Well done on sorting out the insulation first Matt. Is the heat loss figure before or after all your proposed changes?

Posted by: @giryan

↑

Output at designed external temperature: 19.1 kW

Are you going to insulate those solid walls and the loft at all  If so this figure is crazily high and a design based on it will likely perform rather badly.

@bontwoody It's assuming that the loft insulation is done as that's something we're definitely doing, listed as 2023 building regs/0.13 U value. 
The ground floor is listed as it currently is: Suspended floor at 1.75U, and the solid sections at 1.20

We don't want to dig out the solid portion of the ground floor to re-do it all, and exactly what will happen in the suspended floor is kind of up in the air, dependent on what happens UFH/insulation-wise.

@jamespa changing the walls is not on the cards for now.
While it's not a listed property it's a characterfull brick which stands out in our street, and so we don't want to hide all of that behind external insulation.
Interior insulation is something that's not quite totally ruled out, but we're not really up for literally gutting the entire house to insulate every internal wall, and as far as I understand only doing interior insulation in portions is not something you can reasonably do, due to making cold sections which make damp, etc..

@giryan 

Im with James in that the heat loss is very high and you need to do anything you can to get that down. Internal wall insulation would be something I would definitely consider. Its unfortunate that you dont really have any good usage figures to sanity check the calculated heat loss.

Posted by: @giryan

↑

@jamespa changing the walls is not on the cards for now.
While it's not a listed property it's a characterfull brick which stands out in our street, and so we don't want to hide all of that behind external insulation.
Interior insulation is something that's not quite totally ruled out, but we're not really up for literally gutting the entire house to insulate every internal wall, and as far as I understand only doing interior insulation in portions is not something you can reasonably do, due to making cold sections which make damp, etc..

Ok. It still seems high tbh.  Particularly if you do anything to improve the floors.  My 200sq m house of similar basic construction with no floor insulation, partial wall insulation and good loft insulation was assessed as 16kW by two full 3hr surveys, but is actually 7kW.  I would check the figures, ensure they account for the upgrades, and particularly question air changes which have likely been set at 2-3 but, unless it's draughty, may in fact be closer to 0.5-1.

You really don't want to install a 20kW pump if in fact your house is only perhaps 10-12.  That said a cascade could well be ok provided that it's directly connected and doesn't involve buffer/llh/phe between heat pump and emitters.

My advice is check well before you leap, people here will provide suggestions.

Thanks both, yeah, I was thinking they sounded like big heat pumps. I do struggle with what all of the different similar sounding numbers mean though, before the process started I started looking at ASHP with numbers that were the same as the gas boiler, and then got really surprised at the sheer size of 30-50Kwh heat pumps 🙂

Usage wise I do have some data:

For 2024
Our gas use was 22000kwh 
And I just realised I also have some data on other energy use for heat related bits too. 
Our immersion heater used 1870Kwh of electricity
And I looked and in addition to turning it on and off the smart plug I have the electric radiator on said it used 1373Kwh

With the KwH/2900 rule of thumb I spotted somewhere? that would be 8.7 ? I'm not sure if that's actually a good number though?

We did have an air permeability survey, which I realise I missed that off the original post too 😅
Looking that up the numbers from it are: 9.36 m3/hr/m2@50pa, 7.46 ACH 

But that number is really high compared to the 1-2 from the MCS and so I'm not sure if that is comparable with air changes number that the MCS calculation uses? 

The house does feel drafty, and fixing that's definitely on the list too. A lot of what was highlighted was from suspended floor, loft and windows, which are part of the works.