Re: Under-floor heating for a bathroom - hot water or electric, and how to control?



Hi Mike,
If you already have a water heating system, then that would be the way to go.
Tee off from the flow pipe on the radiator above floor level, enough to allow you to fit a thermo' valve (non directional) to this side of the system. Drop from the valve through the floor to a snake of 8mm or 10mm mini-bore pipe under the floor boards (8mm should be enough). The spacing of the loops would roughly be about every foot (300mm). Try to keep as close to the tops of the joists as possible, roughly again, would say about 30 mm down for your drill holes, and remember, when laying the boards down again, to mark on the walls where the pipes are just in case. Try to stay away from exteriors walls as much as you can, say about a foot (300mm) again should be OK. Then it's just back to a tee connection in the return side of the system, which can be under the floor if you want, and the jobs done.
No matter what type of covering you put down, you'll always have the heat rising through it. It should keep the place lovely and cosy for your dainty little feet. Under Floor Heating does the business.
--
BigWallop

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In uk.d-i-y, BigWallop wrote:

Thanks, that's useful stuff. It's actually going to be a *slate* floor which makes a difference in detail - the slate will be on top of a substantial new plywood base, and I assume that the heating snake goes between that and the slates, along with the cement.
I was thinking that slate might feel cold even in summer, which is why I was considering electric heating, or water heating teed into the DHW primary. All I want is to take the chill off the stone, because the towel rail should heat the air OK.
--
Mike Barnes

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Mike Barnes wrote:

No. Bung it under the ply. Its too mucgh faf to lay in the cement, unless yoiu go electric.
You CAN get a bit sexy, and put insulation (probably foil backed celotex) underneath to make sure the heat goes upwards, but essentuially you are creating a warm space under the floor, which warms the floor. Its such a large area the fact that wood and slate are not prefect conductors merely means you get to walk on it with bare feet without taking the skin off them :-) rather than making it so inefficient you won't notice it.

Bleive me, a single pair of 15mm pipes under such a floor heats uit wonderfully for about a foot either side, so going for that suggestion of about 12" apart will be perfectly adequate. Its not the primary heating for the room, so no need to get silly with it. Trust me and the other responder. Snake the CH pipes around under the floor, lay the timber and slates, and relax and have a toasty bathroom. You will be utterly amazed at how nice it will be.

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In uk.d-i-y, Mike Barnes wrote:

Can I take from the lack of response to this remark that everyone would be quite happy with a heating system that only worked in winter when the main house heating was on? The room underneath is the kitchen and is normally pretty warm. If the bathroom floor feels cold to SWMBO's tootsies in summer there will be hell to pay.
--
Mike Barnes

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Mike Barnes wrote:

You could run the hot water primary under there. However, I've got all this, and no one has complained about cold tootsies in summer.
In fact, even on unheated screed floors in winter, the tiles are warm from the room heating, in the one bathroom that is like this.
The 'dog shower' so called because its a wet room designed for shampooing the mangy hound, has got UF heating, and SWMBO uses it more than any other - maybe thats cos the other showers are not finished yet tho :-)

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BigWallop wrote:

This is simply both not a big issue, and not entirely true.
ALL that matters is the relative heat conduction paths upwards through the floor, and downwards through the ceiling, plus the total amount of heat being pumped in - which is a function of teh water temp and how much pipewprk you have.
IF this was a pukka 'in screed' system tou would already have had to REDUCE water temp because of the dangers of screed cracking. In this case here, the water temp an be left at CH normal (60-70C?) as teh airgap between the pipe and the floor will develop a tempeature drop.
Transfer efficiency doesn't matter - in essence the heat is not lost - it stays in the pipes for other radiators!!!
All you want is a bit of heat bleed from the hot underfloor space up wards through the ply and slates. That WILL happen.
If that between fllor space heats up tp 60 C or so, do you really think those slates won't feel warm to the touch?

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BigWallop wrote:

I wouldn't use microbore, as it desn't leak as much heat as 15mm, but otherwise, I am with you on this.
You could even use proper UF heating (flexible butyl) pipe.

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I just don't trust that Butyl stuff under a finished floor NTP, I'd much rather go with copper pipe and good joints for some funny reason.
The suggestion of using the mini-bore pipe was the same reason as your last reply, i.e. it wont be the primary source of heating the room and will only be used to keep your tootsies warm on a cold morning.
Also, your last reply talking about foil backed Celotex to reflect the heat directly up toward the floor would probably work with this system too. I know the Romans used animal dung covered with polished copper to reflect the heat upward more efficiently on some of their UFH systems, so it is an idea I am thinking about trying myself shortly.
Worth a few moments contemplation.
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I
reflect
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On Tue, 8 Jul 2003 17:20:33 +0000 (UTC), "John"

Why not go down to parliament and load up? There are tons of bovine excrement available every single day! ;)
Andrew
Do you need a handyman service? Check out our web site at http://www.handymac.co.uk
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On thing to watch is that UFH is best carried out with temperature limited water ... 38deg C or so as a maximum. The water you are pumping in from your rad system might be too hot and cause hot spots on the floor.
Perhaps a mixing valve teed across your F & R might be worth considering.
If you can't bury the pipes in a good thickness of screed under the slate due to height limits - or on underside of the floor deck - then you may be better off going for electric.
Rick
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Not sure how cold the floor will feel without any heat under it. My sister has a slate kitchen floor onto screeded insulation, and right now this feels refreshingly cool under bare feet. In an upstairs room it should be a whole lot warmer. I can only suggest you get a couple of the slates out, let them aclimatise and see how they feel.

limited
cause
considering.
For small area like that it gets expensive to introduce separate zone controls so combining with the CH is the economic option. If UFH is needed in summer then a separate zone is the only option unless all radiators can be shut off. A pro job would involve a separate pumped and mixed loop controlled by a zone valve. I see in the BES catalogue an "FJVR return temperature sensor" (p/n 15764) plus valve body (p/n 15765). It looks just like a standard radiator TRV except it measures the water temp and costs 25. This will restrict the water exiting the UFH loop so that the water within the loop has the opportunity to cool to the recommended 40deg or so. Of course a bit of adjustment on the flow temperature will be needed, a warm floor in winter, just take the chill off in summer.
Added to a digistat and zone valve this is the option I have selected for a solid kitchen floor of 14m2.
If you decide you may only ever need the UFH infrequently the easier installation of electric mat within the tile adhesive layer will win. You should get about 125 w/m2 from the mat. I believe it's a close call for your requirement.
Toby.
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On Mon, 7 Jul 2003 09:30:44 +0100, Mike Barnes

I'd concur with this. I have slate floor on insulated concrete base over most of the ground floor of the house. Typically I wander round in bare feet and it never feels cold. I think it tends to acquire the temperature of the surroundings and my impression is that compared with ceramics or marble it is a lower conductor of heat anyway - hence not feeling cold underfoot.
Another useful factor is that even if you use calibrated slate (i.e. machined both sides), it is still not slippery when wet.
.andy
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In uk.d-i-y, Mike Barnes wrote:

The experiment has run long enough to know that we definitely need some sort of heating in summer.

I think that's where I'm going to end up. But for now the important thing is to get the pipework under the floor. I can fiddle with the controls in the airing cupboard later.
Thanks to everyone for their contributions.
--
Mike Barnes

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