SimonD

It's not about complex maths. None of the maths involved is complex. It's actually pretty straightforward and it's not about box ticking here. It's just about good old transfer of heat. If you've got a low energy build, lets say 30W/m2 at design outdoor temp, your mean water temp in a screed floor is going to be about 29C, now consider what that needs to be when you're at the average outdoor temp of 7C in England. The mean water temp requirement is then below the required floor temperature to even produce heat into the space because there's then no MW-AT difference and it'll probably below the minimum flow temp of the heat pump. You then have to rely on controls to switch flow on and off, which we all know is not the way to be running a heat pump. TBH finger in the wind look at it and call it, is exactly why we're in the situation we're in here in the UK, because very few heating engineers and possibly M&E Consultants are up to basic design and so many systems are shite and inefficient when they don't need to be. Just a bit of maths and thought is all that's needed at the early stage.

Congrats, must be an amazing feeling. I like the adjustable spot lights you've used, especially on the ceilings - what brand are they and where did you get them from?

Yes, so they should be telling you that they'll produce a heat loss calculation based on your drawings and the SAP report you have. Then you should get a figure of a total output for the heat pump and a report telling you the heat load of each room and that will feed into a design of the underfloor heating (UFH), radiators, fan coils etc. If you're going for UFH on its own or with fan coils, keep an ear out for if they ask about cooling too, as that could be very handy for you.

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.....