JohnMo

Apart from the drawing being wrong - nothing much wrong. I would do - cold water main to PRV, then the check valve, (long bit), flexible, then the short bit. Not sure I would be filling the heating to 3 bar though. Mine currently sits at 0.5 bar.

Easy enough if you plan everything ahead, can get complex and expensive if you don't. Or I just paid someone else to do it, it was fine. Or Never been so knackered, everyone asks you a 101 questions every day. Or I'm broke, house isn't finished, can I have another pint, before I go back to do another 4 hrs, loads to do before the contractors get back tomorrow.

The manufacturers website doesn't even include that detail either.

Your system is reacting the same way as an oversized boiler. The reason it does that, is similar to a kettle, you put a cup of water in it it boils in seconds, fill it it takes an age to heat up. It sounds like you simply don't have enough water volume engaged. Then you have a mixer, this will always mix some of the return water with incoming water, and it doesn't make any different temperatures you set them at they always bypass some return water back to incoming water. If your loops are doing 3l/min, your flow through the Willis is nearer 2l/min. Which is a trickle. The other thing is target flow temp goes out the window with cycling as the target is based on average flow temp. Example, target of 30 degs, is fine if running heat input for hours at a time, but if you are cycling every couple of mins, it's more like 20 then 30, so now your average is nearer 25. So you have to set the target higher to compensate. I would do this first - keep all loops open or closed so run a single circuit. So one control thermostat connected to all loops. And - Crank up the loop flow rate to get more water moved about. Flow each loop at max flow, if making any noise trim them back a little. Then if it's still not ok Delete the mixer, add a pipe stat, for floor protection, to the hot pipe going into manifold. Then finally, add a buffer or volumiser to system.

I've bought DeWalt and Matika, 230V stuff, all good no issues. A Titan SDS from Screwfix, used it plenty further build, no issues.

That is a normal approach, cheap pipe stat.

Your best way to heat almost any house is low and slow, just keep the house heating ticking over. Heat one room, you are really trying to heat the whole house via one room. The external house walls leak heat more slowly than internal walls and doors. So you have one radiator on it runs balls out trying to make up for the heat slipping through the internal walls to adjacent rooms. By low and slow is more about room temps. The normal way is via short time slots, every time to switch heating on, you have to heat the building fabric, all the metal in the heating system etc, so you over compensate with higher room temps. As cold walls suck the heat from you, heavy radiator driven air currents drag cold air about etc. Leaving heating on for long periods with small nightly setbacks, allows the whole house temperature to stabilise, boiler moves to a more tick over mode than flat out for short periods. My sister did exactly the same as you are trying, I convinced to try longer run times with no zoning, the house became more comfortable, rooms that didn't heat well, all became comfortable, heating cost hardly changed.

You can easily heat individual room with enough zone valves or radiator valves. But you get to the point that boiler cannot turn down output far enough and you get short cycling - basically burning loads of gas for little gain. That just means remote access, so nothing smart just an on off device with internet access.

An ASHP feeding UFH will only add heat slowly as flow temp rise is capped by dT between flow and return. If I set mine to batch charge - 35 Deg target point, it will take several hours of full load run to get anywhere that target. So if you set timed windows of one hour for DHW, you will have zero issue, if the 3 port diverter failed. Only cold water coming out the tap.

Can't it - heat loss due to internal 'waterfalling' at pipe connections (tiny little thermal loops where warm water goes into pipe, cools and then falls back into tank, bringing more hot water into pipe). Sounds like a thermosyphon to me. Yes your pipe stays nice and warm at the expense of continuous heat loss, even with good insulation. How many if any seconds it saves you? Just had a shower secondary circulation off, cylinder 20m away from shower, in time it took to take clothes off, I walked straight into warm shower. I will keep the losses in check.

It's a lockable valve. A normal service valve or a motorised valve isn't. @Nickfromwales just beat me to it.

So you need to cut all this back. I would get a suitably rated neoprene flashing/ grommet to tape to your membrane that will get you airtightness back. You may need to add a metal trim plate to get the plasterboard tidy. The insulation around the flue replace with Rockwool as it's classed as fireproof. Warmcell has a fire rating of around E/F, Rockwool is A1. So you may need to box this part of wall in to keep the insulation from mixing.

1 question if you are doing the UVC yourself, where are you getting the sign off certificate from? Assume you are not qualified, as you would ask the questions. Hot water outlet pipe, I would drop pipe downwards at cylinder edge for about 150mm to stop thermosiphon. Your grey and white expansion vessel is a maintained item, so you need access to the Schrader valves to pressurise - do you have access in that location? Do you need one at all, certainly if you have secondary circulation loop you will have issues installing one with out carefully planning. Yes they are approved. You water less on MVHR also. Ideally needs to be on the return line, but an isolating valve or similar device should NOT be installed between the boiler/heat pump and expansion vessel which could isolate the expansion vessel from the system.

It's a bit more than that, physically they have different yield and tensile strength and different corrosion resistance. A4 being better than A2.

So just do long cables, with no breaks between panels and inverter.