Calculating House Heat Loss for a Heat Pump

HI, I am just looking at how I can calculate my house heat loss in order to see if it would be feasible to switch to a heat pump before making any more improvements. Is there any useful guidance that I could look at? I was going to use gas consumption but our boiler has weather compensation (water temp varies a fair bit with outside temp). We tend to use it on/off for certain time periods.
I am currently working on a spreadsheet with room sizes and U values.

I could do with help with the cavity wall U value, it is concrete block internally a 50mm glass wool insulated cavity and external brick.

Hi @Steven_Draper. If you haven’t seen it, a good place to look is Trystan Lea’s video. The description has links to HP calculators.

U-value wise, lots of manufaturers have calculators, though they are often product specific. There are lots of tables of values around, a decent example being this one

Haven’t tried it myself but heard good things about using this:

For a completely different approach, and a lot less effort, you could try


And an article that includes the “gas usage / 57.3” plus some other methods (including HeetGeek’s) is

Hi all, thanks for your replies. @Tim_Gilbert I got 3kw for the rule of thumb from the video you linked, this was based on our current gas usage minus gas for cooking and hot water, I had stated in another thread I thought that was low, but we don’t run the heating constantly 24/7 at the moment.
The main thing for me is to work out if we need to do more improvements before changing to a heat pump or not.
I will look at the other suggestions and report back.

Certainly diversify your results. The “Rule of thumb” is only for rough guidance.

For comparison, I have a 4kW pump for a 4 bedroom semi, 250mm loft insulation, underfloor insulation between joists, uninsulated solid walls front and back, 2005 extension on the side, airtightness 7ach.

Hi all, so far I have tried using the MCS excel calculator, that was a struggle to get to work as I don’t have Microsoft excel. I have also tried the Heat Punk tool, that is very good to say it is free to use. I have got a good layout for ground floor except for the stairs and I am part way in doing the 1st floor, we have 2 bedrooms in the roof so I am trying to get that right. It has come up with a Samsung or Mitsubishi 6kw heat pump at the moment so about 4kw for heating and has highlighted shortfalls on radiators in some rooms. At least I can find out what to change radiators to in the short term and I may see if certain improvements like triple glazing will help lower the heat loss.

Also so far I can’t see a way of setting a hot water usage, this will depend on the number of occupants.

Hi, I do not have a heat pump and probably will not have one for a couple of years. To determine our heating requirements I used a variety of calculations based on what I needed i.e. heat loss (size of room, external walls etc)= justification to do work and having a target in mind for a result, radiator size depending on normal and extreme scenarios T℃ gap (what it is and what I consider comfortable and how quick to warm up). I use multiple temperature monitors to check zones and overall heat loss performance. I monitored temperature drop in the house between switching off heating at night and getting up next morning + external temperatures which gave me a decrement value for temperatures >0 & <0℃. Combining insulation with the way we run the heating system, we can restrict overnight heat loss to <2.5℃ for the coldest temperatures we have experienced since 2019. Hope this helps

Although that is the official calculator for MCS approval (and therefore grants) it isn’t very accurate. There are many stories of it being way out compared with measured results.