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

Posted by: @giryan

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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

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@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.

@giryan I understand your dilemma.  I was in a similar position earlier in the year and was certainly nervous about installing a heat pump.  However, my wife and I took the plunge and so far it is going really well.  We are warmer and the bills seem to be comparable. 

If you are technically minded, I would suggest that you do your own heat loss calculation using a tool such as Heat Punk https://heatpunk.co.uk/

This will help you to review suppliers' heat loss calculations.  You will also be able to test the sensitivity of the various assumptions.

In particular, as you suggest, you should look at the air changes per hour.  This is the most sensitive assumption.  In my experience installers and consultants often disengage their brains when estimating ACH.  If ACH = 1, that is equivalent to taking a perfectly sealed room and opening all the doors and windows to let all the warm air out once every hour.  

ACH can easily account for 25% of your total heat loss estimate.  Your figure of 7.46 ACH is equivalent to emptying all the warm air out of the room every 8 minutes!

Imagine having a shower and opening the windows wide to clear the steam out.  After 10 minutes, the room is freezing cold and you can't wait to close the window.  You don't need to open it again until you have another shower, yet a lot of models assume ACH = 2 - i.e. you open the window 48 times per day.  Sure your bathroom window will leak a bit, but not that much.

If you install good quality doors and windows, then they will match modern building regulations.  Of course, air might be leaking through the walls, floors or ceilings.  However, if you sort those out, then you should probably use the ACH assumptions for a modern house, not for a house that was built in 1910.

Posted by: @giryan

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Looking that up the numbers from it are: 9.36 m3/hr/m2@50pa, 7.46 ACH 

Im assuming the m^2 refers to the outside the area of the house not the floor area, otherwise the calculation appears wrong.  7.46 ACH is really high as @grahamf illustrates - something feels wrong.

With a volume of 600cu m (2.4*250), assuming 20C room temp and -2 design OAT an error of 1 in the ACH value will change the loss by 4.4kW.  As @grahamf says it would be worth doing your own calculation or getting a copy of the calculations the surveyor did, in particular the assumed U values and ACH.  In addition its worth checking that they haven't 'double counted' room to room losses by including the losses to other rooms but excluding the gains from other rooms.  This accounted for 1kW error in the calculation done by my surveyors.

If your house was heated when you were consuming that amount of gas the loss is definitely nothing near 20kW!  For gas use of 22MWh/annum to correspond to 19kW the house temperature averaged over the whole house and the whole of the day/night would have to be roughly the halfway point between 20C and the OAT.  So when its 10 outside the average house temp would need to be 15, and when its 0 outside the average house temp would need to be 10.  Unless you are very cold and the insulation between rooms is good this is not likely.

Something isn't making sense.

thanks, heatpunk looks great, being able to do the calculations and see how the numbers appear will be really useful.

I've started mapping things already.
We do have all of the data from the surveys, so hopefully I should be able to replicate things and work out where my version diverges and ask some more intelligent questions of people who did the surveys now at the very least.

Something that I just realised has been missing in the process with the retrofit co, has been real explanation of what the bits actually mean, they are there to be the subject matter experts and keep us informed about these bits, but prior to the recent refocusing and changes we didn't really get a lot of that. In the last month they really do seem to be turning things around, and all of the bits that come to mind are from prior to that point at least. 🙂