Good exercise.  Obviously I have nothing to check it against but its very plausible.  Do you have your current fuel consumption as a sense check?

At this level, as long as its roughly right that's good enough, you are going to end up with a 5 or 6 kW heat pump anyway because thats the lowest capacity most manufacturers do and you want 5-6kW for DHW reheat.  Thew living room, at a third of the loss, is probably worth triple checking and you are going to need UFH, more than one rad or a fan coil most likely.

@jamespa Hi James, the living/kitchen/dining room is around 40% of the property. My annual gas usage is 9300kw, of which i am estimating 170kw a month for hot water and the hub, as this is my average July August gas usage. However it probably takes more to heat water in winter and we probably cook more on the hob, but conservatively i have assigned it all the summer figures. So around 1800 for hot water and hob, and 7500 for heating.

@jamespa Also this coming winter I am going to run my combi system at as low as it will modulate and see how we get on.

Posted by: @kilgorexv

↑

My annual gas usage is 9300kw, of which i am estimating

That's consistent with your heat loss calculation.

You will end up with a 5 or 6kW heat pump.  Cosy 6 if you go with octopus, probably the Vaillant 5kW if you go eg with Heat Geek, or any of several other models at this output capacity.

Unless you have ufh do some calcs on the rad options for the living room.  It's likely to be either very large, a type 33 (which is rather thick), more than one rad or a fancoil.  I ended up with a fancoiol in a similarly demanding living room which I love, but may not suit you.  Fancoils can do light cooling if the heat pump is set up for it, which neither the Cosy nor the Vaillant are by default.

Just be aware that Octopus are likely to over estimate the heat loss, as could any other installer.  You should have a plan for discussing their calculations against yours.  
My gas usage was 9000kWh and Octopus still calculated a heat loss of 8500kW and a Cosy 9.  They subsequently recalculated heat loss at 7500kW once installed but even so, that would have driven them to a Cosy 9.  It’s not a massive issue - I still managed a SCOP of 3.97 - but you may prefer a smaller unit.  The problem I had with Heat Geek, who calculated the same 8500kW heat loss actually but recommended a 7kW Vaillant, was that they wanted to charge me £26.5k, before grant, for a simple install, almost double Octopus’ price.

@andrewj Gold plated pipework and fittings?!😳 Toodles.

@toodles possibly. The £9k labour charge for 4-5 days work certainly didn’t help.

@kilgorexv, I actually think doing your own calculation before getting quotes is a really useful exercise, even if it turns out not to be perfect. It gives you something to interrogate when an installer eventually arrives with a completely different number.

@JamesPa’s sense check against your gas use is probably the bit I’d take most confidence from. If you’re using around 9,300kWh of gas annually and roughly 7,500kWh of that is space heating, then your calculated heat loss doesn’t sound bonkers at all. It certainly gives you another piece of evidence to put alongside the spreadsheet rather than relying on either one in isolation.

And I think @AndrewJ’s experience is exactly why having done this homework is worthwhile. When you start getting surveys, don’t just compare their final “your house is X kW” number with yours. Ask for the room-by-room calculation and compare the assumptions for U-values, air changes, room temperatures and outside design temperature.

I certainly wouldn’t choose the heat pump from your spreadsheet alone, but you’ve now got something useful with which to challenge the eventual design rather than simply being handed a number and accepting it blindly.

Any idea how efficient your gas boiler is? Is it a 50 year old Baxi Burmuda back boiler from the 1960's that's probably around 50% efficient, or a modern condensing boiler at could be as much as 80% efficient, as this can make a significant difference to the heat provided from that 7,500kWh of gas consumed.

For example, my parents still have their original 58 year old Baxi Burmuda back boiler in their 1960's bungalow, which has a heat loss of around 5kW based purely on gas consumption data, but when we factor in it's probably 50% efficient at best (I don't think it's ever been serviced, only repaired a couple times), that heat loss figure is probably closer to 2.5kW.

If you have a smart meter and daily data, I'd also look at your highest consumption on the coldest days in winter, and divide by 24 for your heat loss figure. I think this is far more accurate than trying to estimate a heat loss from annual usage. If you've got more data, look back further over more winters.

@old_scientist i had a look at this, all my gas consumption for New Years Eve was 85kw. Thats the highest day for two years. Winters have been mild though. -3 was as low as it got, and even then not all day. Even then allowing 6 for hot water and cooking, leaves about 3.33kw an hour.

My boiler is a reasonably odern condensing  combi boiler, installed in 2012, but se3rvices once a year and still seems ok, no major problems, so for my calcs i assume around 85 to 88%

Thanks for your feedback.

@editor Thanks Mars, I have been enjoying playing with the formulae, gives me something to do on long night shifts. i actually really enjoying doing the calcs. i would love to do these assessments for a living, its genuinely interesting and would make a positive impact on electrification.