And here is what the Homely plots say for the last few days. The key indicates the parameters measured. Regards, Toodles.

Posted by: @mairia

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SCOP on the MCS certificate is 3.6, on the heat loss calculation for space heating is 3.02 which the installer explained as being based on 'site-specific conditions'.

That's an interesting use of the phrase 'site-specific conditions'.

I've met it being used elsewhere when a contractor wishes to simultaneously claim that they've correctly implemented the regulations,
but then apply a different calculation as an excuse for why the true figure differs.

For example, if a house-builder wants to develop an area of land which is contaminated with Arsenic, the Planning Authority might have a maximum threshold of 140mg/Kg.
That's 140-milligrams of arsenic salts within 1Kg of soil.

The geotechnical survey reveals much higher concentrations, which means that the land is unsuitable for housing.

So the geotechnical consultants (working on behalf of the developer) analyse the types of arsenic compounds which are present,
and declare that only a proportion of these are capable of 'biological uptake'.
That's called a Site Specific Assessment Criteria (SSAC) which is invariably lower than the threshold.

Here's an example of such an argument being put forward in planning documents:

In short, the term 'site-specific conditions' means that a mathematical fudge-factor is being used.

In your case, the Installer needs to substantiate what site-specific factors are being being used in the calculation.
I would expect this to be an explanation similar that I've just quoted for the arsenic above (ie a fudge)

You might be amazed what turns up...
... including the possibility of those air-vents taking heat out of the 'thermal envelope' of the house.

And that opens up two further questions:

1: were those air-vents properly included in the original whole-house survey?

2: if there's no longer any reason for them being present, why didn't the installer block them off?

You have rights under the Consumer Rights Act 2015 (potentially services not as described; reasonable care and skill) ask the company for a remedy; something like correction at their cost, compensation, refund of part of the installation cost or appointing another company to fix the installation at the original installer’s cost (if they refuse to fix it).

MCS should take mis-sizing, inaccurate certificates, and false documentation very seriously.

With your trusted company:

• Ask them to redo a proper heat-loss survey (your rights don’t depend on this, but it helps).

• Ask them to check radiator output, especially in lounge/hall.

• Ask them to check flow temps, balance, and var flow-rate issues.

9 kW on a 6.7 kW heat-loss house should be fine, so your suspicion about under-sized radiators is very plausible.

Posted by: @swwils

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9 kW on a 6.7 kW heat-loss house should be fine, so your suspicion about under-sized radiators is very plausible.

Is your reference to 6.7kW based on the initial post in this topic?

The OP stated in that post "The heat loss calculation then provided by the installer was not the original, but a recalculation..."
so we aren't trusting the 6.7kW figure in the ensuing discussion.

That's why the OP has been recommended to obtain a copy of what the MCS has on file.

Secondly, it isn't a 9kW unit, but an 11kW one, which has cost more.

Posted by: @swwils

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You have rights under the Consumer Rights Act 2015

I've posted a detailed explanation of that in this earlier post.

@toodles yes I am relying on the Daikin data from the MMI, and have heard before that they are less than reliable. The video you shared gives a really clear explanation, and I can see on looking at my app that all the figures are rounded to the nearest whole number! This whole episode started when I was considering 3rd party controls and monitoring and looking at my documentation - once the current questions are resolved I will hopefully go back to that. I liked the sound of Havenwise as there is no hardware installation involved, but the downside of that is that they are then relying on the Daikin data. I believe they are currently in consultation with Daikin to develop a Daikin compatible system, it will be interesting to see the outcome of that.

@mairia Yes, reliance on the Daikin MMI and thence the Onecta app does suggest Chinese Whispers are involved. Having said that, the data that Homely Connect shows does appear to have less rounding involved and does display totals of less than integer granularity. You will see in the Homely plot that they do state that the calculations may not be very accurate! On a typical day, I will see that Onecta thinks I have consumed 1 kWh over two hours but the Homely will plot this as perhaps ~600 Watt hours over that same period. I suspect that Havenwise may have to use the same source information unless there is an external energy monitoring device to supply the data. There may be a reader of this thread who can advise on this? Regards, Toodles.

@jamespa again really useful, thank you so much. Temperatures tomorrow are forecast for 5-6 degrees during the day, 8 degrees the day after so I'll look at COP over a few hours. At the moment my app shows outside temp 6, water temp 37 and the lounge is a comfortable 20.8. At the end of the day if a poorer COP is the price to be paid for keeping the house at the temperature I'm comfortable with then I don't mind accepting that. And definitely want it to be warm enough on very cold days. But maybe the air vents in the garage should be blocked! I don't think there's any way for me to see if the unit is cycling - any record I've seen of that on the Daikin users FB group is from 3rd party software.

@transparent I feel a lot of the information from the installer in their replies has been 'fudge' and doesn't really clarify anything for me, or indicate a willingness to help. That's such a useful thought about the air vents. The original gas boiler was in the garage so I presume they were needed then, but not now. Should I think of blocking them up myself, or wait until the whole situation is a bit clearer?

Yes, a gas boiler would be a legitimate reason for such air vents.

I don't know what their external face looks like, but I would recommend a temporary solution which can readily be removed if a 'fair test' should be required as part of a complaints process.

Since most walls are built with a 4-inch block, I'd cut a piece of 100mm-thick PIR insulation board to the right size and push it into the hole.
Then 'seal' around the edges with gaffer-tape.

But that's easy for me to say because I have odd pieces of Celotex and Kingspan insulation board lying around!
And also a special toothless saw to cut the board without making fibre-dust.

A whole sheet is at least 2.4m x 1.2m and costs £40+
But it's too big to fit in a car, and delivery charges for a single sheet might be prohibitive.

I see that B&Q stores hold stocks of smaller-sized boards,
which are proportionally expensive per cubic metre, but can be easily carried home!

That Recticel PIR board is only 50mm thick.
So you simply cut twice as many pieces and put 2 layers into each hole.

Can you manage that?

Ask if you'd like further clarification or advice on cutting.

Posted by: @transparent

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But it's too big to fit in a car, and delivery charges for a single sheet might be prohibitive.

If you go on Freecycle or look in local skips near building sites you can usually find offcuts of insulation board.  You need permission to take stuff from skips but it's unlikely people would refuse!

Posted by: @jamespa

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You need permission to take stuff from skips but it's unlikely people would refuse!

If they do refuse,
then they probably didn't intend it to be refuse in the first place 🤗 

@transparent Would that mean they were rubbishing your request?