Fabric and ventilation heat loss calculator

[JohnMo]

I would size heat pump based on a few criteria 

 

Over size ashp approx 1.2 to 1.4 for region of +5 to 0 to allow for defrosting. 

 

Make sure heat pump puts out a genuine minimum of 7kW at you design temp. Take note also of oversize to cover defrosting.

 

Get a heat pump with good modulation.

 

Size system volume based on

20L per kW output at minimum modulation of heat pump. Exclude in volume calculation any radiators with trvs and their pipe work. Exclude any UFH loops with actuators. Any volume needed, add by volumiser not buffer.

[Sparrowhawk]

3 hours ago, timhowes said:

However, as someone with no prior experience, I will have to be quite brave to source my own ASHP, design the system and ask them to fit it.

 

Consider going to a HeatGeek accredited installer, because they know their stuff and will install outside MCS or may be able to get a reasonable system design through MCS. And a HeatGeek installer will appreciate that you've already done a basic heat loss calculation to size up the previous system design!

[timhowes]

Thanks @Sparrowhawk - I contacted a HeatGeek recommended installer back in July who sent me a quote for a mere £25k!!! This included 7kW ASHP (a premium Stiebel Eltron model), all of the UFH I will need, and before the BUS grant. But still...

 

So this sort of put me off. I have gone back to them to ask to discuss, as they are the only company who have entertained the idea of an appropriately-sized ASHP. I have said no buffer, single zone... Let's see what comes back!

[timhowes]

4 hours ago, JohnMo said:

 

Over size ashp approx 1.2 to 1.4 for region of +5 to 0 to allow for defrosting. 

 

Thanks @JohnMo

 

By this, do you mean I should look at the heat loss in the deltaT columns 15 and 20 (assuming I want 20C in room)?

 

 

 

So multiple 5689 by 1.4, allow for DHW and go from there?

 

(Values slightly greater as I had underestimated roof area)

[Sparrowhawk]

1 hour ago, timhowes said:

I contacted a HeatGeek recommended installer back in July who sent me a quote for a mere £25k!!!

Do you have more than one HeatGeek installer in your area? They come with a premium, but there was a 50% difference between the two quotes I got. Some are just taking the mick with their prices (and if people will pay, who can blame them).

[timhowes]

Yes there are a couple of other options - I have today submitted the plans via the Heat Geek portal (seemed to need to be channelled into their "black label" system which sounds even more expensive!) - waiting to see what happens.

[JohnMo]

15 minutes ago, timhowes said:

submitted the plans via the Heat Geek portal (seemed to need to be channelled into their "black label" system which sounds even more expensive!) -

I got some stupid quote, a massive heat pump offered, even with an 'A' EPC. And an actual heat loss for house, close to 3.5kW even at -9 outside.

[JohnMo]

52 minutes ago, timhowes said:

So multiple 5689 by 1.4, allow for DHW and go from there

 

54 minutes ago, timhowes said:

By this, do you mean I should look at the heat loss in the deltaT columns 15 and 20 (assuming I want 20C in room)?

Yes, but you don't need to add anything for DHW. The way they test heat pumps is more for central Europe, which has a dryer atmosphere, so less defrosting goes on.

 

So I would look at nearer 8-9kW (at -3) not a 6-7kW. But as mentioned turn down and modulation is important.

[Great_scot_selfbuild]

On 06/12/2025 at 11:45, timhowes said:

However, as someone with no prior experience, I will have to be quite brave to source my own ASHP, design the system and ask them to fit it.

I’m in this position too. I’m certain I'm being pushed quotes heavily oversized, and I don’t want a system that keeps turning on because it’s not been properly calculated and sized. I just have very little time outside of work to do it whilst also keeping tabs on the work being done (where I keep identifying issues that need my attention immediately 😭). 
 

following, and will update as I progress myself. 

[timhowes]

I had my first non-ridiculous quote yesterday (Cool Energy) and am pursuing this. Will keep you posted

[JohnMo]

2 hours ago, timhowes said:

I had my first non-ridiculous quote yesterday (Cool Energy) and am pursuing this. Will keep you posted

Read the data sheet for the heat pump in the offer to make sure it is doing the correct output for your heat loss. The nameplate sizing as with most manufacturers is not very well representative.

 

And make sure you are being pushed into a buffer when you most likely do not need one. Also don't go pre-plumbed cylinder. Lots of stuff on apre plumbed you pay for but may never use.

Edited December 10, 2025 by JohnMo

[Great_scot_selfbuild]

On 05/10/2025 at 22:57, JohnMo said:

You need a couple of hrs for DHW heating. So for 4.35x24, so 105kWh per day.  To do that in 22 hrs, 105/22, is 4.77kW not 7.5kW.

 

According to the Cool Energy datasheet for the 6kW unit, at -3 outside it has max output of 3.8kW at 35 Deg flow temp.  So isn't big enough for your calculated heat load of 4.35 or 4.77kW. So the only option they seem to have for you is a 10kW unit.  The Cool Energy unit offered has a minimum modulation of 3.89kW, which is fine.  But at 7 degs outside (based on 4.35kW) your heat demand will be around 2kW, so would cycle on off about 50% of the time. Cycling like that is OK, for off time hits CoP, a unit that tick away most the time has great CoP and almost no standby costs.

 

A Panasonic 5kW puts out 5kW at -3 and will modulated lower at 7 degs

Just been reading up on Hisense 4.4 ASHP, made by Hitachi, will do 5kW at -7 and modulate down to 1.85kW at 7 degs. And only £2k incl VAT (Heat Pump Warehouse).

 

 

@JohnMo I've just been revisiting my earlier posts on this thread. I'm so pleased I wasn't rushed into making a poor decision earlier and have revisited this after going through some detail about our UFH design - I'm re-visiting this thread because none (literally none) of the UFH suppliers asked for any details about heat loss in working out their proposed design, and I realise better now exactly why I need to start from a solid foundation of properly understanding our heat loss, the resultant ASHP and then looking for the right emitter sizing/specification.

 

I've now re-read this thread and although I'm close, I'm not entirely there in understanding the modulation (your earlier reply was really helpful in prompting me to pause and start to question this more, but alas I am being slow and could do with it spelling out, if you or someone else is willing to?). I've so little confidence in what I've been provided by the companies (the slightest question to them ends up with them going very silent and figuratively 'pointing' at the results that their over-simplified tool has output; they just want the sale and to move on).

 

So, having read and re-read this thread (not just my questions), I'm of the view that our insulation will be good enough that the down-time required to heat the Domestic Hot Water Tank (looking to get a 250L / 300L one), doesn't really require us to size-up the ASHP noticeably (@jack's earlier reply about the ASHP heating the DHW for a few hours in a day could be almost imperceptible - I think this could apply to our build).

 

I'm currently looking to improve the quality of the floor insulation, and so have re-run the heat loss calculator sheet (2 screenshots - close-up in the text and the whole version is attached). 

 

Summary questions:

1. Going over this calculator, I've just realised that I need to confirm an entry - the internal floor area I have input is for both floors, rather than just the ground floor - should this not actually be just the ground floor? as the intermediate floor won't contribute to heat loss as it is within the warm envelope? (I had populated it as the 'default' approach when asked for house size as being the area of all floors).

 

2. @JohnMo (or others), would you mind explaining ASHP modulation and what I should be looking for in the spec sheets of a heat pump, and how does this translate to what suppliers/manufacturers state as being a '6kw ASHP' (is this in the middle of the modulation range, or the peak performance of the HP? 

 

3. Should the Heat Loss calculator value for 'Floor Area' be just the ground floor, or combined floor areas? (if just the ground floor, the summary table comes out as follows):

 

4. We plan to only install UFH (wet) downstairs, with upstairs having electric mats in the bathrooms. All the UFH suppliers have stated that we won't achieve BR spec with UFH downstairs only and we'll need at least some emitters in the bedrooms. I'm inclined to prefer the option of specifying some limited electric UFH mats in the bedrooms which could be simply left turned off, but provide a backup in case we've miscalculated and find that the rooms do need some extra heat (I'd really rather get the calculations right though, with this group's help if possible?). I know some of you will have encountered something like this - what are your views? If challenged by building control, how do I rebuff their request for us to demonstrate compliance (as suggested by the UFH suppliers)?

 

This post has become much(!) longer than I set out to write. Thanks for getting to the end and I hope some of you are able to comment / advise.

 

Great Scot! Selfbuild

[JohnMo]

1, 3, multi storey building you state the ground floor area.

2. Unfortunately ASHP don't make this easy for anyone and none use the same reference scale. So you to dig about and find technical data books, which in general will give a range of valves for low, middle, nominal, and max output, for various flow and outside temp.

4, if you have downstairs heating only it needs to provide all the heat output for all floors. So by way of example, 200m house with 100m2 on each level. Total heat energy required required is 10W/m2, if supplied over two floors of heating. To compensate for one floor providing the heat, you need to output 20W/M2. But total heat load remains fixed by heat loss, you have how its delivered.

 

 

 

[Great_scot_selfbuild]

3 hours ago, JohnMo said:

1, 3, multi storey building you state the ground floor area.

2. Unfortunately ASHP don't make this easy for anyone and none use the same reference scale. So you to dig about and find technical data books, which in general will give a range of valves for low, middle, nominal, and max output, for various flow and outside temp.

4, if you have downstairs heating only it needs to provide all the heat output for all floors. So by way of example, 200m house with 100m2 on each level. Total heat energy required required is 10W/m2, if supplied over two floors of heating. To compensate for one floor providing the heat, you need to output 20W/M2. But total heat load remains fixed by heat loss, you have how its delivered.

 

 

 

Thanks @JohnMo - really helpful.

 

So, I was thinking that the UFH quote I had for UFH stating 50w/m² was well out, especially as it had the UFH running at 46 degrees(!), but the following numbers would appear to come out at 50w/m² if we want the ground floor UFH to meet the whole-house heating load. It just 'feels' high to me based on the thermal performance figures etc. Could someone have a look and see what I'm missing, or if we do indeed need 50w/m² but at a lower temp?

 

I've revised the figures in my calculator as follows:

• Target room temp of 20 degrees
• Under-slab temp to match the OAT of -5 deg (our house is on a frame supported above ground and so beneath the beam & block floor it is OAT)

This gives the total heat loss (row 39) as 3156W

...equating to 3156W / 131m2 = 24W/m2 (single floor, therefore 48W/m2 to meet whole-house load)

 

How would the UFH firm get to 46deg as the running temp? (they didn't ask me for heat loss or thermal property values); but more importantly, how do I calculate what temp I need to run it at?

 

However in the second section of the calculator, row 65 averages as 2172W total heat loss (2171 / 131 =16.5W/m² or 33W/m² for whole house heat requirement)

 

(for clarification, the UFH plan - based on feedback here actually - is for a single zone across the whole of the ground floor. Expecting to use 16mm pipe at 150mm spacing in a spiral pattern)

 

And so in summary:

1. What figures should I be taking from this to establish the size of ASHP? (noting @JohnMo's answer above about needing to scrutinise the manufacturer's spec sheets to make sure I'm comparing like-with-like)
2. What should the lower and upper modulation / performance values be for it to be able to keep running and be at or close to its optimum CoP? (the more I read, the more I am hell-bent on not over-sizing)

Many thanks! 🤞

 

 

 

[JohnMo]

2 minutes ago, Great_scot_selfbuild said:

This gives the total heat loss (row 39) as 3156W

...equating to 3156W / 131m2 = 24W/m2 (single floor, therefore 48W/m2 to meet whole-house load)

Not quite.your single floor area is 131m², your actual house floor area is 262m². If you had heat on both floor the floor output would be 3156/262. If you are only heating via one floor your floor output needs to 3156/131. So you stop at the 24W/m².

[Bonner]

8 hours ago, Great_scot_selfbuild said:

We plan to only install UFH (wet) downstairs, with upstairs having electric mats in the bathrooms

In theory there was no need so I wasn’t going to put any heat emitters in the bedrooms but decided to err on the side of caution and had (not oversized) radiators installed. Wet towel rails in bathrooms as well. Partly it was to satisfy the installer’s MCS calculators, partly to avoid domestic complaints! Glad I did because we do feel the need during cold snaps.

[JohnMo]

6 minutes ago, Great_scot_selfbuild said:

What figures should I be taking from this to establish the size of ASHP? (

You need a heat pump, does around 3.5kW at -3, but that will also run happily at 10 degs at just above 1kW at around 25 Deg flow temp.

 

Here's a starter data book, full of information. The 4kW monobloc looks right. To buy you need the outdoor unit and wiring centre. Wolesely selling now and a 4kW is £2500 for both units with 6 year parts warranty.

 

https://www.gaf.be/pdf/r290_monobloc_engineering_data_book_20240808.pdf

[Nestor]

@JohnMo 

Just a quick one but what are your thoughts on the actual volume of the property as well as floor area as no consideration for differing ceiling heights.

Could volume be part of a heat loss calculator?

Is it relevant?

 

[Great_scot_selfbuild]

1 hour ago, JohnMo said:

You need a heat pump, does around 3.5kW at -3, but that will also run happily at 10 degs at just above 1kW at around 25 Deg flow temp.

@JohnMo I’m loving the summary, but would you mind explaining where you get these numbers from (in particular the flow temp) - treat me as you would a small child, that just about caters for my ‘speed’  of learning this topic 🤣 (or reference the cell row/column).

 

As for the ASHP you’ve identified - TVM! 😍

 

I can entirely see why people have previously commented about why the MCS grant doesn’t always work out cheaper at all. I may find that I do this myself… now looking into the spec details 😁

[JohnMo]

2 hours ago, Nestor said:

@JohnMo 

Just a quick one but what are your thoughts on the actual volume of the property as well as floor area as no consideration for differing ceiling heights.

Could volume be part of a heat loss calculator?

Is it relevant?

 

It is already by default as it asks for internal wall area and roof area, so is you have vaulted ceilings the roof area will be greater than a flat ceiling.

 

53 minutes ago, Great_scot_selfbuild said:

@JohnMo I’m loving the summary, but would you mind explaining where you get these numbers from (in particular the flow temp) - treat me as you would a small child, that just about caters for my ‘speed’  of learning this topic 🤣 (or reference the cell row/column).

 

As for the ASHP you’ve identified - TVM! 😍

 

I can entirely see why people have previously commented about why the MCS grant doesn’t always work out cheaper at all. I may find that I do this myself… now looking into the spec details 😁

Minimum flow temp is a product of running dT (difference in flow and return temperature) and compressor restart hysterisis. So mild day let's say floor temp is pretty close to room temp at 20-21 degs. So if you are heat soaked the whole floor is at 21 degs minimum. When running ASHP lets say you had it set to run dT of 4. Flow at say 25 return 21. So mean flow temp of 23 degs. Your pipes in the floor will radiate heat outwards while they are hotter than the screed. The further the energy moves from the pipe edge, the lower the temperature gets. So if the pipe starts 25 degs by the time the gets to floor surface it may be 22 degs, the further water travels along the pipe the more energy it gives up to the floor. So once at the end of the pipe, it will 21 degs, starting at 25 degs (this why UFH is rated a mean flow temp, because it's the average temperature).

 

Then you have restart hysterisis, it normally set dT plus a temperature margin to stop fast restarts so say it's 5 or 6 degs with your dT 4.  So now your return temp reads to drop to 20 or 19 degs before the compressor will restart. Which could takes days - so we have an issue. The issue being you get one cycle and then the compressor in the ASHP doesn't get a permission to start 

 

The work around is we reduce dT and reduce reset hysterisis. To some thing like 3 and 4.5 degs. So it's unlikely you will get a flow temp lower than 25 and in a low energy house you can make it work. Your WC curve will be pretty flat as well, maybe raising temp 4 to 5 degs by the time you get to -3.

 

The heat pump I referenced, I have just bought one (4kW) to replace my 6kW one which doesn't modulate well at all. Will need a gap in the cold weather to get it installed. The 6kW will be repurposed as heater for a hot tub.

 

I am doing the simple install, UFH only in heat and cool mode. Doing DHW via immersion during peak.

 

So parts list gets super simple. UFH and manifold, couple of ball valves for manifold isolation, 2x anti freeze valves, strainer/ball valve at ASHP (required for warranty) plus an isolation valve on flow out from heat pump. Fill system with inhibited (corrosion and biocide inhibitors - not antifreeze or glycol). Expansion vessel and relief valves are in monobloc.

 

A direct cylinder (half the price of a heat pump cylinder).

 

I paid £2500 including vat for Hiaer monobloc, you can claim the vat back on a new build.

Edited 23 hours ago by JohnMo
Additional details

[SBMS]

@Great_scot_selfbuild @timhowes I too was nervous of my own calculations using the spreadsheet. It came out around 8.8kW.

 

 

I then sanity checked and used the free and brilliant tool from Heatpunk that calculated 9.1kW. 

 

our MCS installer calculated 12.1kW and was nervous about suggesting a 12kW and was suggesting a 16kW (the range i am looking at is only available in 9,12 and 16. He was using the standard MCS spreadsheet which can overestimate and he had some slightly incorrect u values which was my fault. 
 

I then worried even more and so decided to get a heat loss done that was independent and mcs compliant for the installer. This cost £300 and it came out at 9.15kW using heat-engineer and he even modelled it in spruce and it came out at 9.36kW. 
 

Moral of the story is Jeremy’s calculator is good - but not foolproof and took me a couple tries to get right. Heatpunk is free, very easy to use and almost foolproof. Would Absolutely

recommend you give it a try. 
 

and finally If you’re worrying or your mcs installer wants an independent one the company i used (it’s just one chap I think) was pretty good And fairly cheap. DM me if you want The name. 

 

 

[SBMS]

8 hours ago, Great_scot_selfbuild said:

@JohnMo I’m loving the summary, but would you mind explaining where you get these numbers from (in particular the flow temp) - treat me as you would a small child, that just about caters for my ‘speed’  of learning this topic 🤣 (or reference the cell row/column).

 

As for the ASHP you’ve identified - TVM! 😍

 

I can entirely see why people have previously commented about why the MCS grant doesn’t always work out cheaper at all. I may find that I do this myself… now looking into the spec details 😁

Use an MCS umbrella company? You can still

get the grant but self install. 

[Great_scot_selfbuild]

2 hours ago, SBMS said:

Use an MCS umbrella company? You can still

get the grant but self install. 

This has been my approach recently (prompted by this group), but as you experienced, the MCS calculations from a couple of these companies were recommending 14kw!?! This has rather dented my confidence in them.

[SBMS]

44 minutes ago, Great_scot_selfbuild said:

This has been my approach recently (prompted by this group), but as you experienced, the MCS calculations from a couple of these companies were recommending 14kw!?! This has rather dented my confidence in them.

I’d spend £300 on independent ‘MCS accredited’ heat loss and take that that to umbrella company for heat pump sizing. You’ll get the 300 back and then some if you get the grant! Prob end up nearly free. 

[JohnMo]

9 hours ago, SBMS said:

spend £300 on independent ‘MCS accredited’ heat loss

You still have to careful your not just wasting £300, with sh!te in sh!te out. 

 

I would get UFH in, which I think is done, make provision for ducts for pipes and cables through wall. Seal them up until after air test. Get house air test done, get MVHR in and commissioned, then if going down MCS route go get the survey done. Without all that in place there is a good chance it will be treated as default values for ventilation heat loss.

 

Or just don't bother MCS at all, spend £2500 on a suitable sized heat pump, buy a 200L direct cylinder - install. Power cable and communication cable to outside unit and two pipes to UFH manifold. Then claim VAT back as it's a new house. Little over £3k in materials claim around £600 vat back, so £2400 plus a little labour.