Hi, I don't want to hijack the other user's post so replying here. My understanding is that in order to be mcs registered an installer has to be in a technical oversight scheme, of which there are several including NAPIT and NICEIC. They assess installations based on 3 documents:mcs standards, building regs and one other. It's a different section to the electrical installation inspectors. I had an electrical inspector come and look at the heat pump electrics as part of the electrician's yearly check. Then I had the heat pump inspector come as the result of my complaint.

To be mcs certified, installers also have to be in a consumer code: RECCS or HIES. I think they're the only two.

Call handlers at mcs advised me to save time by going directly to my installer's technical body with my complaint instead of being triaged through them. Also advised to get the technical stuff sorted first and then contact consumer code for any issues around paperwork or customer service.

Thanks for your offer about sound proofing. Here's a photo of the tank in the loft above the bedroom. As far as I can see, it's sitting on two pieces of fibreboard flooring on top of two or three 8" x 2" timbers laid flat over joists, close to but not above brick party wall. Various noises carrying down but the one I can't handle is the high pitched one. I think it's a pump integrated into the cylinder. Installer's came back with a piece of sound insulation to slip under the tank but gave up after draining it because fitting it would have involved disconnecting pipes. The inspector has asked them to provide evidence that the structure is sufficient to support weight of tank, confirm how pipework is supported in loft and sort out safe access for maintenance as currently no lighting up there and if you do go up, you have to stand on bits of beam to avoid treading on pipework or coming through ceiling.

Other noise is a mixture of the noise of the unit coming through the wall and windows downstairs - it only seems to run full pelt. This is potentially exacerbated by the fact that the flow and return pipes are not insulated or sleeved going though wall ( picked up on niceic report) and possibly that they are then slightly chased in downstairs behind dot and dab and behind a stud upstairs fixed with minimal plastic fixings. And there's a big three column tall radiator in front of the stud.  Certainly upstairs in the bathroom and bedroom, I can hear reverberation of unit noise rather than the unit (unless I open the window). Lustening to the heat pump reheating the water while you have a bath is not relaxing. Inspector has asked them to confirm if enclosed pipes are insulated. They said yes during inspection but I know they aren't. In fact I was extremely worried because my builders put drycoat renovating plaster directly over the bottom half of the pipes before boarding. I was reassured by all concerned that this was fine but not convinced.