JohnMo

I doubt that, as you have a heat meter so will pay by the kWh.

But it is unlikely you would ever have to replace the whole contents. Thermostat is low down, the rest of the cylinder will be at a higher temperature. Bottom of our cylinder can read 12 at the thermostat, but it's actually 45 at the top of the cylinder. My average input to a 210L by immersion is 4.5kWh.

Our houses at a very stable temperature, but there are some days that it just feels cooler. We had the fire on yesterday. Small load to get it started, then one small log. Burn was about 3 hours on very low. Soap stone gave off heat until bedtime. Small log shed would keep us going for a couple of years.

Call cool energy they will open now. Have you tried switching the whole lot off and on?

I would just keep it simple One hot and one cold manifold in plant room. Then one of the two options below. Option 1. Looks like a 5 port manifold for hot and cold. A single 15mm feed from manifold to each wet room, in hot and cold. From 15mm pipe in wet rooms, teeing off to users - sink, toilet, shower etc. (this what I did and have seen no issues. 2. Bigger manifold in plant room, with individual (suitable sized pipes) to each user in the house. So a mix of 10 and 15mm.

That assumes your house almost no decrement delay. So you need to be careful with assumptions. In a well insulated house new or old house your weather compensation curve almost flat. Weather doesn't really change much in a hour, maybe drop or increase a couple of degrees. Plus the output of a floor modulates (thick or thin screed) depending on the dT between floor and room. So when room temperature drops floor output increases. You design your system for what you want, it's flexible, there are advantage and disadvantages to every system. No right or wrong answer. So testing done with our 100mm thick screed I have used pure WC and not had an issue, except expensive electricity outside ToU cheap periods. Batch charged the floor overnight and that works fine down to a just below zero (7 hrs charging and rest of the keeping house pretty stable), below that it needs a top up early evening. Running against a thermostat works, but you can overshoot and undershoot a lot with cheap thermostats and higher than ideal flow temperature. With thick screed you have to use 0.1 hysterisis thermostat. Certainly with a thick screed you have no need for a buffer as the floor is a huge one.

You may get a blank look from the window people, they are generally clueless. Do a Google search on here and you will find a few people using different ones. We have MVHR it has humidity sensing and boosts as needed. No idea when it last boosted, fairly steady temps and continuously running ventilation is key. Once the house settles down the background ventilation is mostly all that's needed. No really correct they are always partly open so room always gets some ventilation. No, size is irrelevant really it's the usage, it's there for background ventilation. You will have your cooker hood as well.

Less than half the volume of the house an hour, all very slow. I would really consider dMEV. Greenwood CV3 or CV2, they will only do background ventilation unless needed. They are silent out the box. One in each wet room (including kitchen). No trickle vents in wet rooms. Trickle vents in dry rooms with humidity activated vent control. Undercut all doors for easy cross flow with doors closed. 4 fans cost about £4 year to run. You only have enough ventilation, not over or under vented. No ducts, no filters.

If you have read that somewhere, they generally don't move heat about, the flow rates are too low. Oversizing to do that makes no sense, as although they recover heat, its not 100% efficient, so you are actually pumping out more heat than you should. MVHR is ventilation, not heating or a make shift heat mover.

No you don't, direct cylinders will have two immersion heaters, so you can heat all or part of the cylinder. For the price difference for a normal direct cylinder and sunamp, you could heat a normal cylinder for free for a decade.

Post up you roof buildup, what materials air gaps etc

Post a drawing of your extension/house

So what happens when you sell, if you "need" breathable paint. And the next owner covers with normal emulsion paint? What do you do with ceiling?

Think it needs a big dT, unlike most refrigerants. But you can get them from Mitsubishi I believe.

CO2

No wouldn't work. Trouble is without extracting to the main MVHR you don't have a well balanced system, so not that much energy to recover. So assume not on a new build? If your not super airtight (you may be) MVHR is pretty much a waste of time and you are better with on demand dMEV.

Are you talking 1 Barg or 1 Bara? (as in gauge or absolute).

Should be able too, as there are no breaks in the system.

Have you thought about using solar PV panels. Move so you have one side of roof facing east the other west. 2m2 of PV is about £50 to £60

why not use something like this - other makes are available https://www.ventilationland.co.uk/en_GB/p/brink-indoor-mixfan-co2-controlled-up-to-70-m3-h/17927/ You use the MVHR to dump air in to the hallway instead of the room directly. This fan is installed above the doorway or in wall and sucks air from room based on humidity and/or CO2 levels. Air is drawn into room via door undercut.

First is depth of screed. Then choose a screed. Look at product datasheets Here is a bunch of different materials, some useful most not so. https://help.iesve.com/ve2021/table_6_thermal_conductivity__specific_heat_capacity_and_density.htm

Our is caberboard with carpet off cuts. If you have any plumbing in there, anything not damaged by some water being split is fine. If your plant room is off the utility, just do it in the same as the utility It's a plant room

Who really cares about fast warm ups, prefer a steady and stable temperature, which dips a little at night. Dumped radiators to get away from swings in temperature. If you need fast warm up stay with radiator or install fan coils which are faster again.

Don't really like either. So looking at both layouts, you have loads of pipes transiting through hall all bunched up. Delete the dedicated loops and spread those pipes out. The transiting pipes will heat the hall for you. You could also do the same in the utility by using A7, 8 and 9 in the utility. That removed three dedicated loops, making everything a little more simple

Above sounds about right. Depending on thickness of screed you would need to fiddle with thermostat hysterisis. So get one that can go as low as 0.1 degs hysterisis. Then if your overshooting, undershooting, you can correct with hysterisis. Trying to run two systems with very differing inertia isn't easy. You either accept one is always wrong or put a controller to limit it.