@judith thanks did you install a 7kw and is it working as expected?

So I did my own HTC using 30 days Jan/Feb including the cold snap and it comes avg 3.3KW heat losses. That's even lower than the surveys . Even if I assumed no hot water usage it would be 4KW. So that seems to leave a pretty large room for error and seems to confirm its a quite efficient house.

I managed to chat about some design specs and he's now ok with 9KW LG monobloc now at 55C design. he can't take things like my MVHR into account so there will be a difference in assumptions.

Think that setup would work?

Given the way your heat loss calculations are going, I still think that is way too big. Try and find out how low the unit will modulate down to. Also a design temperature of 55C is going to be expensive to run, are they advocating no radiator changes?

If it was me, I would be going back to Octopus and taking the 8kw Daikin.  If it turns out to be too small, which seems unlikely, then its an easy swop out to a larger unit.

Posted by: @pie_eater

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cold snap and it comes avg 3.3KW heat losses. He's now ok with 9KW LG monobloc now at 55C

Trouble with really over sizing a heat pump is the flow rates required. A 6kW requires 1.0m³/h, while a 9kW will need about 1.5m³/h just to keep it happy. Everything starts to get bigger, piping, pumps, noise inside the house (if you are unlucky). Need for a buffer starts to come into play to manage the flow rates etc.

55 degs, you may as well hand over your wages to your electricity supplier.

Regardless of the scheme that you may get the grant from, YOU are the customer and you pay the bills. Difference in 35 deg flow and 55 Deg flow is at least a doubling of the electric bill. Be smart so NO. Unless he sizes the rads for 9kW, then they are likely to flow at a reasonable temperature.

It's all confusing. He tells me he's beholden by MCS calculations to get the grant, which he knows are too conservative as for instance my MVHR isn't included, and to then get a compatible design that what needs 55C. So it's a bit of paper excercise. In practice he says we should run it at 45c and see how it feels, as my UFH runs on 40C anyway. And perhaps upgrade the radiotors later only if needed.

They will install a 50L buffer tank btw and suggest to keep the current 200L water tank as we never run out.

He's entirely unconcered about the now suggested LG 9KW being too big- He think perhaps at 16kw it would be problematic. They installed 400 pump this year with aleged good experience and he claims he's now retrofitting larger ASHPs for ones fitted for octopus.

@pie_eater TBH that sounds like typical installer bull. I have never come across an undersized heat pump. Loads of oversized ones and I have never heard of octopus installing an undersized pump but I have heard of them changing one to a smaller one at the customers request. My advice is to not use him. Alarm bells are ringing.

Posted by: @pie_eater

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suggest to keep the current 200L water tank as we never run out.

Really depends do you currently store at 50 degrees or below? How big is heating coil, you really need about 3m² or more for good efficiency.

Sounds like I will provide about £3k of equipment, do a couple days work, pocket £4.5k grant money plus whatever they charge on top

There's definitely some bull, but octopus did not cover themselves in glory either: it took 10 months and 2 surveys to even get to this stage, and at the end if they got the radioator specs wrong (they insisted on changing all the raditiors everywhere but almost 2x-3x larger than needed according to MCS and weren't willing to change that).

Between the 8kw daikin and 9kw LG there doesnt seem to be a whole lot of difference , although the LG operates better at lower temps (the daikin is only c 3KW at -7) so it should be ballpark in the right area?

@pie_eater getting bigger rads for free doesn't sound like a bad deal. Might mean you can run the system with a lower flow temperature which is more efficient and costs less to run.

I appreciate that the proposed radiators may have meant other compromises you weren't happy with.

@pie_eater I appreciate its frustrating getting quotes and making sense of them. For a cheap to run system you will want large radiators and a sensibly sized heat pump. Personally I would try to get a smaller heat pump that performs better at very low outdoor temps to get your 3kW heat loss. Remember your heat loss for most of the heating season will be much less than that maybe 1-2kW, unless your heat pump can modulate down that far it will be cycling (turning on and off rapidly - not recommended).

If it were me, I would try to get more quotes, if you dont want to do that I would probably plumb for Octopus, they have a good reputation of trying to put things right if they make a mistake (ergo swapping a heat pump for a smaller one) but its your money and your choice in the end.

@pie_eater our install date is in September we’ve just had some bigger radiators changed so far, (not all rooms needed larger).

it is pure BS to say your MVHR can’t be taken into account. It depends on the software they use, Heatgeek allows for MVHR (and measured air leakage @50Pa, which we have) but only on houses newer than 2006 and we’re 1950s, so one installer fudged their figures on house age. Another could take the MVHR into account in their predictions and magic, our heat loss through air leakage became 2% of the total not 30%. Yet another said it couldn’t be done.

Since our bills align with a <5kW unit I’m not too worried.