SimonD

What's the heat load being carried by the primary pipes? Do you have a plan where the heat pump will be going and its relationship to existing boiler and pipework?

Apart from the rotational effects, there's quite and interesting fact here: If it were us, it would be in bureaucracy for 15 years, then take another 25 years to construct at a total of of an added zero coming in at the bargain price of about £280billion and then we'd be bombarded with documentaries about how exceptional our engineering capabilities are, not to mention our finely honed bureaucratic machinery. Reminds me well of discussions I had with an engineering project manager I worked with who was part of the one of the 1st due diligence assessments of HS2 around about he 2008 financial crash.

I know your dilemma well. It is exactly the reason why I learned how to design and install heating systems, and especially low temperature ones, and now have my own heating design and installation business! The industry is a mess. But yes, there are companies around that can and will do a design and leave it at that. There are a few down south that offer this along side their own installations. Here is one that has a lot of experience and a good reputation, but I don't know their fees. Might be worth a call for @Great_scot_selfbuild: https://robberridge.co.uk/self-build.php

Okay, from the design you posted up you have a classic situation of tail wagging the dog. The ufh you have isn't a heating design, it's a geometric exercise of fitting pipes of a certain spacing into a define space. The heat loads need to drive the design of the ufh, just like the heat loads would drive the sizing of a radiator or fan coil. Really, I would expect the ASHP supplier (should be designer and installer) to be taking the ufh off your hands and co-ordinating to make sure the ufh is suited to both the house and the ASHP.

This. There is a pattern here on buildhub that we see a constant stream of UFH systems all with 150mm space regardless of the house. So: - do you not have a heating engineer working with you to design the ufh as part of the whole heating system? - what has the supplier provided you in terms of design specification relating to floor output compared to heat load? - and have they also specified lots of room stats Get an inkline instead - clearer marking on all types of substrates plus you get water resistant ink too. That way you can use it to do your full markup. https://tftools.com/blogs/product-advise/inklines-versus-chalklines?_pos=2&_psq=inkli&_ss=e&_v=1.0

It's difficult to help without knowing exactly what model you have installed with what indoor unit and cylinder. But there should be a menu for DHW on the controller where you can access the settings - it's in the locked menu IIRC.

Thanks Gus, I think this is going to be the solution. I've got loads of spare joist hangers and heavy duty masonry bolts from installing ledgers on the old bungalow walls. With the metal connector nailer, they'll be in in a jiffy, job done! From the looks of it, the founds are pretty good, so I think it'll be okay. The door lintel is also a beefy one.

GSHP at this depth is actually solar thermal rather than geothermal so the ground used for extraction of heat requires annual restoration by the sun otherwise it does not recover for the next heating season and why the ground array sizing is so important.

I just went to order some timber from one of my local suppliers, Arnold Laver, where I used to get really good prices and service. Website is down and I found this: https://www.constructionenquirer.com/2025/11/14/timber-giant-files-administration-notice/

"the European Development Fund" - oh the irony

Building Regs do tend to be open to interpretation, but what you can lean on and discuss with your BCO if it's a problem is that in accordance with industry practice and guidance (CIBSE Domestic Heating Design Guide) for any new build, a design temperature of 21C should be used throughout the building - because of higher insulation levels and we hope, better airtightness. In this sense, adjacent rooms can be considered as a single zone: 5.21.b

Yeah, that's what I did with the plywood sheets I used for the stringers for my construction stairs. They have a delightful bounce to them but have been plenty strong enough 😁

Ah yes, indeed. I just reread the technical document I had and realised that what I read - Differential Pressure Delta P - was just constant pressure not proportional, which is why I was confused by the chart showing constant pressure. I must read these things a bit more carefully next time! I think it's worth testing pump setting 0 to see what happens. Who knows, it may not work at all but if it were me, I'd like to see variation in flow rate proportional to burner and DT and then see what happens through the system. I wonder what happens when these things are installed with weather compensation, or maybe nobody ever does and they build these things for relay controls only for the UK market, just like other manufacturers like to reduce specs for us?

Good question. I hadn't really thought about it this way but may not be practical given the span is 3.3m so I'd have to sister the plywood sheets too.

Well, yes that was the plan and then I got the look from my wife when I mentioned temporary 😉 temporary that in true self-builder style becomes permanent, so I better plan ahead even though we have planning permission for another 1.5 width garage. So the ties/joists are rough sawn 50 x 100mm with no grading on them at all. They're installed every other rafter at 32inch centres (approx. give or take an inch or so as they vary ) , so I guess I'll either need to add to each rafter missing the ties. Span is 3.3m. As for load, that's a very good question. I'm not imagining anything particularly heavy, but don't really know what's sensible here. Do I go with the ceiling joist span tables by BSW and the C24 (my usual supplier only stocks C24 anyway) and use something like 45 x 145 or 45 x 170 for some extra margin? If I can get them in there, I still wonder whether new joists bearing on the wall plate might overall be the simplest way?