Pigletjohn, anyone - UFH?

[shanghaismog]

I've posted about this before and I'm really fed up with going round in circles with this and I'm getting desperate and hope that someone can put me out of my misery before my brain explodes!

Over 2 years ago we built an almost passive house with a space heating requirement of 20kWh/m2 - it's airtight (0.6ACH @50Pa) and super insulated and we have a 4kW solar PV array. We have no central heating and in the depths of winter use our wood burner to keep toasty - but I'd really like cosy toes...

We laid UFH pipes in the foundation slab when we built it, but didn't ever connect them up to anything - now we're thinking that it would be a real luxury to do so, but not at an exorbitant cost. We're not connected to mains gas (and it would cost £7k+ to do so, plus cost of the gas boiler and install), so we've been looking for ages at our other options. Limited space at the side means that an oil tank or LPG tank is not really an option, LPG bottles could be and an ASHP would be possible. Getting quotes back has been a total nightmare as everyone specs the system for a normal house, which is madness with such a small heat requirement - we can noticeably heat a 35m2 room with several candles on and the door shut...

We always thought that we would connect up and ASHP to stay true to the house's eco credentials but the latest quote has come back at £12k!! This is far too much for a pure luxury item as we don't NEED UFH, we're warm enough without it, albeit just about in a cold snap.

The problem is that a bog standard LPG boiler to run just the small heat requirement for the UFH would run so inefficiently that it's also not such a good idea from that perspective either.

Can anyone help me with the calculations for what size tank it would take and what it would cost to run wet UFH from a direct electric hot water tank (our hot water is taken care of already and will be completely separate)?

My thinking is that if we use a very well insulated tank and the UFH runs on such low temperatures, much less than hot water heating, and it's a super insulated house with nowhere for the warm air to go as it's airtight, then surely the cost shouldn't be too mad?

I don't want to hear 1kW = 1kW, I know that.

I'm looking for the calculations that tell me if I have a 130m2 footprint, with UFH pipes laid at 150mm spacing throughout (in 6 zones, which would be separately controlled), how much hot water I will need to heat to run the system and how much that will cost to run it, using the immersion tank (insulated to the max), based on the heating requirement of 20kWh/m2.

I just can't get anyone around here that can work that out for me and I know it's a case of mathematics... Can anyone point me in the right direction pretty please, as I'm totally losing the will to live with this and about to give up (but I really want warm feet...)???

Probably far too specific and random a question for mumsnet, but worth a try!

Can the alternative technology centre put you in touch with anyone? Or a university department researching energy efficient heating?

Sounds like a great idea to use the tank but if you're designing your own system you'll need a prv and expansion vessel. For that amazing level of insulation, assuming the ufh is supplementary heating, then 1kw would be plenty for winter. With 130m2 then that would be less than 10w per square metre which would give very low water temperatures, hardly above ambient, unless you have thick carpet or something!
Presumably you're only losing about 2w per m2 through the floor in winter so 10 would be an OK boost.

PS the cost would be less than 400 pounds a year assuming 15p per kWh.

Oooh replies, thank you!

Yes, it really is a total luxury in terms of heating but we do need a slight boost in the dead of winter. The woodburner really heats the place up but doesn't help the kids coming in from school earlier than us from work, who are obviously not allowed to light the fire, so it would be great to have another option. A couple of panel heaters for £50 each would also do the trick, hence my reluctance to part with such large sums of money...

I really don't want to use the alternative energy guys we used the first time as after 6 months of trying to get them to come and service the Ecocent hot water tank (for which they wanted to charge me £400!!!), they eventually turned up without any warning at all...luckily I was working from home that day. Needless to say, I'd rather not tie myself any further in with them... They did spec an ASHP with 50lt buffer tank and manifold/controllers for £10k, but as it's a pure luxury this seems pretty extravagant and will take a lot of years of immersion heating to pay back in comparison.

I was thinking of something like this, with an inbuilt expansion vessel as due to all the rest of the kit we're at risk of squeezing out the beer fridge in the plant/utility room (heaven forbid!)

www.kingspanenviro.com/albion/ultrasteel-plus-direct-cylinders

130 m2 house requiring 20 kWh/m2 means your annual heating requirement is 2600 kWh of heat. If you get that water electrically as you propose then you will pay approx £390/year to make the water warm enough for that, plus the pumping cost (not substantial- call it £450/year all told)

That is a very, very low heating requirement though. It must be massively insulated. And if you're below 0.6 ac/h then you must have some sort of heat exchange system or you would have the most terrible damp issues. In which case I would opt for an ASHP to work with your exchanger.

As for how much water you need to heat it depends on the bore of your pipes but if you've got 130m2 and that has 100% coverage in UFH pipes, then do 130/0.15= 870m of pipe (roughly). Volume of a pipe is pi r2xh, and the bore of UFH pipes is about 12mm I believe. So, 3.141x6x6x870000 = around 98 million cubic mm or 98L of water.

So I would guess you need a cylinder that can heat probably 200-300L. that said I have no idea if it's possible to hook such a contraption up or if it would work well. Immersion heaters work very well and if you can heat your house with candles then it probably will work.

But any UFH plumber should be able to tell you all this with more accuracy than I've given. Probably for free while he plans the heating system with you as well.

HTH

I think you should do a real-life experiment to see how much heat input the room needs through a typical winter. You could do it with about two electric panel heaters, one towards each end of the exterior wall, fitted with energy monitors to track their usage. You'd want to use their thermostats to maintain the room without you jumping up and down to change the settings, because that's how your UFH would work.

Since you would in future be heating the room electrically, you can't moan about the cost of the real-life test.

I have a feeling you won't be able to justify it economically but perhaps as a lifestyle choice.

The other option is to get radiant heaters, the ones that glow red like patio heaters. We got one for during our building work when we had no ceiling in a few rooms and no outside wall at the back, and it really throws the heat at you rather then up at the ceiling.

You're a star thank you so much.

You would think that the people who I've asked to design and quote would be able to tell me but I'm just getting nowhere with it (probably because they're trying to sell me £12k of kit...).

Yes, it is pretty much passive house standard, so super insulated and we use a MVHR system for ventilation, using the exhaust heat already to heat our hot water.

If the pipes are laid at 150mm spacing, so not 100% coverage, that would then reduce the amount of water to be heated (and therefore size of tank)? Is that right?

hiddenmnetter the cost of pumping the water would be included in the heating cost so no extra there.
Also the size of the cylinder isn't relevant to the amount of water in the pipes. In a gas boiler the pipes can be 50+ litres but the boiler only holds a litre or two. It's the heating capacity that matters so it needs to be able to put a kw into the water.
Agreed with piglet John about the lifestyle choice thing though, but to be honest you'd struggle to make your money back by putting in any other kind of boiler, given it's expected life. Plus in a standard house your gas bill would be around that. And furthermore that assume you're heating the whole place routinely.

Sorry missed a couple of posts.

You're absolutely right, it would be a lifestyle choice and at this stage it seems madness to sink such a lot of money into something we would use so little, especially when as you say a radiant or panel heater would suffice. Do you have any idea on what cost we'd be looking at for such an immersion setup? £1k manifold, £1k tank and thermostat controllers, ? Pump and ? Installation

We've already done 2.5 winters and the lowest temp we've had inside in the morning was 15/16 degrees in a really cold snap.

Any suggestions as to what else might be an affordable option?

Thank you all so much for your help though, much appreciated.

Our UFH pipes are on top of the foundation slab, not in it. This will
make the heating inefficient because they are too insulated.

It's a passive raft foundation with 300mm of insulation below it, so the only way the heat is going is up, so not worried about a few mm of extra concrete!

If only I could work out how best to power it (and run it) without breaking the bank...

if electric, my preference is some kind of oil-filled radiator, because the surface never gets hotter than a teapot, and even if you accidentally let a curtain touch it, or stupidly put socks or a towel on it, or push a sofa against it, you can't start a fire. They also give a more even heat due to the thermal lag.

Sorry if I'm misunderstanding but this is only for supplementary heating to take the chill off isn't it, with most heat coming from your log burner? So the heating requirement really is pretty small. You'd think you'd be able to come up with something small and simple to do it, just maybe not a standard solution (I'm thinking like those immersion elements for towel radiators and a pump, just scaled up a bit...)

if the stove has a HW ability, it could warm up a heatstore ready to warm the UFF the following day... but the complexity is ludicrous

Sorry pipes laid 150mm apart is the norm, 100% coverage meant that every part of the 130m2 had UFH piping at 150mm separation (there were no unheated larders or anything)

johnd2
The reason I thought an immersion tank would require a fair amount of stored water is because it's not a boiler type "heat as you call for it" situation is it? You'd need a reserve of hot water that the system could pump around to add warmth. So I supposed by having double to triple the water being warmed while the house is empty (using those PV cells) you would create a store of warmth that could then be pumped when a thermostat called for it. I don't know though- I've never heard of UFH supplied by immersion cylinder before.

In a house of this insulated standard then any heating cost is minimal...minimum temp you've had is 15/16? If you actually want warmth then a few space heaters for £50 will more than supply your requirements.

If you want warm feet then #agreewithpigletjohn. Lifestyle choice - the only question is can you scope it? I would probably personally go for a GSHP as the RHI can make the cost pretty close to zero over 7 years, but your house is already very eco so no idea what that does to the payments...

Hidden that's a really clever idea, I didn't realise you meant a heat store. That would help even out the fluctuation well too!

Thanks all, so if I got one of those immersun diverters to heat the water from the PV that would help too, although only marginally as we wouldn't be generating much in the winter when we need the heat?

How old are your dcs? I was fully capable of safely lighting and managing my parents' wood burners from about 10 (but do realise no one would have called social services in the 80s!)

Am Immersun costs in the region of £250, plus some electrical installation.

I've calculated that the amount of money it saves would take me more than 20 years to recover the cost. In my case I do have gas as well, so my calculations are based on energy saved at the cheapest available source. Solar electricity generated in winter months is practically nothing. In summer months your need for heating is practically nothing.

You will find marketing calculations which assume you will save energy at the highest possible price, which make it look a better bargain.

p.s. I did actually buy one, on a foolish whim, in my early enthusiasm. It was money down the drain.