Radian

Sure. Do you have a plant room with an outside wall?

That's a serious amount of condensation right there - I think mould has formed on the rafters. That needs treating. But this is a cold loft with vapour permable roof membrane. My understanding is that these don't require ventilation at eaves level, with sufficient evacuation taking place on the upper side of the membrane. The gap in the membrane at the apex doesn't look intentional given the sloppy bunching. Again, ridge vents aren't always required for permeable membrane but they seem to be provided by default with dry ridge. I'm spouting all this in the hope that someone will step in and correct me if I'm wrong.

I honestly don't like to think about it. I added the loop to an existing rad only system and all the advice was to have the rads power flushed before connection - but I hooked it up last November just when it went sub-zero and I decided not to take any risks with the main heating. Seems OK but I never quite get all the air out (open vented system). I was going to seal it this year but didn't get around to it. I always seem to end up in November when I have time to do big jobs like that - then chicken out for fear of leaving everyone shivering.

But you went and said it anyway didn't you. How's it been doing over the weekend?

A SE designed UB to pick up the ends of existing joists and carry the new 1.5m lengths on the other side?

It's so subjective. I used to build PC's with passive cooling chassis just to eliminate the last vestiges of noise that disturbed me when trying to think. Yet now we're in an energy crisis (of multiple makings) I seem to be fairly oblivious of the noise the indoor units make. Although they're very quiet and have multiple options for fan control) It's actually a sound I now find soothing as I'm being warmed for minimal cost. Air movement is probably the biggest difference that has to be reconciled but the units that have thermal occupancy sensors do a good job of keeping the airflow away from you. Placement is worth thinking about very carefully. The most comfortable area seems to be at the furthest point from the wall unit. Pretty obvious except it also seems the warmest (not quite so obvious).

I'm coming round to the idea of buffering for my radiators as well as our single UFH zone (I use a hare-brained scheme to use our HW cylinder to buffer the UFH which works like a charm). The use of TRV's always leaves the boiler short cycling for ages and no amount of tweaking setpoint and balancing rads seems to get round it. As with my UFH hack, any surplus water seems to get used by natural HW demand.

I expect he's too busy trying to get it all ready to move in by xmas.

Resistive heating is completely off the table now as far a I'm concerned. The only use for it in my reckoning is for cycle-by-cycle dumping of excess Solar PV. A2A now fulfils the roles I had for electric convectors although I've kept one as an overflow dump load in the workshop for when the HW immersion divert max's out. I doubt this will happen because in the summer the A/C units will be cooling at the hottest times of day. So's mine and it's been a nightmare heating it with gas. The A2A units are in an outbuilding that was recently extended and has cinema and hobby rooms. It's been tempting to shut down the house and move in over there since the 60m2 can be kept at 18oC all night and day for just 10 to 20 kWh whereas we've been sinking 150kWh of gas into the house on a couple of days this week. It's kind of odd really as the house has EPS beads sandwiched between lightweight concrete blocks and EPS sheet insulation under the floors. But it is kind of big. Even odder is the rental property we have - the tenants had me over the day before yesterday to look at some damp on their walls. I had a good look around and the house was very cosy (cosier than mine). It was built in the mid 80's - barely any insulation and a few badly fitting windows but not too shabby. I asked about how they heated it and the answer was just an hour first thing in the morning and three hours in the evening. Jeez - I wish! Mines on pretty much 24/7. Turns out the condensation was in one room they keep cooler for their kid who was complaining his bedroom was too hot!

The flow after the mixer is 43oC which is the minimum set point for my buffer so probably a bit less. It has been taking 6 or 7 hours to get the room comfortable since it went sub-zero recently. Incidentally, if you use the Flir tools App you can call up multiple spot readings retrospectively. Lousiest bit of software imaginable apart from that but that one facility is worth having.

It's easy spending other people's money (AKA being a government). But this one could be reasonably cost-neutral depending on the work for either approach being contracted out (time being money).

Dodged a bullet then 😆 'Twas the first one to come up when I googled 10:1 modulation. I knew a few do this but couldn't remember the makes.

You seem to have the right sort of ideas and are realistic about your options. One question springs to mind: have you considered stripping the roof(s) and re-laying them as warm roofs? This would simplify the ceiling as fitting a vapour barrier (and making it good) below the rafters is difficult. It would keep the MVHR happy (you already know the pitfalls in cold lofts) and it would be easier to make a continuous insulated envelope that connects with your EWI. Getting the detailing of this right with a cold loft would be a nightmare.

Vokera go on quite a bit about their 10:1 modulation The 30oC shouldn't be a problem for any boiler.

Do you have a Flir type imager? Post a photo. Even with spreaders it would still tend to show up hottest on top of the pipes but I'm guessing the temperature in between loops shouldn't be more than a few degrees lower. I only know what it looks like in 50mm screed and it's even to about 2.5oC

The only insulation product that isn't flammable that springs to mind is vermiculite. You can buy it as a solid board or loose in a sack If you got a 100L sack, that would fill a 0.5 x 0.5 m aperture to a depth of 0.4m for example, and you can pour it in after panelling over the hole.

I was surprised to see it actually maps the altitude of the terrain for your chosen location and azimuth. Doubt if it models next doors shed roof though.

I'm heating 60m2 over two floors with a couple of mini splits in an outbuilding and I really can't believe how little they cost to run. Keeping the space ticking-over at 18oC used just under 10kWh yesterday with an average outdoor temperature of -1.5oC Discovering that they can heat the space up by 5oC in about 20 minutes, I'm going to drop the temp to 15oC and use the App to boost before I want to use the rooms to save a few pennies. But yes, A2A has to be the best thing to run off batteries.

https://www.chimneysheep.co.uk/

We only started generating properly in late October and in November, our first full month, got nearly the same yield as predicted by PVGIS. That was nice to see. I don't know how PVGIS assesses cloud levels (if at all) but I'm guessing it must do something statistical for it to be so close to reality. It's been frustrating though waiting since March for the roof installation while the best ever summer for PV came and went. Fortunately I did get some from a bunch of panels propped against a fence to assist with the development of my home-brew PV diverter setup. That means we've been self-consuming all but 0.02% of our generation. Come next summer I'll have to hunt around for some dump loads that aren't heating devices. Our A/C units look like my best bet.

Are you sure the acuator pins are returning to their fully open position when you remove the actuators? (I'm assuming these are like radiator TRV's where sometimes the pins get stuck down in the closed position) Also you didn't mention spreader plates in your buildup - there has to be a way for the UFH pipes to conduct their heat to the floor surface. The way you describe it there's nothing around the pipes to do that.

No, what I mean is you're putting X Watts in and nature is taking X Watts out, and it's all happening with a mean inside/outside delta of 20oC If it the outside temperature dropped another degree and so did your inside temperature, that would be the biggest hint. It's unlikely to be this but you meet all sorts in this game.

-2 shouldn't be a big challenge but it sounds like it's reached thermal equilibrium. What's your floor build-up?

My experience is limited to using a one strike ready-mixed filler over the foam. The filler gives a little so is more resistant to cracking and adheres to foam very well. It's hard to tell how big an area you will be filling from your photo though.

And also "chimney sheep"