SimonD

And yet the pipes are all still at 150mm and no variation between rooms. Whilst we have seen the design we don't know the heat losses the design is supposed to cater for. If this is a properly insulated house with low losses and in particular low down losses, the 150mm spacing is frankly silly, It'll end up overheating the house and then the installer will start telling them they need a 1000l buffer or something to prevent the heat pump from cycling when all the actuators start closing down and it all unravels from there. Instead they could do a proper job of UFH design. But show us the design numbers and I'll be open to correction! 😉

What are they talking about? Of course MVHR is taken into account in the designs and you can use whatever flow temperature you want as long as you can demonstrate in the design that the system provides the heat to heat the house properly. With MVHR you use the calculation methodology from SAP where the efficiency of the system is taken into account to calculate ventilation heat loss. This is really important because often ventilation losses make up more than the fabric losses. Where do these people come from? BTW I know this because I'm MCS Certified for design and installation of ASHP.

SAP is about calculating energy use not about sizing the actual heating system or emitters (Radiators / Underfloor Heating / etc) . But the new standards for heat loss and system design do use aspects of SAP (e.g. designed or measured air permeability). You do need a heat loss calc and system design.

I'm not sure what you mean? MLCP pipe is absolutely suitable for hot & cold supplies and at 13mm insulation thickness it's good enough for your purposes, but if it's exposed to external temperature it needs to be minimum 19mm insulation. Here is a link to maincor's technical brochure about MLCP: https://www.maincor.co.uk/wp-content/uploads/2022/02/Maincor-MLC-Pipe.pdf You can then also buy the pipes that are designed to be buried and are preinsulated and ducted as complete units, either with one pipe or 2. Here is maincor's sheet on that kind of pipe. https://www.maincor.co.uk/wp-content/uploads/2025/09/Maincor_AUSTROFLEX_2025_web.pdf The issue is that with water regulations you shouldn't really embedd the pipes unless you absolutely have to. So use some ducting and thread MLCP pipe through the ducting to the outlets. This is high quality stuff.

Okay, here's another link - 9 13 or 26mm insulation https://www.maincor.co.uk/pipe-systems/products/ or this: https://wavin.com/gb/p/f38c1cf7-cf05-4844-9e65-3529628340c8/tigris-pexcalpe-pins13bl-25x25-l25?systemId=C05_F001_S278 All manufacturers I will use from time to time.

Ah, yes, if no insulation below, then it's a problem. Oh, you do test me 😉 https://www.bes.co.uk/riifo-multilayer-pre-insulated-fire-rated-pipe-coil-16-x-2mm-x-100m-blue-27351/

I think you've just described properties in the opposite of the order most people put them in. Most people will think less about spending huge amounts of money on kitchens and bathrooms, but will then skimp on a heating system, unless it's a flashy column radiator..

It's fine for this kind of use as it's only to reduce heat transfer between pipes and not to insulate the pipes from external conditions. The MLCP insulation is usually 13mm which is in line with indoor pipe insulation. My assumption is the pipe in the OP will be well surrounded by the insulation being installed around them though?

I'm guessing the proposal is to lay all the water pipes in ducts? As Water Regulations does state that no fittings should be embedded in a floor or wall, but people do. I don't know what they're talking about regarding barrier pipe unless they're referring the plastic pipe that has an oxygen barrier as I see no need for the barrier pipe you're referring to - it's confusing. You can, however get pre insulated MLCP which is going to be easier to thread through the ducting, just do it before you introduce bends to make installation easier.

We're still 4, two teenage boys who disappear for an eternity when they have showers! But, with DHW there is actually a detailed calculation method which should be more accurate, but I doubt a lot of designers will spend the time completing those calcs. Now, for a moment I was about to respond on your question about the rethink, but I then wonder whether the industry really is that responsive? It's only taken them 9 years to implement a hybrid version of the 2017 standards....and even then they still haven't ratified the national annexes to the standard, so you can still, according to MCS just use BS EN 1283-1:2017 for your heat loss calcs, for example, and ignore the new CIBSE guide because MCS only require calcs to be to that standard. So still a bit of a mess, but a step in the right direction. It'll be interesting to see what the fallout will be because if I now update a design just completed for a customer, the airing cupboard is not going to fit a 400l cylinder!

I've just got my hands on a copy of the new edition of the domestic heating design guide and it's a major revision. Totally different approach to calculating ventilation heat losses, comprehensive tables for calculating floor perimeter to area ratios and then applying those with fabrics of up to 400mm insulation. And even new guidance on DHW cylinder sizing, which is probably the easiest to illustrate here. Typically the DHW cylinder size calcs use for heat pump training courses is either 45 x number of rooms + 40, or 45 x number of occupants x 40, so in our house we have 4 x 45+4= 220l so we had a 250l cylinder installed. The new CIBSE guide suggests that for our house/occupancy/bathrooms and storage temperature of 50 we should now, with 4 × bedrooms, 2 × bathrooms or 5 × bedrooms, 1 × bathroom (maximum occupants, 6 persons)according to their table have a minimum size of 577l. Now, we have no problem at all with hot water provision, but is this new sizing table taking things too far?

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