JohnMo

Why not look at a cylinder with heat pump built in. Needs an internal and external air supply, something like a Dimplex Edel (other makes available) Will use a 1/3 of the electric you use now. Instant water heater will use peak rate electricity so you don't want to go there.

It is big, keeping me busy. Roof overhangs most of it. Unfortunately as the sun moves west it still gets in during the summer. Structural work just about finished. Bitumen paint coating on top of the structural wood next week, if it's dry enough, then a big stack (210) of composite decking boards to install.

Why? Not in contact with soil, well ventilated, treated for external use. Top of wood will be further treated. Our 70s (sold in 2012) house had a wood balcony and 40 years later I had to repair a decking board.

I have just ordered glass balastrade, 26.5m long, that's is just under £9k that will give you a starter for ten. Photo of our planning drawing. Uprights are basically steel at top and bottom, sandwiched between wooden upright. Then lots of wood to build the structure.

I would put the cylinder on a stool, and infill the stool with insulation. Cheap and easy to do. My cylinder. Made pads/cushions out of leftover airtightness vapor control membrane and left over glass wool. Boxed in piping, heat exchanger with leftover PIR. Mine is on the floor, but the floor/ceiling has 100mm of insulation in it.

Put it on that roof, but route the power to your house?

Our floor was power floated, it's not what you would want as a finished floor. You certainly would have any hard skin on your feet if walked about without socks on. Make sure your contractor is aware he is doing a finish floor, power float and finished floor are not the same.

You may need to drop a size or two, plus a buffer. My boiler turns down to 6.7kW feeding a max of 3kW and I have a 180l buffer to stop it short cycling.

normal cement, and hydrated lime. I mixed mine as 1x 25kg bag of cement, 3x 25kg bags of building sand, and think it was about 2 to 3 shovels of lime. The lime gives a creamy consistency. Add enough water to make a double cream. You need to play with the mix slightly. Makes a mess, wear safety glasses.

Strangely you say a free port, but all ports have pipes connected. The one without an actuator/ manual valve has pipes connected? The two in parallel could just be 2x units, one duty the other standby.

If it for a holiday let, why put such a big array on the roof. Put 1kW or so on the roof, then most would be self consumed, with no effort and not worry about export.

You could that way. Or fill with some concrete/ cement once positioned.

My pipes are at the bottom of a 100mm concrete slab, directly stapled in to the insulation. So an over pout could be done, but you don't need to. Where you want your shower tray form a void so the concrete is missing. Make a plug out of 50mm foam or PIR slightly bigger than the size you want your tray, position exactly (some wiggle room as you are making plug oversize). Wire down to where your UFH pipes wil be. Do pour and after a day or month or two break out the plug and you have a void for your shower. Make sure you have space and have for drainage.

Do you need three phase? sounds like you don't and only got because it wasn't that expensive - are making things way more complicated than they need to be?

+1 as above

Why not, just take the wall out between boot and plant room? Russdl beat me to it

Big buffer will sort it out. As said upping flow temp will help. Your cylinder, assuming you have one, will have coil in it designed for flows in excess of 70 deg. If the flow temp is below that, the coil cannot transfer enough heat in to the cylinder, so the boiler will trip on excess return temp. Then keep repeating until cylinder thermostat is satisfied. CH, if you have UFH you could batch charge the floor, so run the flow temp a couple of degrees hotter for 4 or 5 hours, then timer the flow off. Or fit a something like a 100 litre buffer (you would need to size it correctly).

Our door is made a similar way, I think the insulation is 15mm PIR and has a certified U value of 1.0

The crawl space looks like a 70s house we owned, I looked and worked on something else and never went back (that was 20 years ago). No-one can see it, so who cares if it looks pretty said the trades men.

Are you eligible if you live abroad, I assume your also not a UK tax payer? From what I have read, would make you NOT eligible. "Who’s eligible for funding? You could be eligible if you fall into the below criteria: Resident in England or Wales"

So you have loads of solar, and Aircon for cooling and heat. Why is this a reason to have a sunamp? No logic to join the decision process. Again no joined up logic, what has a hot water storage got to do with dormers. If you want a sunamp, just say you want a sunamp. Nothing to do with anyone else. Your money your choice, wouldn't be my choice, but it's your money.

Just open a window a bit. I have no issues with the recirc unit. Only have it on if frying which isn't very often. MVHR looks after the rest on boost.

Or don't fry

We have all vaulted ceiling and used pre made standard size doors. They don't look odd or small. You may be other thinking. A previous house we renovated needed new doors and every one was a different width and height, cost a small fortune to replace, as each was made to order.

A bit of a brain dump The structural design of our decking, (part of my structural design certificate) with a big fall in some areas (7m drop), has 4x M16 bolts/stubs at 8.8 strength rating at each metal to wood interface. All wood is C24 with external preservative treatment. External framing is 220x45 all doubled up. All joists are on hangers. Long joists are 220x45 C24 and the shorter ones 170x45 C24. Balastrade design ratings depend on how far you can fall. But is all rated at 0.75 (forget the units) on ours.