ProDave

That switch has a reed switch. It would not be my first choice. Though it does say it is rated for 3A which ought to be enough, and the usual trouble with reed switches if you overload them is the contacts weld shut.

Yes to support the pug mix and to fix the UFH pipes to.

You should not be dropping 20V across the switch. I suspect a wiring / measuring error somewhere. Especially as you are getting similar volt drop with the new switch. Post some pictures of where you are taking your measurements and with what. Is it a real flow switch (I have seen some strange setups with a thermostat acting as a flow switch)

Do you really mean a 10M wide side extension? That is wider than most houses?

I used 300mm I joists. 300mm Frametherm 35 between the joists. Then air tight membrane taped and sealed to the building walls. 12mm OSB. Battens following line of joists. UFH pipes between battens, dry pug mix for heat dispersal, then finished floor (engineered Oak) Design joists for the extra dead load of the pug mix and allow for the height build up.

What type of insulation is it (link?)

I read the issue as lack of sufficiently talented people. He might be a brilliant joiner, but to design it such that to replace a switch you have to remove the whole think is just lousy design. I am sure you electrician could have designed the location of the switches better than that, but he is not a joiner and not making the cabinet. And this is the problem when you ask 2 different trades to contribute to one finished product. When there is a problem, they will each blame the other and you will get no satisfactory resolution. I know on this job your expectations and standards are high, there is nothing wrong with that, and sadly it seems they have been unable to meet those standards. The only way to determine if it is an installation problem (the "bodged" joint) or an equipment problem (either a dodgy switch or a dodgy LED strip) is by substitution. I bet nobody is offering to do that? In a perfect world you would have been able to get the cabinet, with lights fitted, supplied and installed by one person with a performance guarantee on the end result. And there is you lack of skilled people because nobody would offer that. I am in fact doing this with a neighbour. He has a brilliant German cabinet maker building the shelving unit and I am installing the lighting. We are of course testing the lights are up to expectation before installing them.

I am not convinced the issue is the soldered connections instead of a plug in connector. Before installing and connecting the strips I would have given them a dry run on their own to make sure the light they give and the startup characteristics are to your satisfaction before fitting them into the cabinet.

The teeth on that one look a bit coarse to me for cutting metal.

You are right to check. Planning means you have a legal right to build a house there. It does not mean it is actually physically possible to do so, or that you can connect to all services to make it habitable. So as well as ground conditions, check you can get at least water and electricity, and if it is a rural plot away from mains drainage, then you will need a treatment plant and some form of soakaway which may require a lot of space. And check the site measurements. It would not be first time a plot turned out to be smaller than on the approved plans and the approved house would not fit on the actual plot.

It sounds more likely the air bladder in the expansion vessel has failed and on the higher heat cycle it has ran out of expansion capacity and the over pressure valve has opened.

If you hit your head it is too low, no excuse for that with a 3M ceiling height.

Already been invented. It's called a "hot tub"

Your 2 options are a good steady hand and a good paint brush, or a good quality masking tape such as Frog Tape.

If you have the digger there, dig a 1 cubic metre hole in the ground, and then in the bottom of that dig a 300mm cube hole. Fill that small hole with water and time how long it takes for the water to drain away.

I had quite a "discussion" with SEPA when I registered the TP on our new build, in fact I actually raised a complaint. The gist of my complaint was I paid what I thought was a high fee just to enter some VERY basic details on a website and immediately got my registration number. They NEVER asked for any details about the treatment plant whatsoever. During this discussion I raised the question as to why there are at least 2 septic tanks discharging into a watercourse near me, and surely now they are registered (both houses recently sold) they should be enforcing their upgrade to a TP. The reply was we only do this in individual cases and only if there is a reported pollution incident.

If I am reading this right, the ONLY room thermostat is in the hall which is often a cold a draughty place. What is the room temperature in the living rooms? I take it there are no individual room thermostats as is sometimes common with UFH. My point being if the rooms are warm enough, and it is just the silly placing of the only thermostat that is the problem then you need to move the thermostat. You can get wireless thermostats if re routing the wiring is too disruptive.

^^^ ALL that is saying is a TP can discharge to a watercourse, but a ST can't so must drain to a soakaway. If you have no watercourse available it will need a soakaway. In Scotland SEPA are less keen on discharge to a watercourse and only allow it if no suitable land drainage option is available. Yet SEPA are not enforcing existing septic tanks discharging to a watercourse to be upgraded

I would organise a percolation test. Only then can you calculate the area of land required, and then with that, and the limiting distances to buildings etc, work out just where it can actually fit on the plot. Our previous house required 85 square metres of soakaway.

This should be checked before you bought the plot. You WILL need a drainage field. And depending on the percolation test results it might end up using a lot of land. I do hope you have enough land to accommodate it, or an agreement with a neighbouring land owner for the drainage field to be under their land. It is perfectly possible to get planning permission for a house you cannot actually build or cannot occupy if you cannot get a drainage solution.

That will be down to the amount and decrement delay of the insulation. Our house is all timber, plenty of insulation but of a type with a slow decrement delay. The house only ever heats up or cools down very slowly. That is not because it has lots of "mass", it doesn't, but because the type of insulation.

With a properly air tight house, very little draught enters the house when you open just one window or door.

Perhaps the rogue current your smart switch draws reduces once it's storage capacitor is charged, so it is only an issue when the switch is first connected from new?

Is this your no neutral smart switch?

In many ways your downstairs layout is similar to ours. I suggest you make the living room just a little bigger (shrink the plant room which rarely needs to be large) and then have double doors into the living room directly opposite the double doors into the kitchen / diner. Ours are double glass doors and we like being able to have all the doors open making the whole downstairs feel like one big open space, or closing them when we want it to be snug. Edited to add a picture looking from the kitchen / diner, across the hall and through into the living room