What's the square footage of your neighbour's house?

The other alarm bell is why the heat loss report wasn't shared with the homeowner. If they've hired and paid them, they're entitled to that report. They should demand it.

@editor Sometimes, things can be even better; Octopus Energy’s Plumber and their Electrician spent three hours each assessing our property for a heat pump installation and shared their thoughts, reservations, suggestions on how to maximise siting and routing of pipes etc., provided us with the Heat Loss Calculation (verbally anyway) without any charge whatsoever, (They are our energy suppliers but there was no obligation involved for this survey). As it happened at that time, as they were the new boys on the block (early 2022), they were unable to provide an installation matching our desires due to using a narrow set of formulations and they did say that in due course, they would be widening the scope of installations. Regards, Toodles.

@editor 

Here is the report size. I made her ask for it once I got involved, they said they usually provide it after installation!

and the losses

@bontwoody,

it does seem excessive for a 100m2 house that has a heat loss in the normal range.   

@kev-m well I guess it would be nearer to 120m2 after the possible extension, but by my calculation still twice what is required. 

@bontwoody holy ***t batman, yes! 16kw unit for a 5.8kw heat loss? even with a bit of growth room for an extension, thats bonkers.

I just had a look at the Acond range, there are two units. The N is rated at 1.5-7 kw. The R is rated at 3-16. Don't know at what temps, or how accurate the "plate rating" numbers are, not found that level of detail yet.

If the installer really felt that that brands "small" wasn't enough then surely the professionally appropriate thing to do would be to look at and discuss with customer alternative brands that have a more "medium" unit? Not just stick in the "large" because they have a deal with that brand?

Additional thought whilst getting coffee - we have "design conditions" weather right now. If the system is installed and running, it should be possible by looking at the systems stats to derive the house's actual heat loss in design conditions.

@iancalderbank That mirrors my feelings Ian. Unfortunately the owner has not been given a manual yet or even instructed how to turn off the heating. Its costing her £10-15 a day at the moment compared to my £5-7 a day in a similar sized worse insulated property just down the road, and I like it a lot warmer than she does.

The company want her internet installed to monitor it from there end but this is proving problematic in her property. I haven't been able to get an answer as to whether it can be controlled just from the controller unit or if it is an internet function via an app.

Posted by: @bontwoody

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@kev-m well I guess it would be nearer to 120m2 after the possible extension, but by my calculation still twice what is required. 

do you think excessive enough to hit the not fit for purpose clause in the sale of goods act? 

I'm not sure about the relevant consumer law.  It might be hard to prove it's not fit for purpose if it's able to heat the property and especially if it can modulate down to 3kW.  An undersized system would be easier to prove.  Also, if it complies with MCS I would think it would be deemed fit for purpose.  

However, I don't think a twice oversized ASHP need cost twice as much to deliver the same amount of heat as the correctly sized one (as suggested by your own figures) so maybe it's the way it is set up and used? 

Posted by: @bontwoody

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@iancalderbank That mirrors my feelings Ian. Unfortunately the owner has not been given a manual yet or even instructed how to turn off the heating. Its costing her £10-15 a day at the moment compared to my £5-7 a day in a similar sized worse insulated property just down the road, and I like it a lot warmer than she does.

The company want her internet installed to monitor it from there end but this is proving problematic in her property. I haven't been able to get an answer as to whether it can be controlled just from the controller unit or if it is an internet function via an app.

Has the controller been correctly optimised?

I always find posts and cases like this very disconcerting. Our floorspace is around 380m2 and we have an 18kW ASHP which is probably the right size for the entire property. This nonsense about heat pumps scaling down is such mumbo jumbo in my opinion, because there have been times when I've shut off half the house (UFH and rads) and there's very no drop off in electricity consumption.

@bontwoody, I don't think there's much of a case for it hitting any thresholds under the "fit for purpose clause in the sale of goods act?". I've looked into this when we ran into issues with our Samsung fridge (which is 10000% not fit for purpose), there's an awful lot of subjectivity involved in the definition of what is and isn't "fit". I have, however, been researching the complaints process of badly installed heat pumps and will now be adding incorrectly sized heat pumps to my research.