ProDave

That's an in line fan. You need to oncouple the ducts both ends and work out if it is the fan itself that has failed or something in duct catching on the fab blades.

A 2 port motorised valve and a room thermostat wired or wireless, but you will need a power supply to that area for it.

Shame you have to buy the whole lot just to replace the broken / missing bits but that would be your easiest solution.

I suspect that thermostat knob is supposed to connect to the grey bit on the left hand pipe, looks like a TRV for the UFH? Not seen one like that before but if it is supposed the be attached but is not it could explain why not working. Will it just clip on like a radiator TRV does? What was the thermostat in the original post that appeared to be mounded on a wall?

Yes definitely get a static caravan as large as you can find. Then spend the time and effort to add external insulation and clad it if you want it to be warmer and look nicer. It will comply with all the rules it has to and be fit for habitation. I don't know the law in Ireland, but in the UK a static caravan comes under a different set of rules to building control. Assuming you have planning for the house build, it should be easy to get permission for the static and do it legally. Alternatively as you are building a house, why not design it so you can build one wing of it first and kit that out to live in while you build the rest? That way all your work is permanent and gets you to a finished house quicker. You can think of outbuildings for holiday use after the main house is built, but do them properly.

I am not sure what you are measuring there, your 2(n) on everything confuses me? You want to measure terminal 10 on the wiring centre or terminal 1 on the boiler, with respect to neutral which can be found on terminal 2 on the wiring centre. This is the call for heat to the boiler that should be on all the time HW, CH or both is demanded. This is the one I think you will find is pulsing on and off. Verify that first. And testing on someone elses system won't get you closer to finding what your system is doing.

The obvious thing from the photos is that thermostatic valve is turned down to minimum. Turn it up to max and tell us more about what you find. When the system is running and the radiators are hot, do any of the grey pipes get hot? And remember UFH works slowly especially when turned on from cold, it is likely to be a couple of hours at least before you feel any warmth in the floor.

Tarmac you can drive on straight away even if still hot.

This is a good description of how the 3 port valve works http://wiki.diyfaq.org.uk/index.php/Three_port_mid_position_valve I still suspect yours is overshooting and not reaching a stable mid position "In the "water and heating" state, white is energised. If the valve has port B open, the motor will wind it until A+B, whereupon SW1 changes over, DC is applied to the motor via SW2, and it will stall. If it overshoots, or if it port A was open, SW2 will be operated as well, removing all power from the motor, and allowing the spring to pull the valve back to A+B. It is fun to watch this happening: as the spring pulls the valve back from A open to A+B, the motor acquires quite a momentum and overshoots. It then winds forward a little, and stops in the correct position." Check the voltage coming out of the orange wire. (boiler demand) I bet you will find it is pulsing on and off.

Or just wait until it next floods and it will be obvious which bit is sticking out of the flood water = that bit is too high.

Y plan = 3 port motorised valve. the work of the devil. I bet of you go and look when it is selected for DHW and HW, the 3 port valve has not settled in the mid position but is hunting backwards and forwards, and in the process the call for heat to the boiler is turning on and off. Replace valve control head or bin the 3 port valve and use two 2 port valves instead, FAR more reliable.

We only have the one triangular window in our sun room. It has no blind and never will. I would be very interested so see pictures or a video of any bespoke blind for this sort of window in use, ours have a 45 degree slope and I just cannot imagine any sort of blind or curtain that would actually work. But we knew this at design stage and knew there would be no blinds needed on this window.

That is more or less what I did with my electricity connection.

Looking at that drawing above. What height difference if any is there between the treatment plant and the soakaway? Can you add to that drawing or produce another one showing what happens at the "output" of the soakaway, including levels of important features.

Two points here. The 2 houses feeding this with septic tanks should not do that any more. But sadly SEPA don't seem to care about old non compliant systems. So on the face of it you have a soakaway and rubble drain acting as a partial soakaway and ending up in a burn. So in theory this should never back up to the point of flooding the treatment plant. So something is wrong with the levels when it was installed that is causing the plant to flood, or something is blocked. THAT is what you need to concentrate on finding, what is the weak link in this that is making it possible to back up. Probably an unpleasant job of paddling about in wellies next time it does back up to work out where the problem is. It surely should not be hard to sort out when you have found it. With an eventual destination of the burn, it should not normally back up.

Fred Dibnah was on television several times filling his boiler from them and telling us it was illegal to do so. I wonder how many times he got fined?

Can you post an actual model number of the thermostats, there must be something on them somewhere?

A friend of mine bought a house like that. He had a survey during the buying process and the surveyor warned the corner of the landing like that was cantilevered from the rest of the building and was a structural defect. My friend of course answered, well it has not fallen down or even moved in the last 60 years so I think it is okay.

It sounds like an RF interference issue, or the non working stat has become faulty. Does the manual say what frequency they operate on?

Yes I too fitted a pressure reducing valve to protect the (not fit for purpose) lousy plastic pipes and fittings used on my 'van.

That has narrowed it down. Are the wireless stats mains or battery powered? If battery can you physically move it closer to the WB9? Have you checked the state of the battery? how far away is it? Any thick walls in the way? Any NEW equipment added like new wifi router?

Replace with your own system assumes you have enough land to do that. Often a shared system is because you don't, especially if they have bodged the soakaway. I am hoping @BigYin will come back and tell us more please.

If it is condensation, then I would expect it to have dried out completely over the summer as the walls will not be cold enough to be below the dew point. Do NOT invite a "damp specialist" around. I have yet to hear a good outcome from that.

How does that work? Do they fit a separate meter for the heat pump? It sounds like hark back to the "total control" tariff that gave cheap rate 24/7 for heating appliances but that had it's own meter for that.

Can you clarify is the floating corner the bottom string going down to the ground, or the upper string going upstairs? If the downstairs one as the drawing suggests to me, then the dod leg stringer joint will be in compression so the hidden fixing might be less difficult, but that will rely on the bottom of the stringer being properly fixed to the floor so it can't move and spread outwards.