Whilst I'm not the best person for advising on house design for efficient heating, one thing does spring to mind. If ever there is an ideal time to consider MVHR, it's at the house design stage, so do get your friends at least taking a look at heat recovery options. The more you can conserve heat whilst maintaining reasonable ventilation, the less work the heat pump will have to do and therefore the smaller the heat pump that needs to be installed.

Additionally, albeit not something for the architect, if they're going to be getting the builders in for that scale of work, the digger for the foundations could possibly also be used for digging trenches in the garden for a ground source heat pump rather than an air source heat pump. Once again, just worth investigating; a GSHP will be more expensive but perhaps not that much more if installed at the same time as the house build.

One detail that is often overlooked, your friends might well want to get the home kitted out properly with structured network cabling (i.e. computer network sockets dotted around in the same way the leccy sockets are). It might not be that closely related to heat pumps and so forth but modern heat pumps, solar inverters, EV wallboxes and so forth are all Internet-enabled and wired network connections are far more stable and reliable than wifi, so getting said network incorporated right from the start is a good idea.

Now I've gone down several rabbit holes, I'll step back and let someone better qualified to talk about heat pumps jump in and answer your original question.

Posted by: @mikef

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However even with K3 rads these would need a higher water temperature which would then be detrimental to the whole house COP.

Has anybody got any suggestions about the best way for them proceed?  

From first principles I would say:

Definitely design for a single low flow temperature, not different flow temperatures.

Definitely calculate (to the extent possible) the heat transfer from downstairs to upstairs, and adjust accordingly.  I get the impression most heat loss programs attempt to do this (given the right data) but only for conductive transfer not convection.  With a big stair well convection is probably rather significant, but I have no idea how to calculate it.

Several people on forums report that, with a very well insulated house, no heating is actually necessary upstairs.

Possibly a bit of calculation and a bit of suck it and see if the convection transfer cant be calculated

I just noticed that @majordennisbloodnok mentions MVHR.  100% agree but but the heat transfer from MVHR will need to be factored in if air is moved between floors.

@majordennisbloodnok

I agree that they should look at MVHR, but will need to get their builder onside, some builders don't think air-tightness matters, so they look for the quickest option, not the best option. The more airtight the house is the more successful the MVHR will be.

They are already looking at the cabling, i used the same electrician and he was very pro active in this .

@jamespa 

I will see what the plans are for the hallway, it should be possible to use it to get heat upstairs. When balancing the UFH flows the hallway would need a greater flow because of the volume of air it is heating, maybe push this even further and have the hallway at a higher temperature than the rest of the ground floor. This would have to be done through the flow rate so keeping one zone, and not using room stats and multiply zones.

I think getting a plumber to agree to no heating upstairs, would be unlikely. Heat loss calculations use a standard figure for the build materials, type and thickness of insulation etc  but don't account for the quality of install of said materials. The same materials installed poorly, or well will make a substantial difference to the end result. Most self builders i have talked to say their property exceeds their efficiency expectations, its not the same for the mass build property's.

A little bit of thinking out loud, if the plumber says they must have radiators upstairs to cover all eventuality's, on my heat pump(Viessmann) its easy to push up the WC, to a maximum flow of 40°. The pump would cycle a lot from the UFH but flow upstairs may help to keep it running. The COP would fall substantially when run like this, but it may only be for a couple of weeks each year, thoughts anybody?

Posted by: @mikef

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A little bit of thinking out loud, if the plumber says they must have radiators upstairs to cover all eventuality's, on my heat pump(Viessmann) its easy to push up the WC, to a maximum flow of 40°. The pump would cycle a lot from the UFH but flow upstairs may help to keep it running. The COP would fall substantially when run like this, but it may only be for a couple of weeks each year, thoughts anybody?

Im not sure what your thought process is here.  Are you suggesting running with a max FT of 40 rather than the max of (say) 35 that the UFH is likely designed for.  If so how will you stop the downstairs overheating?  And what will happen at moderate OATs - the requirement to heat the upstairs, if there is one, doesn't magically vanish!

If the amount of heat needed for the upstairs is small, once you take into account transfer from downstairs, then maybe you can design for rads at 35!  If not put a fancoil on the landing and in any upstairs rooms where you dont sleep, or even in ones where you do but then turn the fan off at night.

Fundamentally you want, I think, to design for the same max flow temperature throughout and make that as low as possible having accounted for heat transfer from down to up.  That shouldn't be impossible at all because its early enough in the process and you are aiming for low loss.  The numbers (loss accounting for heat transfer) are key to this; once those are available the answer should emerge.

@jamespa 

They will see what the plumber is suggesting when he gets back the projected heat loss, Maybe with a fancoil on the landing, the design temperature can be 35 or less

cheers  

Posted by: @mikef

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@jamespa 

They will see what the plumber is suggesting when he gets back the projected heat loss, Maybe with a fancoil on the landing, the design temperature can be 35 or less

cheers  

A single design temp of 35 or less really would be the way to go.  If it were my newbuild I would be insisting on it unless I had concrete proof (not just plumber BS) that it was not practical. 

Bear in mind that the domestic heating industry, for the most part, hasn't got a scooby about heating design (ie they just don't understand it).  The same seems to go for 'M&E consultants' often employed by architects to design heating systems.  Unfortunately the industry has got used to decades of the lazy approach of showing in a (grossly oversized) 30kW+ condensing gas boiler set to a flow temp of 75C (with the result that it doesn't condense), putting oversized rads and TRVs everywhere and letting the system 'sort itself out'. 

With dirt cheap North Sea gas and a population that knew no better, it could get away with this, even though the result was a lower standard of comfort and higher bills than a properly designed system.  The chickens are now coming home to roost but the industry is slow to change.  Thus anything you/they are told should be considered suspect unless backed up by a coherent explanation that can be related directly back to the underlying physics or a properly explained mechanism for the alleged phenomenon.  Im sorry to be so critical of the heating industry and there are of course some very good guys, of which the plumber in question may be one, but its abundantly evident from what we see here and what I have personally experienced that it is, at least in large measure, this bad.

We self built in 2011. The ground and top floors are each around 100m2

The house is extremely well insulated with U values as follows, walls=0.15, roof=0.1, floor=0.1, windows and doors=1.0

We have MVHR with about 0.3 ACH.

We have a very heavy floorslab downstairs with UFH heated by a 5 KW Vaillant heat pump.

The pump more-or-less never cycles and generally only runs at night on an E7 tariff. If we have very cold winter weather then we might run the ASHP during the day using free solar electricity

There is no heating upstairs but heat rises via the open(ish) hallway.

If the downstairs is around 21C then the upstairs landing is around 20C, the family bathroom is also about 20C as the air extraction pulls warm air in, the bedrooms are around 19C, if you leave the bedroom doors closed then they settle at about 18-18.5C

If we were building another house then we would do the same again.

@bornagain 

Sounds a perfect fit, I am in a 200m2 Bungalow, unfortunately i didn't know about this site so allowed the heat loss calculations to dictate the size of the pump. I have a 10kw which is way over-sized, the records, over 3 years, show the most we have ever needed was 6kw,  and this was only for 5 days over 3 winters, a 5kw would have been fine. I will try to get my friends to challenge their designers if they start saying that they need anything bigger than a 5-6kw. Thanks to everybody here, now i can suggest the questions that they need to ask, rather than take an "experts" view as gospel.

The definition of an Expert, a Ex is a has been, a Spurt is a drip under pressure.

Cheers

Posted by: @mikef

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The definition of an Expert, a Ex is a has been, a Spurt is a drip under pressure.

That's a great one for the Uxbridge English dictionary (I'm Sorry I Haven't a Clue - Radio 4)

Posted by: @mikef

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@majordennisbloodnok

I agree that they should look at MVHR, but will need to get their builder onside, some builders don't think air-tightness matters, so they look for the quickest option, not the best option. The more airtight the house is the more successful the MVHR will be.

The belligerent part of me says they don’t need to get the builder onside at all; they just need to tell the builder that’s what they expect, since they’re paying for it. I realise the real world is more nuanced than that and that the builder may well have other qualities that your friends wiuldn’t want to jeopardise, but it’s still worth rememvering that if you say what you want that’s what the tradesperson should be delivering.

Posted by: @mikef

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They are already looking at the cabling, i used the same electrician and he was very pro active in this .

That’s good. Network cabling is a bit of an art in itself and so not all sparkies understand all that’s necessary (like the need to ensure unshielded cat 6 is kept separate from electrical wiring), but if the person you’ve used before knows what he’s doing then getting structured cabling and electrical wiring done at the same time is an excellent idea.