Just an update to this:

Went away from the Eco 4 for complex and frustrating reasons. I suspect in the long run we'll be happy as the scheme seems more and more shocking the more you look into it, but it has delayed the house being ready.

We're now using another company, and their engineer came out today to measure up for the heat loss calcs. Everything looks better already, he was definitely more knowledgable and didn't just scribble the measurements on the back of an old piece of paper! They fit Valliant ASHP, and I appreciated the reasons he gave for choosing them over other companies. We discussed the design, and overall I was happy. He said they have to design for 45c flow temp which isn't what I want but is certainly better than the 50-55 we would have got through the eco 4. That is also based on a temp of 18-22 whereas we tend to keep the house cooler than this overall, and the woodstove will help the system run lower again. We decided not to have a manifold as space for pipework is at a premium and obviously a manifold would add to this. I think he was right in saying that a small house like ours would be easy to balance. He was reticent to not fit TRVs, if necessary I presume I can just go back and either take these out. From the looks of it it is a requirement for trvs to be fitted to every rad in a new wet system. Why would the effect not be the same if the TRVs are open fully all the time? He didn't want to fit 2 zones which is good news, and whilst he said they would be putting in some sort of buffer he was happy to put in duel flow valves for balancing the flow. He was also fully onboard with WC, and was happy to only fit the original controller and use that for the house thermostat instead of any seperates. One thing I can't quite decide on - Currently the house has single glazing although this will be upgraded in the summer. If we did the heat loss calcs based on the single glazing we would obviously get bigger heat emitters, would could presumably enable the system to run lower temps when the double glazing is in. The reason I'm not sure about doing this is that it would be more expensive for the rads, and there is a chance that the rads would be oversized at that point. He said that because the house is a couple of miles from the sea the calculations require a 10% expansion on the heat calc, although presumably if the house is 10% better than the number he comes out with the system can run lower. I'm in the middle of doing my own calcs to compare to his. I suspect that we'll end up with a 7kw, which seems like the right size for the house. I'll post my calculations when I have them.

Follow on questions are:

I don't quite understand the chat about 'does the heat pump control the circulating pump flow rate based on load'. What does this mean and what do I have to ask the designer about this.

I think we'll end up with a sunamp heat battery and pv diverter in the system which I'm happy about. I'm hoping that this will allow me to minimise the time the system spends heating up the DHW. I'm also looking into defrost cycles and how to make this as efficient as possible.

Incidentally, with regards to TRVS, I've been reading this paper.

NGL I don't understand much of it! One thing it seems to be saying though is setting the reference rad with no TRV carefully. Would I be best placed if this was in the lounge where the wood stove is (which would presumably confuse the system), a rad in the connected kitchen (which is further away from the woodstove) or the hall, which is presumably colder and tbh a room I don't mind being a bit colder.

 Nothing wrong with 45 degree flow, that will be at the design outside temperature, which in your area might well be -2 or -5. After all, there needs to be the capacity there to heat the house when it gets freezing out.

I designed our system for 45@-4, and it’s been running at that this week. House is nice and warm and I’m not bankrupt.

I would get the windows done asap… double glazing really is so cheap these days, and even bargain stuff is 1.4Uw, it’ll make a massive difference.

@hughf Thanks, ODT is -4, which doesn't happen much around here except at night.

Waiting for the Home Energy Scotland loan to come through then the windows will be whipped out (and whipped back in again).

@jwilliams89 sensible…. We’ve just completed an upgrade to triple glazing, it has made a big difference. Mainly because I supervised the measurement and installation of the new windows. The previous double glazing was installed by a bunch of cack handed idiots who couldn’t read a tape measure.

@hughf I thought about triple, but for me it makes more sense to replace only the units and not the frames as these are already upvc and getting someone in to rip them out and put new ones in would lead to damage to the (concrete) walls as they can be tricky to work with. I've also already pumped foam in around the frames. These had been in for years and behind the fascia was some pretty dodgy holes. I'm dreaming about italian style shutters for the winter, but these are way down the renovation road....

@jwilliams89 ah, ok you’re changing the sealed units. Gotcha. It might be possible, with a simple change of glazing bead to go for triple, In the existing profile.

Many standard 70mm profiles will take two thicknesses of glazing with just a change in bead.

@hughf I'll check but I think the frames are too old and too narrow. You say you think the triple has made a difference?