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I just noticed no-one actually tried to answer any of your questions. I'll give them a go.   I haven't quite done exactly what you plan, but I did handle the MVHR penetrations during the build and have run various additional cables etc over the years, through a very similar structure.   How did you detail the penetration through the OSB airtight layer? Cut a slightly undersize hole in a piece of membrane (thick plastic sheet or film is fine) and push the conduit or whatever through it. Seal the edges of the membrane to the OSB with airtightness tape. Cut the membrane square ven for round conduits - much easier to apply airtightness tape in a straight line!    Did you use proprietary airtight service grommets/collars or tapes around the pipe bundle? Generally not. They're expensive for what they are.   At different times I've used offcuts of EPDM and DPM I had lying around. I've also used neoprene insulation a couple of times for cables. For example, I used a disc of thick neoprene insulation to run satellite cables through a snorkel on the roof, where I didn't want to disconnect the cables. I cut the disc a little oversized for the inside diameter of the snorkel, drilled some undersized holes through it for the cables, then cut from the edge to each hole. Slid the cables into the holes through the cuts, then slid the disc along the cable into the end of the snorkel. Neoprene is closed cell, so it was perfectly airtight. It's not a bad approach for running cables.     Is it better to install a larger airtight sleeve through the wall and pass all the services through that? If I understand what you're saying, probably not. It's straightforward to seal a single cable, conduit, whatever, passing through a penetration. It's really hard to seal multiple cable etc passing through the same hole. We have one penetration like that, from the house into the attached garage, and it's a hideous nightmare that's probably responsible for half of the air leakage in the house. I suppose you could have one larger airtight seal over multiple holes in the OSB - if that's what you're saying then I think it'd work fine.   How was the sleeve/penetration dealt with at the breather membrane and external cement board? Same as on the inside, but use membrane similar to whatever is currently in place. You could use non-breathable if that's all you have (e.g., just repeat what you did inside), as long as it's resistant to damp and whatever UV it might be exposed to.   How did you deal with the condensate drain without creating an air path? We didn't have to do this, but consider whether you can run the copndensate drain to an internal drain. Alternatively, you could use a dry trap. We have one of those under our MVHR and it's worked fine for 11 years.    Would you pressure-test the house again afterwards to confirm the airtightness hasn't been compromised? I wouldn't. Just pay attention to the detailing as you go and it'll be fine.   On the underfloor cooling, we have a very similar Panasonic ASHP, probably an earlier model of what you have. It's very effective downstairs, but makes only a modest difference upstairs. Personally I'd consider installing aircon upstairs first, and seeing whether you can get away with mild underfloor cooling downstairs. It helps a lot if you have hard floors - tiles and concrete (we have the latter) are ideal. Not sure whether I'd bother if I had a lot of carpet.   With wood floors, I'd be extra careful about making sure the temp is below the dewpoint. We had an incredibly humid couple of days in one of the hotter periods earlier this year, and we had literal wet patches appear on the surface of our concrete floors over the course of one day. They disappear overnight. I think the floors were probably only a couple of degrees below the dewpoint. I'm not sure I'd have been happy having that with wood floors.

Braver man than me. Still if it's not given you any problems yet. That invisible one is shit. 

Hi   We are at Groundworks stage on our build we get a monthly report from our private building control company with any outstanding queries.   they have commented on Part O overheating to show functional compliance.   Ive tried researching this online and the requirements aren’t fully clear I can see some companies online create a report for a fee? Is this just the case of getting them to confirm the design complies.    we don’t have masses of large glazing and I understand you can’t site AC as a way of keeping the building cool.      

https://www.theguardian.com/business/2026/sep/11/speed-up-electricity-grid-upgrade-or-else-face-higher-bills-warns-uk-watchdog   Fingers need to be pulled out - but we knew that anyway.

We have an 11 year old Brink Excellent 400, and the criteria are similar (with the possible exception of the first one - I don't recall seeing anything like that in the manual).   I seem to recall that some of the settings are adjustable.