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Hi,

This is my first post, so please be gentle with me !

I want to install an ASHP, using the BUS Grant.

I’ve had lots of different ‘suggestions’ about my Heat-loss, so I decided to do my own calculation, using the MCS-approved ‘Heatpunk’ system.

We live in a Victorian property that has been heavily renovated, including triple-glazed Doors & Windows throughout.

If I tell Heatpunk that those renovations bring our house to a post-2000 level of insulation, it gives a figure of 6144W for space heating, whereas if I tell it that the same specifications only bring it to a ‘Pre’-2000’ level of insulation, it gives a figure of 8455W

I am happy to go with the lower figure, as I am aware that under-sizing is generally less of a problem than over-sizing, and Samsung do an 8kW that nearly meets the higher figure, anyway.

But my installer has the higher figure, and so feels he can only install a much bigger (12kW) unit, as that is the next size up in the range … (Samsung R290)

So, to put it simply, can I ask him to use my figures and install the smaller 8kW unit, - and if he does so, will MCS have a problem with that, and stop him receiving his £7,500 BUS Grant ?

Thanks !

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Hello @tokenpom it sounds like the difference here is being caused by the default air change rate assumptions for pre-2000 properties vs post 2000 properties? Your installer I assume is assuming CIBSE column A air change rates, which are in our experience a significant over-estimate.

If you want detailed reading on this, you might find the following that I’ve written on the subject useful:

and also discussed in my blog on heat loss calculations here:

Many installers are aware of this issue and are selecting lower air change rates but not everyone is aware or confident to deviate. MCS do allow the installer to use lower values (their standard does not specify what values should be used and their guidance highlights that age is not always a good guide to air change rate levels).

You could try forwarding the following CIBSE approved guidance from the Passivhaus trust to your installer https://passivhaustrust.org.uk/guidance_detail.php?gId=72 that suggests “Do not use default air change rates for a retrofit. The default figures are often very high and it is recommended to do an air test and use the actual figure.”.

If you want peace of mind you could always get an air tightness test done, the cost of a test should be around £250 and would save more than this on the cost of the heat pump. That said if you dont feel that the house is draughty it’s very unlikely that the air change rates are high… 0.5-0.7 ACH is quite typical.

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Hi Trystan,

thankyou very much for that, that could be really useful !

But I do have to ask, however, can I then ask my installer to accept ‘my’ (Heatpunk) calculated figure, - and if they do, can they then be punished by MCS or anybody, in any way, for doing so ?

And what might they be punished for ?

All the installers I’ve spoken to over the last year seem absolutely terrified of using my lower figure, despite the fact that it would be me asking them to do so, - on the basis of using my (MCS approved) ‘Heatpunk’ Calculation ?

For instance, I had this as part of a reply I received today ?

“Having spoken to our Group Technical Manager, we are unable to install an 8KW unit, as this will be below our heat loss calculation. The only way to make your heat loss below 8KW is to change the build date to post-2000, which you did tell me, but we’re unable to do so due to our MCS requirements.”

So is there some sort of disclaimer that I could write to MCS to explain my choice of an 8kW unit over a 12kW ?

Full disclosure: This Heatpump will be heating a Sunamp Heat Battery for our Hot Water, we are therefore limited to the Heatpumps that can do that, and for me it’s either the Samsung R290’s, which only come in 8kW or 12kw, or the Panasonic ‘Aquarea’, which does 9kW, but which my preferred installer can’t install (!) ( - and the other Panasonic installers I’ve spoken to have been appalling, - one clearly knew nothing about the Aquarea’s and the other just insisted that he ‘knew’ we’d need a 16 kW unit, just from the ‘look’ of our house … !)

Kind regards,

Nick.

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The only thing that selecting pre-2000 or post 2000 in Heat Punk changes is the air change rate assumptions and MCS guidance is very clear that air change rate values do not necessarily reflect the age of the property and that the installers engineering judgment can over ride these default CIBSE values.

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See what they say if you are very specific that you want them to use lower air change rate assumptions. E.g in Heat punk select pre-2000 even and then manually change the assumed air change rate in each room to ~0.6 ACH…

If they are not receptive, perhaps it’s worth trying a different installer?

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Thanks very much for that Trystan, that’s terrific !

And Yes, I would like to use these people, because they did a good job installing my solar PV recently, but I have absolutely no interest in sticking with them if that means lumbering myself with a stupidly over-sized ASHP !

I’ll let you know how I get on !

Kindest regards,

Nick.

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Hello Nick,
I was in much the same situation as you - the surveyors put my renovated 1980’s house in category A air tightness and calculated a heat loss of 9.64kW which made their recommended HP above an 8kW Daikin to an 11kW - which is widely accepted here to be a less desirable HP in terms of efficiency.
I pushed back on the installer saying I thought the air tightness should be better than CatA but they wouldn’t budge, quoting “MCS regulations / obligations” etc - same as yours.
I had a blower-door test done which reported air tightness better than Category C which brought the heat loss to 6.99kW which is within 8kW HP territory.

Still no movement from the installer.

I then phoned MCS asking if they did in fact insist on blind adherance to building date when categorising air-tightness in heat loss calcs. The rep was very open and helpful, saying they had no such proscriptive rules. She went away and dug out their paperwork and emailed me a relevant extract, saying “We base our ACH’s on CIBSE Domestic Heating Design Guide”

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…and highlighted the note:
“For buildings where extra attention has been given to reducing ventilation losses…it is recommended to seek specialist advice”

In this context, I presented the MCS guidance and the blower-door test as specialist advice to the installer and at this point they had a change of heart and became able to fit the 8kW HP, which is working well for us :slight_smile:

Everyone has signed off on the installation and the BUS grant has been paid.

So my advice to you is : get a blower door test done and then you can be sure you are not mistaken when you try to convice the installer that you calcs are valid and that MCS will accept them.

Good luck, David.

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Thanks for that David, that’s a great help !

It’s a bit crazy that we still have to jump through hoops like this in order to do the right thing, but yes, as long as my installer is happy to go with the results of a blower test (and perhaps deduct the cost from his price !) then I think that that this will be the logical next step !

I’ll hope fully let you know how I get on !

Kind regards,

Nick.

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Hi,

My installer is still having issues, but he has now been convinced, it seems, that I could be considered to be a post-2000 build date.

I have a Heat Punk project plan that appears to be completely accurate, but the Heat loss figure it gives is for a pre-2000 build date, - so is there any way that I can change this project to a post-2000 build date, without having to start all over again ?

Thanks

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Hi Nick,

Did you find this MCS heat loss calculator spreadsheet?
https://mcscertified.com/wp-content/uploads/2021/04/MCS-Heat-Pump-Calculator-Version-1.10-locked.xlsm

It describes the values of ventilation heat loss and gives you the option to specify the category of property A,B or C and hence change the ventilation part of the total calculation.

Depending on the detail of room-by-room information you already have from heat loss surveys you could find ways to apply the 3 different ventilation heat loss figures.

Cheers, David.

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Hi David, thanks for that !

I did, but I haven’t gone near it yet, as I’m going to assume that I wouldn’t know what I was doing with it (!)

But I have passed it onto my installer, so hopefully they might be able to use it to give me the result I want, anyway ! :wink:

Kind regards

Nick