Installer Fitted 9kW Instead of 11kW Heat Pump and Changed MCS Paperwork - What do I do?

MairiA · 2025-11-03T15:10:14Z

Hi, I'm hoping to get some guidance as to how to proceed with a heat pump issue. My ASHP was installed 2 and a half years ago. Recently I realised that rather than the 11kW unit discussed with me and quoted for, a 9kW unit was in fact installed! The MCS certificate issued after installation was for an 11kW unit. I feel a complete idiot for not picking up on this before, so please be kind. My SCOP isn’t great, and in the coldest of days in the first 2 winters the unit has struggled to heat the house adequately. I changed to a different company earlier this year for servicing as I was unhappy with customer service/communication from the installer. I mentioned the performance issues to the new company and they suggested getting the heat loss calculation from the installer and reviewing things this winter. The heat loss calculation then provided by the installer was not the original, but a recalculation, as the installer’s ‘process had changed’, and I had to provide radiator sizes for them! Design temperatures seem low at 20 for the lounge and 18 for all other rooms, they assure me these are ‘target set-points based on industry standards’. I didn’t notice at the time but they also sent me an MCS certificate ‘version 3’ which now shows a 9kW unit. I thought the MCS certificates were issued by MCS, but I now assume they are actually issued (and can be revised) by the installer? The company have been glacially slow to reply to my questions about this over the past 2 months and are insisting that it is just a simple failure to communicate a planned change in size of ASHP following heat loss calculation. They didn't comment on the issues reaching a comfortable room temperature, and suggested, without justification, that the poor SCOP was due to me changing settings! I don’t know if they are incompetent or dishonest or a bit of both. I asked about it on the Daikin Altherma Help and advice FB group, consensus was that the ASHP is likely large enough - if I'm reading the heat loss document correctly heat loss is 6.7kW - the house is a 4 bed 35 year old villa with loft and cavity wall insulation. I'm now suspecting that the radiators in the lounge and hall may be undersized rather than the heat pump, they were relatively new and not changed in the installation. I think I just want to cut my losses with the installer, and am prepared for some expense to get the system performing well - possibly ask my new service company to redo the heat loss calculation and assess the system. I did think I might raise a Subject Access Request with the installer just to see if their records match with what they have told me. I would really appreciate any thoughts on how best to proceed.

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Mars

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14/11/2025 8:47 am

@mairia I think your case is going to be very interesting. I did my own MCS data/info request pertaining to our own system and I’ve been gobsmacked by MCS’s replies to me yesterday. I am extremely interested to hear what MCS tell you.

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MairiA

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Topic starter 14/11/2025 9:57 am

@transparent yes I'm sure I could do that. I can cut it outside and wear a mask so hopefully no concern about dust. I had a Google about ventilation in garages. The other concerns about no ventilation seem to be condensation in an unheated garage, and car fumes. The door is an insulated one but there are inevitably gaps so the garage is definitely not airtight. And my only car is an EV which I rarely put in the garage as although a 'double' garage it has a wall down the centre and the only side big enough for a car is pretty tight for a gen 2 Leaf. I'm happy to fit the insulation and monitor things, and as you say it can be readily removed.

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