Is a 5kW ASHP suitable for Heat loss calculation of 5kW?

[Berkshire_selfbuild]

Hi All,

 

I have recently received my heat loss calculations from my local ASHP/UFH installer. It’s for my new build which is currently under construction. I’m not really going above a beyond building regs when it comes to insulation/glazing etc however will have MVHR.

 

Heat loss shows 5kw heat loss and they are proposing a 5kw Vaillant Arotherm plus. Property is 387sqm of which 39sqm is the garage and this element has not been used for the heat loss calcs as is not heated. Please see details attached, I would appreciate any comments and whether the proposed heat loss with proposed ASHP looks about right?

 

Thanks! 

Edited March 30 by Berkshire_selfbuild

[Conor]

Without commenting on the calculations and assumptions, a 5kW will struggle. If the max output of the heatpump is the same as the heat loss, when it's cold, the heatpump will have to work 24x7 to keep the house warm. So no time for heating water. And if the COP isn't at what the point needed to produce 5kW, then your house won't maintain temperature. I'd be looking at a 7-9kW heat pump.

[SteamyTea]

You have to check that the heat pump can deliver 5 kW (now kw) at -3°C.

 

Not sure if this is the model you are getting.

https://www.vaillant.co.uk/product-images/0020330791-03.pdf.pdf

but it shows a power output of between 2.1 and 6.9 kW.

The 2.1 will be when the temperature is cold.

 

Update

This

https://www.plumbnation.co.uk/documents/vaillant-arotherm-plus-information-sheet.pdf

Shows different numbers.

Edited March 31 by SteamyTea

[RobLe]

I’m impressed, required insulation values are pretty good these days - that’s a big house for a 5kW heatpump.  
How confident are you that the insulation and airtightness will be done right?  Is there an airtightness test, and consequences if not met?  Who checks thermal stuff is done right?  Any guarantees kick in if the heatloss actually is double the design value?

 

 

[Berkshire_selfbuild]

Thank for the comments. I’ll query the points raised and also find out the exact model. I’m addition, the installer mentioned following -

“If we were to apply the true efficiency at best case scenario we would be looking for a unit around the 3.7kw output range (the 3.5kw arotherm is the same as the 5kw just restricted via software). We would be proposing a 5kw Vaillant Arotherm Plus with adequate water volume to make the oversizing factor irrelevant to any oversizing negatives”.

 

Edited March 31 by Berkshire_selfbuild

[Nickfromwales]

I’d be fitting a 7kw minimum here, but also a buffer for when the heating just needs a tickle.

 

Fitting a 5 currently for a home that prob needs less than 3, defo needs a bit of headroom. A worry that your installers are proposing this tbh.

 

If you’re not under 2 ACH for airtightness and you’re fitting MVHR then you can assume some additional heat requirements for compensating for poorer heat recovery and the input of cold(er) air from atmosphere through the delivery of fresh air to habitable spaces.

 

Fitting a very good door between the house and the garage for draught-proofing will help a lot, as the last thing you want is cold air getting pulled / blown in from there.

 

Do you have a target airtightness score?
 

Cold air infiltration will be a massive factor when sizing heating, but most “heating engineers” (lol) won’t look further than the very basic stuff, let alone make adjustments for (actual) MVHR performance. 

 

[Nickfromwales]

  On 31/03/2025 at 07:04, Berkshire_selfbuild said:

Thank for the comments. I’ll query the points raised and also find out the exact model. I’m addition, the installer mentioned following -

“If we were to apply the true efficiency at best case scenario we would be looking for a unit around the 3.7kw output range (the 3.5kw arotherm is the same as the 5kw just restricted via software). We would be proposing a 5kw Vaillant Arotherm Plus with adequate water volume to make the oversizing factor irrelevant to any oversizing negatives”.

 

Expand  

How are they oversizing if it’s 5/5? 🤷‍♂️

[SteamyTea]

Just had a look at your heat loss calculations.

Says that there is no insulation under the solid ground floor.

Is that correct, I though floors had to have a minimum of 0.13 W.m^-2.K^-1 I think.

Better to have more if UFH is installed.

[Berkshire_selfbuild]

  On 31/03/2025 at 07:17, SteamyTea said:

Just had a look at your heat loss calculations.

Says that there is no insulation under the solid ground floor.

Is that correct, I though floors had to have a minimum of 0.13 W.m^-2.K^-1 I think.

Better to have more if UFH is installed.

Expand  

I noticed this too. I Will query this as we are definitely having 130mm insulation above the beam and block floor.

[JohnMo]

  On 30/03/2025 at 20:50, Berkshire_selfbuild said:

Heat loss shows 5kw heat loss and they are proposing a 5kw Vaillant Arotherm plus

Expand  

Heat pump manufacturers label heat pumps differently. Vaillant 5kW at -3 kicks out quite a bit more kW than 5, you need to look at data sheets to get the correct figures. See no issue with what is proposed. So long as you aren't being ripped off.

 

Installed system should be simple, cylinder, 3 port diverter valve, UFH or radiators. Minimal zones ideally one, no buffer.

[SteamyTea]

  On 31/03/2025 at 07:24, JohnMo said:

Heat pump manufacturers label heat pumps differently

Expand  

I wish there was a standard.

Not as if it is difficult as all manufactures will know the ΔT curves of their products.

[Alan Ambrose]

There is an argument that an oversized heat pump may tend to cycle which is inefficient, so you may want to deliberately not have much headroom. Also, how many days are at -3C for 24h in Berkshire? - so there’s already some headroom built in. Ask them to re-run the calc (should take 2 minutes) for a more realistic cold day, to see how much real headroom you have?

Edited March 31 by Alan Ambrose

[JamesPa]

@berkshireselfbuild

 

If the heat loss calculation is about right then the 5kW Vaillant is a pretty much perfect fit.  It claims 6.5kW at -3/35C (as you have UFH I presume you will be operating at 35 or less), but note these figures may not include defrost..  Its a nice unit with good and very capable controls - better than many).  As your installer says there is no point in going for the 3kW model its just the 5kW one software limited at the top end but identical at the bottom end.  Your installer sounds like they may be a goodun!

 

As @Johnmo says keep it simple 3 port diverter valve, UFH/radiators. Minimal zones ideally one, no buffer/LLH/PHE separating heat pump from emitters (2 port volumiser OK - but with the house area you have even that probably isn't necessary - but no harm if it is)

 

Edited March 31 by JamesPa

[Berkshire_selfbuild]

  On 31/03/2025 at 07:32, Alan Ambrose said:

Also, how many days are at -3C for 24h in Berkshire?

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Far and few between. I would say maybe only 2 days like that over the past year. 

[Berkshire_selfbuild]

Thanks all for the comments and the education. I have some questions to run by my designer/supplier/installer, however I now have some confidence that they are trying to right by me by not over specifying the unit and using data that is realistic. All these figures and calculations etc go right over my head, so I’m glad to have you bright people to run these things through with. Thank you 😀

[SteamyTea]

  On 31/03/2025 at 08:36, Berkshire_selfbuild said:

  On 31/03/2025 at 07:32, Alan Ambrose said:

Also, how many days are at -3C for 24h in Berkshire?

Expand  

Far and few between. I would say maybe only 2 days like that over the past year

Expand  

 

  On 31/03/2025 at 08:39, Berkshire_selfbuild said:

All these figures and calculations etc go right over my head

Expand  

A heating system is designed to supply the correct amount of thermal power, 99.5% of the time.

2 day below -3°C is 0.55% of the time.

So that looks good.

 

Most of the heating season time the outside air temperature will be between 10°C (very little heating needed) and 5°C (not that much in reality).

We do get weeks where the temperature hovers around freezing for 4 or 5 days (when the Thames freezes over) but not seen any of those weeks since the early 1980s (worked in Marlow back then).

 

The worse that will happen is that your internal temperature will drop a couple of degrees, say to 18°C, then plug in a fan heater.

 

If the domestic hot water becomes a struggle, turn the immersion heater on.

 

Don't fret over the extremes, they are called extremes for a reason.

[siletto]

  On 31/03/2025 at 07:11, Nickfromwales said:

I’d be fitting a 7kw minimum here, but also a buffer for when the heating just needs a tickle.

 

Fitting a 5 currently for a home that prob needs less than 3, defo needs a bit of headroom. A worry that your installers are proposing this tbh.

 

If you’re not under 2 ACH for airtightness and you’re fitting MVHR then you can assume some additional heat requirements for compensating for poorer heat recovery and the input of cold(er) air from atmosphere through the delivery of fresh air to habitable spaces.

 

Fitting a very good door between the house and the garage for draught-proofing will help a lot, as the last thing you want is cold air getting pulled / blown in from there.

 

Do you have a target airtightness score?
 

Cold air infiltration will be a massive factor when sizing heating, but most “heating engineers” (lol) won’t look further than the very basic stuff, let alone make adjustments for (actual) MVHR performance. 

 

Expand  

2ACH, isn't that a terrible number? 

 

Or is it different than the european ACH50, 50Pa passivhaus measurement method? 

 

The goal is 0.5ach there, and as far as our construction goals go, it can be easily achieved with RAL window installs, sealed vapour stop foil barriers etc, not some space tech solutions. 

[Nickfromwales]

  On 31/03/2025 at 19:17, siletto said:

2ACH, isn't that a terrible number? 

 

Or is it different than the european ACH50, 50Pa passivhaus measurement method? 

 

The goal is 0.5ach there, and as far as our construction goals go, it can be easily achieved with RAL window installs, sealed vapour stop foil barriers etc, not some space tech solutions. 

Expand  

Yes, it is. But better than 3 or 5!

 

I’ve got AT test scores in clients builds as low as 0.25, so I’m not advocating a poor (terrible) number, just using a minimum threshold where any worse than that should mean you don’t bother with MVHR as there’ll be no heat to recover after cold air infiltration. 

 

 

[sharpener]

 

To me the big unknown is the 50% discount in the heat loss calcs from the assumption that the MVHR will recover 50% of the heat that would otherwise be lost through ventilation, I don't know how realistic this is.

 

OTOH they have assumed all rooms will be at 21C. For most ppl this is too hot for a bedroom and 18C ensures a better night's sleep. Ditto Utility Room and some others perhaps.

[JohnMo]

  On 02/04/2025 at 16:56, sharpener said:

MVHR will recover 50% of the heat

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It's fairly conservative, as long as your house doesn't naturally leak a lot. Passivhaus standards are in the high 80s 90s%

[sharpener]

  On 02/04/2025 at 17:22, JohnMo said:

It's fairly conservative, as long as your house doesn't naturally leak a lot. Passivhaus standards are in the high 80s 90s%

Expand  

 

That's reassuring, it's a new build so presumably can be made to meet modern standards. 

 

But losses through walls and roof must be very low with a total net 348 sq m only needing 5kW, which is under 15W/sq m* so the accuracy of the air change prediction is probably quite important. So I would seek assurances from whoever specified the MVHR and its mfrs rather than rely on a guesstimate from the HP installers.

 

 

 

 

*that's 4x better than our barn conversion at 60W/ sq m

 

 

Edited April 2 by sharpener

[JohnMo]

  On 02/04/2025 at 17:46, sharpener said:

But losses through walls and roof must be very low with a total net 348 sq m only needing 5kW, which is under 15W/sq m

Expand  

Pretty similar to ours at -3, and our house has a really poor form factor. We get down to -9 and have a 6kW unit and it pretty big for the house. But depending on screed depth (ours is a 100mm) oversized can be useful, but with shallow screed depth may be a pain. Ours is useful for buffering heat that is very slowly released later.