JohnMo

Be worth watching this, it's talking about heat pump but equally applicable to gas. System used is easy and doesn't take up much space, you need to screed over, but you get good heat transfer. If you are keeping the rads as they are you will need 2 flow temps one for radiators and the other fir UFH so a mixer is needed. You can several different mixers done good some not so good. An Ivar mixer is good, link to first one that came upon Google, shop around for best price. https://underfloorparts.co.uk/product/ivar-uni-mix-underfloor-heating-ufh-pump-mixing-kit-underfloor-heating-manifold-control-unimix/?srsltid=AfmBOoqyNurwUf8R844Nbc0pdaRMSUPkMqZy8wvhKLBpAY1c1CW1wiun Link to discussion here Problems you could have Zones - the more thermostats you put in the system the worse its going to perform. Small zones and short cycling is an issue. I would suggest you explore weather compensation, you would have TRVs only on the bedroom radiators, everything else would just tick away on weather compensation. Your boiler does allow weather compensation, adding an external sensor, will allow heating water temperature to change in response to changes in outside temperature. This has a big impact on gas consumption for the better.

Not sure where the 75m² comes from they said 150m² footprint. Plus they are doing up the existing house so who knows the final footprint available. No one said average build (except yourself) I am just applying stuff I know to the OP questions.

Think what is going on and I said this on another post today is Installer knows you really don't need radiators in bedrooms when they are above a heated area in a new build, well insulated house. But to tick a box they are specifically specifying a high flow temperature, to allow smaller radiators. The installer and architect have basically said that. Reality is you run UFH as you would (no mixer hopefully) the radiators just tick away at UFH temp. Not really doing much, but everyone is happy(ish). Room upstairs take secondary heat from below anyway. Many state on here you just don't need them in bedrooms.

Is you current floor insulated? If so how much insulation or do you know the U value? What is the motivation to move to UFH? What boiler do you have (make and model)? Will you still have radiators elsewhere in the house?

But the OP is talking about adding 150m2 to the existing house so could easily have that much (or more) concrete in the slab if he wanted.

It really depends there are many variables. Heating method is one, depth of modulation another. Thick screed UFH is way different to radiators. The difference in heat capacity of air 'v' concrete is huge. So a radiator has very little place to dump heat and the air is soon up to temperature, a thick screed UFH system has a mammoth buffer store built in by the nature of way its built. Agree with a radiator system you need a heat pump capacity close to the max heat loss AND a good volume of water. The ideal for a radiator system is 20 to 25L per kW at minimum load output of the heat pump. This ensures a good run time and good control of modulation. Our heat pump 2x oversized for the heat loss, but only really heats a thick screed slab (57 Tonnes of concrete), so the heat pump will chug away for as long as you want without cycling. Then when it does go off, it's off for the rest of the day.

Just had a look at our quotes and kits were wanting 20% deposit and everything else a month before scheduled delivery. If we had gone that route, delivery would have coincided with the start of COVID, having paid lots of money to them prior to knowing this. But the whole factory closed for 6 months plus, so money and kit would have been trapped for six months at least. Good for me I didn't go that way.

Part L of building regs actually states the following for heat pumps 6.39 The heat pump should have external controls that include both of the following. a. Weather compensation or internal temperature control. b. Timer or programmer for space heating. So you don't really need any temperature controls. Heat pumps should always be set to run WC, it is normal for the manufacturers controller to have a timer or programmer built-in. Part L is full of contradiction, so use which you feel suit you. But if push comes to shove, a trv with a timer is a zone.

I know that feeling. I eventually went down the path of what I can do well. Plasterboard - no, too heavy, walls ok but not ceiling. So got someone else to do it all. Joint Taping, the same. Electrics - no Second fix joinery - no. Roofing - no, likely to kill myself. Windows - no one company does it all Everything else was fair game.

MCS rules will not allow you size heat pump based on something you MAY do on future. Until it's done, it isn't done, so may only get done.

Why do you need to sell you house to design the MVHR? Ideal first step is know how big a unit you need, then look out for a bargain

£200

Ours said similar, but the requirements are to be commissioned with instruments with a valid certificate of calibration and a certificate to be issued. They guy who did our air test, did the MVHR final commissioning and issued certificate with calibration certificate details. I did everything else, design, purchase and install.

Didn't see anything, so presume nothing

No, mainly because at time of design I didn't realise you could. Plus I didn't order enough to do it as a change. A guy down the road did his with Durisol below dpc, full filled with concrete. He even did all the internal walls with Durisol.

Durisol install manual says

So he is just playing with numbers, to fudge the design, for your benefit. The likelihood of a new build actually running at 45 degrees to UFH is none existent. So you are getting small radiators you want. You will just operate the same temperature as the UFH - so low temperature. So long as he does the design without buffer and mixers, the reality is you will run low 30 or less on the coldest day. Do you actually need bedrooms with rads in a new build airtight well insulated build? Unlikely, but they will add capacity to system when on which is good.

At least it's easy to bleed!

I would be aiming at same as ground floor, no additional mixers or additional pump required for UFH. Then all you need in the house is a diverter valve between DHW cylinder and heating system. Add TRVs to rads only if you really need them. Or fit fan coils to bedrooms.

I found with a diverter I was only getting to mid to high 50s. Not sure if it just the lower power input that screws thing up. Now divert differently, so the immersion only gets full power (3kW) and now can get to 70 degs. Have you tried switching the immersion on diverting past the diverter to see where it ends up?

My first quote was £10k, hence I started to learn quickly.

But would you not be excavating approx 5 to 600mm from the whole area of the house to replace with MOT anyway even for a passive slab?

I just pointed you towards an MVHR unit, about £1k off full retail. I bought everything else from Paul Heat Recovery and EpicAir. Everything except MVHR units is either Zender or Ubbink, about £1500 plus my 2x MVHR units (£550 for the two in total delivered)

So what's so special about a raft, that suitably detailed normal strip foundations and slab cannot achieve? I just started the structural engineer and architect in the direction of travel to get thermal bridge free design that any ground worker can follow.

Not just MBC - That's exactly how I did mine, but did the membrane before the studs to make life easier. Before After, stud walls do not attach to external walls either, gap then filled with mineral wool to prevent noise migration. External walls were later battened for service space.