My Heat loss calculation is about 6kW but installer insists i need a 16kW heat pump!

Hi all. I am in an unusual position (I think!) of having bought a new house that is being built at the moment.
The Builder has no Idea what the heat loss will be!
I have done my own calculations and come up with about 6kW including hot water and air infiltration.
The house will be 4 bedroom and obviously up to Building regs so I used the regs as the basis for my calculations.
I have spoken to someone at the company that will be installing the heat pump and basically he just wasn’t interested in talking to me and it was obvious to me that he thought I was a numpty and that I just couldn’t possibly understand such a complicated subject.
Perhaps he is right I don’t know !
Anyhow he said that the Fujitsu pump can modulate from 16kW down to 2kW output so It can’t possibly be over rated for the job.
Can anyone put my mind at rest and tell me if he is right please?
All of the information i have seen is saying that, over rated pumps short cycle all the time and are disastrous for COP.



Hello Mike, 16 kW for 6 kW heat demand sounds crazy, perhaps suggest he watches this video which sounds like a very similar predicament to that you are in

You could also try validating the modulation claim, does it really modulate down to 2 kW that’s an unusually large modulation range… If this is the right datasheetési_segédlet_Waterstage_SuperHP_HMV_1f_3f_2020.pdf it suggests 6.6 kW heat output, which is roughly 40% of the badge output which is closer to the kind of minimum modulation range that we expect.

On the heat loss side of things can you share your calculation, I’d be happy to have a quick look over it. What air change rate did you assume, I assume this is not MVHR?

Sorry about late reply it’s been a hectic time of late as we have been moving into a rental.
I’ll have a look at those sites and get back to you.
And Thanks for offering to look at my calculations, that would be great!
I’m very busy today but I’ll try to send you my calculations tomorrow if that’s OK.


You’re probably quite correct to be sceptical!
Another thing to watch is pipe diameters, if they’re than inept on heat pump sizing then they may have installed microbore. You want 15mm (copper) minimum.

I’ll put your mind at ease but perhaps not in the way that you might like: I think you ought to find a new heating engineer pronto.

Dealing with somebody new to heat pumps is fine. They often go to town and can do a good job once they’ve read, re read, and obsessed over the details on their first install.

Dealing with somebody who is both new and untrainable / unteachable is probably not something that you want to be funding.

What else have they screwed up given that they can’t read a datasheet and can’t do heat loss calculations? Perhaps trust them to join pipes; but I wouldn’t trust anything they’ve done using a calculator (heat loss, pipe sizing, etc) given the attitude you’re describing!

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Hi again.
My heat loss calculations.
I added up total wall area of the house externally minus windows and doors = 292.62 sq m @ U-value 0.18
added the ground floor 175 sq m @ U-value 0.13
and the loft area =175sq m @ U-value 0.15
windows and doors = 48.92 sq m @ U-value 1.4
totaling this gives me 150.74
adding Infiltration Heat loss =Volume of whole house(658.88cu m) *.05 = 32.94
equals 183.69

Multiply that by 22 which gives 4041w
I’ve added 10% for initial heat up period which I’m not sure I need, as this calc derives from my old house with an oil boiler
this gives 4445w divided by 1000 = 4.445 kw
than added 0.87 for hot water to make a total heat loss of 5.32 Kw/Hr. max required boiler output

I really hope that’s clear enough
I am probably missing something or got the whole thing completely wrong so would really appreciate your input.


Unfortunately I don’t think I can do that.
I am buying the house from a builder who is actually still building it .
I have exchanged contracts on the house so am committed to buy.
He employs quite a large local company to install the Heat pump so the contract is in the hands of the builder not me. I have talked to him about this but even though this would save him money he has built many homes with this company but has “Never had any complaints” ! so he want’s to play it safe (from his point of view) after all it doesn’t matter to him if I have to rip it out and replace it in the future!

I have never bought a new house before and didn’t realize what a weak situation you are in if you want anything changed.


If you’ve exchanged against a plan the builder will want to deliver exactly what was exchanged against then take the money and take a holiday.

If it doesn’t work…doesn’t matter…you’ve already agreed to buy.

He may also have borrowed against an agreed design…and can’t change it without renegotiating the finance arrangements.

There’s no requirement to follow even the MCS standards on a newbuild so very little to hold the developer’s feet to the fire I think.

Would he agree to a heat meter being installed in the pipework whilst you’re at it? Would he agree that it’s “at least up to MCS standards” (out of arrogance) That may create some comeback when it’s clearly overkill.

You are under contract but for it to be built to a sufficient standard and quality, so they have to meet their end too.

16kW is far too big.

How did you get .05 here? Is that taking into account MVHR recovery efficiency somehow?

I guess either way the heat loss is still very low and this is heat loss before internal gains (heat given off my electric appliances 200-400W worth, heat from body heat 4 people 240W, solar gains on the coldest day is likely very low).

I guess the other side to this is how well do you expect the insulation to be installed and will those U-values actually be achieved, what will the attention to detail be on air-tightness etc?

To be honest I did that originally for my last house many many moons ago and I can’t remember where the 0.05 comes from!
I think I can only assume that the work is done up to Regs ! I am making regular visits with a camera and from what I have seen the wall insulation is good at least. They have not started the floor yet as the roof isn’t on.
Do you think my calculations were correct apart from that?

Also you sent a link to Fujitsu’s Specifications but I could not find anywhere where it mentions the modulation range being from 16 down to ? I would love to be able to show the builder that they are claiming a modulation range that is wrong or even better that they are correct because maybe then I would not have to worry.

I have seen the Heatgeek video you linked to before ! I do follow him and think he is a very clever chap! It was only watching this video that really got me worried about oversizing although he has mentioned this before many times. I would love to have them do the installation instead .


I guess if we’re going for ballpark calculation and there’s no MVHR:


For MVHR it depends on the recovery efficiency and infiltration test result, you could use the calculation in the ventilation section of SAPjs (based on SAP 2012 method) together with a blower door test result value to give an estimate of winter ACH: E.g achieving 5 m3/hr/m2@50pa (building regs <=8 I think Part L Building Regulations: What is the 2022 Update? - Building Energy Experts ?) and 90% recovery results in 0.42 ACH effective air change rate in terms of heat loss in January:

Page 7: Heating capacity > Minimum: 6.59 kW

It would be worth double checking the actual model number of the proposed heat pump as this datasheet may not be the right one…

It might be when they say it modulates down to 2kW they meant 2kW electricity input, which would probably align with the 6.5kW minimum output.

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Great thanks for all the help.
they gave me the model number for a recent house they just finished on another site WPYG160LJL for the outside unit and WSYG160DJ6 for the inside unit but until I pop round to the site and look on the only house that has had the unit installed I won’t know definitely if it’s going to be exactly the same . I imagine it will be though.

Hi Michael, I have a 14 kW Daikin Altherma installed against a 7 kW heat loss. As it’s a conversion from a previous oil fired boiler the radiators are probably a bit undersized - I just changed out two in the open plan downstairs area. The Daikin appears to run down to about 800 watts input (electrical) so it’s just about OK, but do get some cycling when the room ‘stat cuts off when the temperature exceeds my limit (21 degrees).

It was originally set up with a fixed set point flow temperature of 55 degrees, but I’ve got them to switch on weather compensation now so I can tweak the compensation curve to try and better balance the system. Watch out for that in case they have a gas/oil boiler fixed temperature mentality!

Hi again :blush:
I am have been examining my calculations on hot water and am having a problem. Firstly I forget where I got the figure I used from so wanted to check it.
Mostly because I know I calculated it for just two old pensioners (my wife and I) and I realise that the Heating should be calculated for the potential amount of people that could use the house>
So I found a site with this formula .
calculate the power of heating element needed to heat a specific volume of water by a given temperature rise in 1 hour.
volume in litres x 4 x temperature rise in degrees centigrade / 3412
(4 being a factor and 3412 being a given constant)
So I am now using this to calculate for a 150litre/day usage ( I can’t seem to find an average per person/day ) does this sound tight?

That seems similar but not so bad as mine. sounds like you are just about on the edge though.

Have you ever worked out your COP? I have been looking into how to do that but it looks like you have to have extra hardware to work it out?
Watching heat geek, he seems to think you MUST have weather compensation on. Definitely not a fixed flow temp. I think they do that just to avoid having to think about it. :face_with_raised_eyebrow:
The first thing I do will turn weather compensation on. If I can find out how to do it :upside_down_face: