My understanding of cables is to derate them to 0% at 70degC. The belief is that below 70degC PVC won't creep sufficiently and above, it will. You're going to have to assume that the joists and air around the joists will be whatever temperature your boiler is set to unless you use a mixer to reduce temperature.
Floorboards at or above 50degC are going to hurt!!
I will have a mixer - but it's not necessarily the case that the air will reach pipe surface temperature, except in a later very close to the pipe. Calculations based on IEE regs data and other factors limit my design to max
50, preferably 45C ambient (edge case, 2.5mm2 T+E cable, required current capacity of >=20A)
Don't be daft, the *air* below them is at X degrees, where X might be 35,
40, 45 or 50 (the calculation of which is non trivial and certainly beyond me).
The heat has to transfer through air then wood, where the surface of the wood is being cooled by losing heat into the room, so it stands to reason that the air under the floor may be considerably hotter than the upper surface. But it could be 10 or 20 degrees below pipe temp, especially below the pipes: In Andrew's scheme, the pipes are about 1" below the floor surface.
Cables must be > 2" below, generally the might be near the mid point, so
3-5" below.
It's all going to be speculation other that a real life observation or a complicated set of calculations.
The relevant BS, BS EN 1264-31998, recommends a maximum floor temperature of 9 degC above the room temperature, i.e., 29 degC, 33 degC in bathrooms. You could work it out from that.
No-one uses copper for underfloor. PEX is preferable. The Rehau or Uponor crimp fittings are much preferable to push-fit.
There has been some criticism of 'staple-up' used without radiant plates in the US trade press; the design heat emissions often aren't achieved. .
Like U values. Probably try a heat loss type calc where the temperature differential is the difference between the assumed floor void temperature and the room temperature. Increase the floor void temperature until you get the required heat emission to the room.The BS gives temperature drop through floor construction graphs.
Heat transfer plates would be best, the pipe would be in contact with the floor, the pipe would have a larger effective area and the insulation could be close to the pipe.
I don't have any way to measure it now -- there's no access under the floor.
I may be there later in the week -- I'll take an IR thermometer with me and take some measurements. There was only one wire there, which goes to the hall light. It was threaded through a mixture of the floor joists and ceiling supports (bottom of floor joists wasn't level so there were extra ceiling supports fitted under the joists). What I did do was to unthread bits of it that looped up and layed it all on the ceiling, so it's going to be pretty much same temperature as the ceiling, which should be the coldest part under there.
The home owner is extremely pleased with the scheme, and has commented on how well it works. The tiled floor doesn't feel hot on the feet, but if you stop and think for a moment, you would expect it to feel cold, and it doesn't. I've been curious about what temperature it actually runs at, so I'll check with the IR thermometer. I didn't know how much or closely spaced 10mm copper tube I'd need to get the right heat transfer, and had no time to try it, but it seems to have been pretty much spot-on.
Ah yes - that might get an approximation - forget the air and just see what the board lower surface would need to be to get the surface to say 25-30 (which is a reasonable guess for UHF surface temps). I'll have to go and dig out some U-values for wood.
Undoubtedly that would be best - would rather nadger access under the floor though.
Andrew Gabriel coughed up some electrons that declared:
I realise my request is a PITA - if there were any indicators between it being "warm" or "hot" down there, it would be very interesting to know.
I'm short of space for rads upstairs (leaving fan convectors as the next option), but I do have infinite underfloor space. I'm quite inclined to do UFH downstairs, so I'll have water available at full temp and reduced temperature anyway (maybe your system will need higher than normal UFH temp but less than 80C which is what I may run the thermal store at so I might need another mixer).
All my cables are going to be there - my design sheet shows it's the rings on 2.5mm2 which are the bounding case.
That would help, though I'd not be happy to do that with all my wiring for obvious reasons. I did think about suspending an inch of wool under the heating pipes, but it rather destroys one of the obvious benefits of your design which is it doesn't make access to the underfloor void impossible (like a plate system would). The other benefit is your system is 30-50 quid's worth of copper tube and elegant simplicity :)
I guess, I'd take a risk and try it and monitor it closely.
The system has a condensing boiler, but it's never operated in condensing mode because it has a user-accessible knob on it, and the owners just turn it up if they feel cold (not that it changes the house temperature anyway) and of course never turn it down again. I've given up explaining what it means to have a condensing boiler. The radiators are not sized for condensing operation in any case, so it would probably need turning up during cold spells (and again, would never get turned down).
I rather suspect that's how most condensing boilers are really used, other than by a very small minority of users who understand what they are.
Yes, indeed. I have got a TRV at one end of the circuit though, poking out from some boxing. The system relies on not having a heat spreader and having poor thermal conduction to the floor, rather than using a mixer to regulate the floor temperature. Now if I was installing it for me (with a correctly running condensing boiler), I would probably have had to use a spreader (and hence also a mixer), as my boiler is normally run at 45C flow, and otherwise there may not be enough heat transfer (or more pipework would be needed).
If you have the luxury of time to try it, I suggest you do so. I didn't have that luxury. The nearest I got was knowing the radiator pipes under the floor in a few places did locally heat it.
But not very efficiently. It needs a spreader plate and a mixer to lower the return temperature. It appears to be keeping the return temperature to the boiler up reducing efficiency.
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