Long ASHP explainer with questions

Using the 2900 rule my 1930's externally insulated 3 bed semi, 110m square needed 5.5kW. Using heatpunk, it came in at just over 5kW. Got a property survey done by Heat Geek approved and it came in at 5.5kW. The estimate then proceeded to size up to 7kW and add buffer storage. Post grant was £8k. At least £3k of that is heat geek guarantee as far as I can see. That cost isn't justified imo. As touched on above the quality of installers is not great yet, the good ones are hide to find. There's also a degree of pocketing some of the grant money.

I'm choosing to go multi split air to air, half cost to install, run costs similar, and I prefer the flexibility of temperatures across the rooms. Bonus I can install myself relatively simply.

Using the 2900 rule my 1930’s externally insulated 3 bed semi, 110m square needed 5.5kW.

Using Heatpunk I am finding that the recommended unit size is >8kW and an 8kW unit does not provide enough heat for my house. I need to spend more time playing with it and making sure my numbers are right although at the moment it seems to bear out my concern, that it is near the limit.

Interestingly I was running with a group yesterday and one of the guys is senior in grid renewables and lives near me (his house isn't comparable, a large Victorian with high ceilings and large rooms) and he went for a 16kW ASHP and is very pleased with the performance.

I’m choosing to go multi split air to air, half cost to install, run costs similar, and I prefer the flexibility of temperatures across the rooms. Bonus I can install myself relatively simply

My brother, architect in NZ, has been specifying this way for the last decade in NZ, albeit in a warmer climate.

He's just built the base for his own pump, which sparky is due to install this week. 60 year old 100m2 Aukland home with new windows, new insulation throughout and MVHR he self installed.

And as a comparison of how expensive the UK is on heatpumps - Daikin 5.5kw multi split pump at £1550 / NZ$3000 installed, on the base he prepared. UK cost is around £1800+vat unit alone, £2000 as multi split, and at least double that installed apparently...

He also has solar and battery due to be fitted in a month - looking at again half what the UK cost is...

Update in case anyone else finds this useful:

Decided to follow the installers guidance (they provide a minimum heating guarantee) and go for the 8kW unit. I used the Heatpunks website and that appeared to show 8kW was underpowered but I wasn't able to populate it fully with things like our specific cavity wall insulation type, DG type etc and I suspect it was over estimating our heat loss as a result. Other less sophisticated models were indicating a heat pump size of around 5kW would suffice. Had a long conversation with the planner yesterday and he came across as extremely knowledgable and answered every question first time. He explained that the heat loss calcs after their detailed survey show the 8 is appropriate, after we increase our loft insulation as per their recommendation.  I asked to see the loss calcs and unsurprisingly he said he would send them over once we had signed the order. Made a number of mods to radiators types and sizes all of which they were happy to do at material cost. Just need to set an install date and have the house ripped apart for a week!!

Update:

install went ahead mid June and leaving aside a few minor issues the work went ahead as expected. They made an error with their calcs and had to do more pipework which set them back 2 days, but no grumbles from them.

Up until the last couple of weeks we havent needed to draw any heat from the pump and only used it for hot water, 3 of us at home and we have a 250ltr tank so that energy was all drawn either from solar or battery charge at one of the lower Octopus Cosy "windows" I know its pretty meaningless to estimate any savings over such a short period with so many variables but I cant help myself... Love Octopus but their billing is a nightmare (not helped as we change tariffs through the year and feed back which buggers things further)

So from 22/7 to 22/8 2023 (with our old boiler) vs 22/7 to 22/8 2024 (ASHP) we seem to have made a net saving of £52

Of course the winter months will be the killer but of our annual gas bill of £1608 in 2023, £904 of it was Dec, Jan, Feb and March so as they are the worst for solar, it will be interesting to see how high our electricity bills are then. I have an 8.7kW battery so may boost that by another 4kW to allow us to bridge between the 3 low cost Cosy windows.

In terms of the ASHP itself it is not silent but I have to be outside to hear it, and the only indication I have that it is heating water is my power usage spikes on my Home assistant. I know it is heating now as the outside temp drops but the house is comfortable (at the moment) at 18C and it seems to hold that rock steady hardly using any power.

Anyway though a bit of feedback may be useful