🤣 Forget Narnia, sounds like Octopus found a whole wing of Hogwarts in your house!

😀 😀 

@davesoa have you raised this with them? Will be very curious to see what they say.

I saw an Octopus promo today, as an aside, where they're claiming that they can fit heat pumps in up to 80% of UK homes. Bold statement.

Yes I’ve sent them the results from the initial survey that shows room by room calculations and required radiator sizes. They are considering whether they will undertake a further survey. It’s really curious how the results vary so widely. And why they think I live in a very big house. Something is amiss. However this is not a quick process. Update - they’ve just come back and asked me to reiterate the insulation in the house and speculate where I think they might be in error with heat loss.

Hello @davesoa, some things I would cross check between different heat loss calculation estimates, in order of greatest influence on heat loss estimate. You need the granularity and detail to do this, i.e.: the heat loss report. If they don't want to share that, avoid them.

1. Design indoor temperatures for all zones [rooms] - you probably don't require all rooms to be 21 or 22degC. This concise NHBC guide reiterates CIBSE design values, see link below. You might want some rooms cooler of course. I would design bedrooms to 18degC for example, mindful that warm air rises, and most bedrooms on a 1st floor dwelling will end up nearer 20degC anyway even with emitters sized to achieve 18degC

2. Design reference outdoor temperature. It varies from South to North dependent on location latitude. See NHBC guide, is yours correct?

3. Infiltration and exfiltration rate per room. This is commonly over-estimated. We don't really 'do' whole house permeability and leakage rate tests in the UK, save on new build sign-offs, but there's increasing evidence the default air change rate values in CIBSE, Part F et al are over-estimated and too conservative. This can lead to heat losses being over-estimated. Sometimes a wood burning stove in a lounge, which is room sealed appliance with air dampers closed, is assumed to be an open fire and chimney for AC/h calculations. That's bollox, with too high a resulting heat loss estimate for the lounge.

4. Insulation U values of the fabric; walls, floor, ceiling, windows, doors, etc. These are not usually too far away from reality. They are always a guess with older properties that don't have architectural details available to cross-check. Your newer 2002 extension should have those details however. Windows, doors and loft insulation can be easily checked and a competent surveyor would do so.

Has one quotation heat loss estimate added a large factor of safety [FoS] on the sizing selection? This is usually done because of inexperience and ignorance, rather than calculation uncertainty. For heat pumps, it should be avoided. If you intend to run your ASHP 24/7 then 'right size it', don't oversize it. If you want to switch it on and off via a scheduled timed control, a-la a fired boiler, you'll need it oversized by some 30% to allow sufficient capacity for pre-heating from cold. The latter is an inefficient and expensive method of controlling a heat pump however, as it will always be playing catch up from a cold start to try and achieve required internal conditions. It never gives the ASHP chance to modulate down to really efficient inverter output with highest COP, which relies on stable control conditions. Oversizing a ASHP increases the minimum turn-down output in kW, which will lead to on/off cycling of the compressor at lower load demand conditions. A right sized ASHP mitigates that so far as is practical.

NHBC.CO.UK "https://www.nhbc.co.uk/binaries/content/assets/nhbc/tech-zone/nhbc-standards/tech-guidance/8.2/tg-8.2-01-heatpumpdesign.pdf"

Thanks for the advice I asked for (and got) their room schematic. I can only count 16 rooms (the stairs/landing count as two rooms). Maybe 19 is a typo. I’ve also asked for the heat loss by room to try and account for the 30% difference in heat pump size. 

I think the Narnia analogy is very apt.

Like Edmond, we are overawed by our first encounter within the magical world of taking heat from thin air.

But by chapter-4 we find ourselves in the freezing cold castle of the White Witch, and regret being taken in by a heat-pump system called 'Turkish Delight'.

Oh dear. Always winter never Christmas?