LaChab

Which unit are you thinking of? (Google failed me!)

@Alan Ambrose I used this company. https://www.air2heat.co.uk/mcs-umbrella-scheme. As you can see, they're based in Oswestry but they were happy to travel down to North Somerset for me. They did a first visit to chat about the design, then a second one to "commision" the system. I had a few queries on the way (should have taken notes!) which we handled by 'phone and email. As I did all the work myself I'd already got everything running by the final visit, so there wasn't much for him to do. I was a bit concerned about not having the fan coil connected when testing, but just having a volumiser was acceptable. I would have considered choosing a Panasonic rather than the Samsung, but they have an indoor "hydro" unit which I wanted to avoid if possible. And the Samsung's widely available at very good price!

You pay the MCS umbrella scheme guy a fee up front to deal with the admin of getting the grant, then you get the work done either by yourself or using a plumber. The final commisioning is then done by the MCS guy. We only got one formal quote, which was a pretty excessive £16700 before grant, so I'm really pleased to have done it and made a small "profit".

It's probably not very relevant to you, but I've just self installed a 5kw Samsung Gen 7 with a 300l Oso using the "umbrella" scheme for an overall "profit" of about £1000. Free heat pump! If you do the course you can get a 7 year warranty for it. Our build sounds rather different to yours, but as we're in North Somerset you're welcome to visit for a look.

Probably OTT, but I ran a bead of foam down each batten as I fixed it.

We did option 3, 'cos it seemed easier to DiY. We cut slots in the battens for the horizontal cables. We used metric ICF so the panels matched exactly with the ties, but that probably doesn't matter much.

I had an idea that Panasonic used Oso cylinders, in which case it's possibly not a good idea to use any screws as Oso use vacuum insulation, and state "no penetrations".

If it's just a straight stair, then dead easy to make (as long as you've got a cordless drill! and a saw!) Couple of bits of 225 x 50 for the sides, short lengths of 50 x 25 batten to hold it all together, and decking for the treads. Mine supported 4 guys and a 150kg triple glazed window with no problem. Have a go!

@SimonD Thanks for the observations. Hadn't thought of utilising ing the "normal" valves. I was thinking of using end feed tees rather than a pre-made unit so the insulation would be less disturbed. Outside gives me access to a convenient hosepipe!

I feel rather tempted to add a few bits to one of the outside pipes from the heat pump to make up a fill & flush function. (28x15x28 tees, 15x3/4 service valves, 28 full bore valve) BUT I haven't seen it on any schematic, they always seem to be indoors, which doesn't seem as convenient. Ok idea? Bad idea? Is it likely to be a problem, particularly freezing? Thanks,

That's exactly how I built our 2 "garden rooms", except ours are 4.8 x 3.0, and I used Rockwool (not as good, but cheaper and easier). Been 2 years now and no problems at all. Have the osb on the inside and covered the outside with membrane and then featheredge, which seemed the cheapest solution.

+1 for Superlec, and internet-electrical are good for Fusebox CUs.

FWIW, I've used a 3 x 360 Huepar for a year and been very happy with it. I'm only a self builder though!

@JohnMo Thanks for the thoughts. No UFH (sorry!) or radiators. I'm hoping that over time the house will mostly be at the same temperature. Not planning cooling with the heat pump. I thought this KS might be ok. Says it goes down to 30c. And I can always disable the flow temperate relay! https://kitchenheaters.co.uk/product/thermix-kph-1-5-smart-wifi-heat-pump/

So that I can get the wiring installed, I'd really appreciate some guidance with sorting out the controls for my future heating system. It's a fairly well insulated and airtight new build, and the spreadsheet shows I need < 2kw. I'm planning to fit a heat pump, with 1 or 2 kick space heaters (fan coils) in the lounge / kitchen and a towel radiator in each of the bathrooms upstairs. I was thinking I could have 2 zones, one with a (portable) room stat for the kick space, and the other with smart TRVs that can switch the heat pump. I've spent ages on the Tado website, but still not sure that it could do what I want. There must be others............. Thanks for looking,