this done as a service.  They basically replace an external door with a piece of wood that has a fan inserted, about gbp 300 I am told.  Openenergymonitor has a way to do it (for a single room I think) using a carbon dioxide detector.  Basically inhabit the room for a while raising the CO2 level, leave from a while so it naturally decays, measure new level, calculate ach.

3. Judging by the gaps on manufacturers ranges probably not too much, but are you confident the excess is only 25%?

@jamespa thank you so much for addressing my questions so clearly.

1. I knew MCS standard ACHs were unjustifiably high and
2. I will go ahead with a blower test to prove it
3. if the actual ACH is less than 1 the difference between my installer quoted size and the new calculated size will be significant, perhaps even a clear pump size different.

You have made a huge difference to me, cheers

@matwin 

If you plan to commission one it might be worth checking that at least one installer will take notice of a blower test.  Some can be a bit jobsworth.  

As in almost all fields there are people who follow a simplistic interpretation of the rules, and those who actually read the rules and interpret them in a way which allows them to do the right thing!

I find it worth bearing in mind the fact that MCS is funded by installers, and therefore I personally assume that the principal purpose of the MCS rules is to protect (1) MCS themselves  and (2) installers, with consumers as a poor third priority.  Of course I could be wrong, and if so look forward to seeing the evidence that I am wrong!

It's interesting to contrast MCS and the IET (which is responsible for electrical regulations).  The latter puts much more responsibility on the contractor to do the right thing and assess the situation, the former essentially constructs a 'painting by numbers' rulebook which, if the installer follows it, so far as I can tell from what's reported here, indemnifies the installer against almost all claims.  

Please continue posting and asking questions.

update - my old installer insisted on a heat loss of 11.1kW, mainky because of the MCS air change values. I got a £350 blower test this week which demonstrated the draughts throughout our house (ceiling sunken light fittings, poorly fitting double glazed window frame, loft hatch, various cable holes etc) and still reported ACH 0.49

obviously those draughts need to be dealt with but the heat loss now looks like 7.5kW which is perfect for a Vaillant 7kW heat pump which can do over 8.4kW when necessary.

the savings of not buying a 12kW heat pump and associated pipes, radiators etc to match the overstated heat loss will easily pay back the blower test fee and the whole scientific process is valid unlike the crazy misguessed process before.

@matwin Quick question, do you think you would be more likely to have gone with a provider that included air testing in the building heat loss survey so they were the most accurate with the design?

I am wondering if we should start offering air tests on all homes from that survey point as this is the largest unknown or over designed element, this part of the design could be largely improved but like all these things that costs money. You are correct in the fact that the difference between a unit a few Kw lower in power can be more than the price of the air test but the cost of the air test is up front to design the unit selection and most "but not all" customers are likely to go with the free or cheapest heat loss survey as they see this as an upfront costs before they even know the final cost of the installation.

@ashp-bobba  

I definitely recommend offering the blower test for the benefits stated. Unless it is a relatively new build with an already known ACH.

If I were an installer I would show older home clients that the heat loss and design elements of a heat pump system are crucial and the ACH is the most unknowable input which increases the total heat loss and system requirement by 50% - 200%. 

The blower test should also be throughly used as a draught diagnostic (think how much eliminating draughts could save over 20 years!) as well as a system sizing tool. 

The £350 I paid will pay back in correct ASHP sizing, let alone unnecessary pipes and radiator upgrades and draughty heat loss. Then there is the comfort of a draught free home. 

Speaking as a new client I say that smart installers should include the correct measuring of ACH as an option to build a trusted client relationship from the start.

@ashp-bobba

Firstly, well done for raising this!  

It occurs to me that, if the blower test were done at the same time and by the same person as the survey, it would probably add relatively little to the cost.  Obviously its then a commercial decision how you price it, but the added value of detecting leaks as well as the certainty in the design surely means its worth something to householders. 

It also occurs to me that, once any installer has done a few, they will get a pretty good idea of what the variation is.  

For what its worth and with the objective only of assisting your deliberations, I didn't have a blower test and did find two installers who were anyway prepared to use 0.5ACH, on the strength of my gas consumption figures and their personal belief that ACH is generally overestimated.  This worked well for my case, but I appreciate that both I and the installers and were probably taking a risk.  Like @matwin case, it made the difference between the '10/12kW' and the '7kW' (Vaillant) model, material in terms of price, physical dimension and noise.  For various local reasons the alternatives in the 10/12kW class weren't as attractive, so it was an important 'pivot' in my design.