LaChab

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,

I'm using this stuf to bond wooden battens onto PIR and concrete. Seems to work really well. Possibly cheaper from elsewhere.... https://www.amazon.co.uk/dp/B08NK8CHJV?ref_=ppx_hzsearch_conn_dt_b_fed_asin_title_1&th=1

@Nickfromwales I'd really like to take advantage of BUS, and I don't like the idea of using "straight" electricity to heat the hot water. A small heat pump is exactly what I'd like, but it seems impossible under BUS, so I think I'll be forced into at least 7kw. So using the UVC as a large (giant!) buffer seemed a good idea to reduce cycling. Agree completely about A2A, and we've had the pipework fitted for a multi split. Just daren't fit it until I get BUS sorted! Then in practice the HP should be almost entirely for producing hot water.

@JohnMo Thanks for all the suggestions. Hadn't thought about affecting the hot water temperature. Suppose I thought the heat pump would just be using the UVC as a large buffer, so it would always be within a few degrees. That's a very interesting suggestion about a "direct" UVC. I could have a tank with no heat pump coil and two output coils (middle and top) to provide heating and hot water. (prefer a coil to a PHE if there's a choice) Think I've got room for a 500l cylinder, so would be able to "carry over" quite a bit from the early morning charge up. Ordinary days 1kw should be enough.

The BH heatloss spreadsheet says that I should need well under 2k for space heating in our new build. We aren't having UFH, so I'm thinking about using just one or 2 kickspaces (fan coils) in the lounge / kitchen. To get over the low water content issue, I was wondering about buying a UVC with an extra output coil at the top to power the space heating. Effectively a thermal store. C2Go say no problem specifying one. Then the heat pump would simply keep the tank at the set temperature, and the space heating cirulation pump would be controlled by a room stat. Good idea? Daft? Pointless? All thoughts appreciated.

Personally, I just put DC Isolaters in the attic to terminate the cables from the panels. No MC4s needed. Then you can sort out the "on going" safely at any time. Similar to @JohnMo's approach I think. I find it hard to see "never switched" DC Isolaters as a fire risk. Less than an inverter I would have thought.

@Markuz Thanks. LOts of choices! TP are close so I'll give them a call.

@Nickfromwales Thanks, we'll give it a try.