Due to various issues with tree protection and mains gas lines not adoptable by Cadent if laid in a duct I may well not end up with gas in our new build. Assuming this is the case I have to rethink my DHW and space heating plan.

On the plus side - no gas boiler and no gas supply charges (say £5000) and no standing charge (say £90 per year).
On the negative side no access to energy at around 3.5 p/kWh.

I calculate the house will have a heat loss (inc, ventilation and infiltration) of around 140W/K - that will give a peak heating demand of around 3000kW. (location UK, Cambridgeshire)

There will be two occupants 99% of the time, no baths, but the DHW should be designed to cope with visitors so 6 people in total. The house will be occupied most of the time.

I can site 2.5kW solar in an OK position if necessary (about 20 degrees south of east on a 40 degree pitch).

Cost is the main factor here - we do not have an unlimited budget so a balance between capital and ongoing costs is a factor.

At the moment UFH and ASHP seems like the space heating of choice for this situation (correct me if there are other solutions) - and it looks like an 8kW Ecodan will do the job.

But what about DHW. I'm prepared to implement almost any solution - from tank-less/on demand electric heated water everywhere, or, at the other extreme, a small TS with ASHP and solar PV assist with an inline electric boost or other fancy system. Which way would you go/have gone?

We put in a duct for gas, which is in the road, but didn't bother connecting it because of the capital costs and onging maintenance. We have 4 kWp solar and just went for direct electric heating for the house (via MVHR post heater and some radiant heaters - I had thought ahead and laid in radial power cables for these) and for the DHW (via immersions in a 250 L thermal store). It works well for us.

Our space heating is about half yours, DHW is similar (2 + guests). We use E7 plus the solar to reduce ongoing costs. I couldn't justify the cost and complexity of an ASHP and I committed the design just before Sunamp arrived, though I might have chosen that instead of a TS. One bonus of an ASHP would be the cooling capacity. Your own sums & preferences might work out different to mine with a larger heat load and smaller PV system.

Posted By: goodevansI calculate the house will have a heat loss (inc, ventilation and infiltration) of around 140W/K - that will give a peak heating demand of around 3000kW. (location UK, Cambridgeshire)

Could you explain 3000kW. 3000 kilo Watts is a lot. I suspect a typo ! Also is 140W/K a continuous load of 140W

The link above shows a prodigious amount of insulation. Are you building to the same standard? If you are then a gas boiler on LPG would be OK.
Quote from the above link "for space heating the indoor temperature is not expected to fall below 16C even after five days without heating ." They also calculated the difference between mains gas and LPG to be about £165 pa more for LPG so on those figures mains gas with a 5K fit bill looks expensive.

Posted By: Nick Parsons140W total (??fabric??) heat-loss and delta T of 21?

In that case a bit of 'heavy breathing' and there will be no need for a heating system

''In that case a bit of 'heavy breathing' and there will be no need for a heating system''

@ 1 deg delta T, yes!

oops - yes 3000 kW is clearly wrong - 3000W or 3kW. I have assumed a 'waste heat' output of about 500W and I'm prepared to use a little direct heat from electric fans if it gets really cold but 3kW of heating should be good down to approx -5 centigrade.

140Watts per centigrade is my estimate for fabric (there are two houses in fact and the smaller one will be lower than that).

I think propane would be more expensive to run than ASHP (I would need cylinders).

I like the idea of TimSmall - use ASHP to keep a warmish tank of water and boost the DHW with an in line heater. In the summer heat that tank to its limit with PV, In the winter draw the heat from that tank for the UFH.

Any losses from the tank in winter are useful and as cheep as the UFH. In the summer the DHW can be considered free whist the sun shines and although the losses may heat the house a bit it won't cost me.

So, assuming a monoblock ASHP - my thought is to have a twin coil TS- one coil for the ASHP (with antifreeze) - one for mains pressure DHW (for luke warm water in winter) and the store feeding the UFH. This avoids the legionella risk.

Or I can have a twin coil indirect tank - one coil ASHP, one coil UFH, and the tank for DHW with weekly boosts against legionella.

or a single coil and a zone valve which would mean running the UFH with antifreeze.

The DHW output would need both a blending valve (because the DHW could be very hot with solar PV) and a booster. - presumably put the the booster in line first with an output temp of say 45 deg C and the blending valve ensuring temp no greater than say 48 deg C.

I have no idea what the physical size of your place is, but if it is small, then even a 200 lt cylinder will take away a fair amount of space once plumbed in and put in an insulated cupboard. There never seems to be a sensible place to fit a cylinder.

2 people having a shower a day will use about 50 litres of hot water (@65°C storage temperature), so a 200 litre cylinder should do 6 people (depends how greedy they are with water), though it may need 'boosting' with a simple manually switched element.
Space heating could use fan assisted heaters, where the hot water comes from for them is then up to you i.e. gas, ASHP, electric, just depends what you can find going cheap. The advantage of fan assisted heaters is that they are flexible and can cope with different input temperatures easily. Disadvantage is that they can be noisy.

The fan assisted radiators are fairly expensive - I was thinking of using them upstairs (or at least plumbing in the water / electric) but downstairs UFH will probably be cheaper and not wreck the usable space..

It looks like we will probably qualify - the criteria we would fall down on is "Where the first owner financed the construction of the property, this can be through the use of a custom-build mortgage or loan." it depends if the word "can" means "must" or "may" - it can be read either way (we don't need a loan).

The next issue is is it worth the hassle - looking at the rates for RHI and FiT - it looks like it - but then knowing my luck it will be phased out before we commission. Lets leave RHI/FiT aside for the moment and focus on the correct solution assuming no subsidies.

Say 2000degreedays x 140W/K = ~7000kWh/a heating.

Over say 10 years that costs very roughly:
£10k with direct electric heating (15p per kWh) plus say £0.5k for installing some panel heaters

Or about £4k to run an ashp with cop2.5.

So you won't want to spend more than say £6k on installing your ufh, cylinder, ashp and plumbing and controls, plus any repairs or maintenance.
Could be a bit tight to be worthwhile going for ASHP.

If you do get RHI of 10.5p on the renewable part (1.5/2.5) of 7000kWh/a, that would be ~£450/a over 7 years = ~£3k total, which might tip the balance to the ASHP?

Other options could include E7 together with storage heaters, or a store, or using the slab, I have no experience with these.

And don't forget your cost of capital - if you sink say £10k upfront on ashp/cylinder/ufh/plumbers/etc, that ties up capital that could maybe have been invested, or saved off the mortgage if you'd had one.

If your alternative home for that money could earn/save 3%, then over 10 years you'll lose 10*(1.03^10 -1) = £3.4k by tieing it up in heating kit.

OK, Wills numbers look about right - 7000KWh heating p/a.

taking ST's numbers...

50 litres of 65 deg C water is a 54 deg C increase. Energy req = 50*54*4200 = 11.34 MJ = 3.15 kWh times 365 days = 1150 KWh/a. We use dishwashers and washing machines that seem to take cold feed only these days - so this number could be about right (equivalent to two 10 minute showers with a 10KW heater) - but may need to be a little higher with the wife and the flow rate she would like - lets double it to say 2500kWh/a

So yes, heating dominates. So as a minimum don't spend much money (if any) connecting the space heating to the DHW.

E7 has always been a problem for me - unless you can shift half the load or more to over night it doesn't pay but with solar PV reducing daytime use, and using an immersion storage tank it may make sense (with extra insulation on the tank). It's striking that about 1.7kW/day (70 watts) can leak out of a tank if it's not got extra insulation installed .

I Looked again at electric in-line water heaters - unless they are specifically designed to boost heat I don't think it will work (due to flow rates). So storing DHW for shower use looks like the way to go - it seems wasteful (the sunamps would work but are not economic).

So we are looking at 9500kWh/a - 7000 of which for heating. Just too expensive to go for resistance heating so I'll look at ASHP. (it also feels wrong to use that high grade electric power just for heat - doubling it or tripling it via ASHP seems better).

Posted By: goodevansSo as a minimum don't spend much money (if any) connecting the space heating to the DHW.

Why would you even want to consider linking UFH space heating and DHW?

E7 has always been a problem for me - unless you can shift half the load or more to over night it doesn't pay but with solar PV reducing daytime use

For us, E7 is used in winter and PV only happens in summer (or NOT-winter to be more accurate). But then paid for consumption only really happens during winter. Your situation may be different.

I Looked again at electric in-line water heaters - unless they are specifically designed to boost heat

The Stiebel-Eltron units?

storing DHW for shower use looks like the way to go

If you want to make use of PV and don't want a Sunamp, it's the only game in town.

So we are looking at 9500kWh/a - 7000 of which for heating. Just too expensive to go for resistance heating so I'll look at ASHP. (it also feels wrong to use that high grade electric power just for heat - doubling it or tripling it via ASHP seems better).

Sounds good both economically and morally. You will use enough energy to justify the capital investment.

I know it is unusual in the UK (and won't do underfloor heating), but air-to-air heat pumps are much cheaper (to install as well) and also very efficient (especially the good ones). Depends on the overall layout how many heads you need (i.e. open plan downstairs would help). That plus some bathroom towel warmers would be enough for 4.5kW, wouldn't it?

Separate HP system for the DHW would also be possible but the space heating is the main thing.