ProDave

A Sunny boy will report on it's display the string voltage and string current. Next Sunny day you get, record those peak values that you see for each string.

Start by fitting an accumulator, that will maintain the 2 bar pressure until you have emptied the accumulator. A pump would need a break tank and gets messy. you can't just suck water out of the mains pipe with a pump.

Run the towel rail direct off the 1 pipe system, and just the UFH off the buffer with my idea of a 2 port valve and tank stat to limit the temperature in the buffer. Then for now you don't need a manifold and blending valve. You can add a proper manifold and blending valve when your next room is refurbished and ready to go, some time about 2025?

This is only temporary. @Onoff definition of temporary may be a bit longer than mine.

Surely only if it is designed as an unvented cylinder. A header tank, even without a ballcock would seem better. Just keep an eye on the level if you do that.

Agreed it needs a header tank.

I don't like the stairs pushed off to one side, they would be better in the central entrance hall. Definitely needs a door direct into the kitchen from the parking area.

Better would be a 2 port on the input to the tank, driven by a tank stat so it only heats the buffer to the required temperature. Normal "1 pipe" system bypass when the 2 port is closed.

You don't need to go "off grid" Just do as I am doing and fit your own system. It does mean anything you export you do not get paid for, so just do all you can to self use as much as possible. That will almost certainly mean battery storage at some future date.

So the TOP is open (i.e nothing to fix to) Are BOTH ends attached to something, or is one end floating with nothing to fix to?

I would be building that of 4 by 2 minimum with 2 straight runs across the top. Might also line one side with OSB before the plasterboard.

Photo of the manifold so we can understand which valve was the issue?

That sounds like a good plan. You will need a header tank would that go in the loft or just above the cylinder?

Well I now have the Unistrut. Could not better £24 per 6M length locally. Transporting that on the roof of the car was interesting. Good job the cheapest supplier was also the closest, and has a handy back road route so I could drive slowly without being a nuisance.

What do the flow meters show? They should show water is actually flowing through all the loops. If return temp = supply temp then don't discount the slab being up to full temperature.