SimonD

Numpties.....

You're not alone. I think there must be a gravity well here. I keep on unsubscribing to the thread and as my finger moves itself to click on the mouse button, my head is going, no, no don't do it, and then I'm back in again, uh!

No, I have his website in my bookmarks. I don't know him personally, but he is known in the industry, and I'm in the industry - IYKWIM? He was actually interviewed by HVAC Education hub recently, so you can listen to him talk - he very much talks the language of buildhub in his design philosophy.

I asked for that back on page 16...maybe it was missed. Important information particularly since the surveyor said no.

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?

Solid wood for me all the way. For us, I've bought par redwood and machined with a router as I don't really like the standard profiles. Everyone I know who has had MDF has regretted it, especially if you have hard floors that get mopped.

Yes, but there is a rule of thumb to divide this (q50) by 20 to arrive at a leakage rate under normal conditions.

That's a very good question. I simply don't know. In my heat loss calcs I keep on going back to ventilation like a revolving door to find more accurate answers to more accurately calculate ventilation losses, but we know that standardised heat loss calculation methods almost invariably over-estimate ventilation losses. And the industry is set up to lock us in to the proscribed methods. If you deviate from the national figures, you have to be prepared to provide a rationale for the deviation, especially when it comes to retrofit. So again, this is more complicated the it first seems. Yes, assuming that the MVHR system does not impact pressure, then it would be in addition to infiltration. But from a ventilation heat loss perspective, it would be wrong to assume this simple addition given that, 1 the industry standards tend to over-estimate infiltration ventilation losses anyway, and 2 the effectiveness of the MVHR system. MVHR systems can suffer the same fate as heating systems in that many are over-sized (I know from data that this was definitely the case a while back and I would hope the situation has improved). The question then remains in each individual case as to the flow regime within the heat exchanger. If it is in the transitional or laminar state, then the efficiency of the MVHR is greatly reduced, which obviously impacts the ventilation losses significantly. Now if someone finds that they back off the flow rate after installation, they'll still get the benefit of fresh air, but without all the benefit of heat recovery. Obviously there are systems that deal with this by cutting off flow to parts of the heat exchanger to maintain turbulent flow, but tbh, since I decided to go the natural ventilation direction I haven't followed the MVHR market for about 5-6 years, so don't know the current market situation, but I would cynically doubt that this function has become a standard. Maybe I'm wrong?

I know the dilemma well. I actually used to work in Microsoft back in the 1990s. I have a dual boot system and spend most of my time on Linux with LibreOffice, but then boot up on Windows and use Excel only when I absolutely have to. It makes life a bit of a pain, but for me it's worth it. Ventilation losses are a big problem for the whole industry. Unless you've got the results from a blower or pulse test, it's pretty much guesswork anyway and then you've got just a few options on methodology - you've got SAP based on storey height, location, exposure etc. or BS EN 12831-2017 or CIBSE methods and they usually treat MVHR differently so I'm not sure MVHR would correctly be an extra leak on top of ACH.

Most of these boilers are thrown on the wall with configurations never touched by the installer and in default I believe the 8000 DP proportional pressure 250mb. In most scenarios, it provides sufficient head to supply most houses where they're installed. I'd be very surprised, given the pipe oversizing indicated earlier, that the index circuit is anywhere even close to needing 4m residual head. This approach to me is tantamount to the plumbers who set the pump on 3 and run out the door. But I agree that the system behaviour isn't right - you've got constant flow rate of 21.5lpm. and modulating burner output and reducing DT until it seems to find equilibrium at burner output of 49% during the test. If the pump is on Delta P proportional, the flow rate shouldn't be the same at 49% burner output because it doesn't need it and it's therefore creating unnecessary pressure drop through the system - e.g. the system fighting itself. The tech info is a bit ambiguous here because it says the pump can be set to proportional Delta P and if this were indeed correct, there needs to be a change in flow rates somewhere sometime - on any decent modulating system there should be variations in both DP and DT as heat load and output change. Why not test in fully proportional linked to burner, which is what most sensible boiler manufacturers do out of the box? You can probably see now why I don't have anything to do with these things from this brand.

Both the ods and xlsx file work fine in LibreOffice calc. Best thing to do is download and install a copy of libre office - https://www.libreoffice.org/ But beware, once you start using it instead of Excel, you may never go back 😁 Now need to do some interpretation.