SWA in U/F floor slab

Do you have any idea of the operating temperature for the UFH? Mine works at just below 40 degrees (and barely perceptible on the floor surface) - but that's under timber floors in a well insulated house - under concrete it could be a fair bit higher especially if the thermal insulation of the building in general isn't great. Even at 40 degrees there's be reasonable de-rating factor (compared with the tabulated ambient of just 20 degrees for a buried cable).


I'm not sure how much benefit there's be putting the cable below the insulation - it should be a little cooler there of course, but with insulation and a higher temperature layer above and a long way sideways to free air it's hard to see where the heat will go (other than downwards into the ground). It's not that long ago building regs didn't require insulation for floors with a large area compared to their perimeter - since once it had warmed up the heat loss downward into the ground was considered small.


There's no alternative route, I suppose?


    - Andy.

Andy,

When you say 40', is that the actual temp of the slab? Am assuming 'C rather than 'F! The new area will be heated from the existing GSFH set up, so just another manifold and pipework to add. Don't know what the rest of the system is operating at in terms of final slab temp, but will try to fins out. Am hoping 16mm will do because there is an area of the route where it will have to come quite shallow, and 25mm might not be able to be buried fully enough.

All is a bit ambiguous at the moment. I know there will be a high power all-electric range cooker, a couple of washing m/cs and tumble dryers, plus the general kitchen kti like kettle/toaster etc, then there will be a lounge area with the usual TV stuff etc, so am thinking 1 x 50A cct for the cooker and 2 x 32A rings for the rest, plus a 6A general lighting circuit. Nothing is finalized and no plans or other specs for kit etc exist. The job has been thrown at me at the last minute, and all I know is that concrete is booked to be poured next Tuesday!

Am thinking something like a 70A supply via 16mm 3 core to a new cu and then await further developments!

SWA in below the insulation. If there is any significant heat transfer downwards, it's not doing its job!

When you say 40', is that the actual temp of the slab?

On my system that's the water temperature (from the blending valve) - as it's below a timber floor there's no concrete slab as such (pipes just clipped to the sides of the joists). With a concrete slab it's not uncommon to run it at a higher temperature to get a better heat output - the limit being not so hot that the surface is uncomfortable to walk on in bare feet. Either way there'll be a temperature gradient between the surface and the pipes, but close-ish to the pipes it won't be much less than the water temperature.

 

SWA in below the insulation. If there is any significant heat transfer downwards, it's not doing its job!

But similarly where does the heat from the cable go? It can't go upwards or a little sideways and upwards like it can with a conventional buried cable. Going downwards there maybe a large thermal capacity initially, but also a large thermal resistance (like an old house with very think masonry walls - take ages to warm up but once they're warm they stay warm...)


   - Andy.

Hows about some M.I.C.C? Perhaps even two parallel runs to ensure reduced Voltage drop and reliability. The cable is physically small and can tolerate higher temperatures than plastic insulated cable.


Z.

No chance for M.I.C.C - Too expensive, plus I no longer have any tools for it. After FP200 came out, I no longer bothered with 'pyro', generally it is now only used in Hot Work areas, Chemical plants and petrochemical wks and filling stations.

I think ideally Id install it under the insulation. (Its cooler there)


But another option maybe - I don't often do houses anymore, but when I have done, I sometimes had this problem with Kitchen island units getting installed with wet underfloor heating - the insulation I've seen favoured for floors is 2" Kingspan (maybe it was 3"?) (I think its called Kingspan but might be something different for floors)) - and I'd be tempted to install 2" metal trunking on the floor slab (surrounded by kingspan on two sides, cold on the bottom. Heating pipes on top of that and then they tend to install quite a thick layer of (levelling compound?) - some type of concrete - 50mm or so I'd guess ontop of the insulation/around the heating pipes........I cant really remember the specifics of the build but the 2" metal trunking worked quite well for me. 


Surely the cables in metal trunking would be cooler than the SWA buried direct? Allows for future cable runs or comms/networks maybe too..


Regards


Tatty

My own UFH system is wet.  It comprises concrete slab, insulation, UFH tubing, engineered wood floor.  It is driven directly from a combi boiler, the boiler water i.e. the UFH water, cycles between 35 degrees C and 45 degrees C, the thermal inertia of the floor smooths this out and the floor is pleasantly warm to the foot, probably around 38 - 39 degrees.


I think the cable needs to go in the slab under the insulation which should be near the base soil temperature if the insulation is doing its job, it will be hotter above the insulation.


David

the electric mat underfloor heating (100-200 watts per m2) typically sets the floor temperature limit stat in the range 28 to 35 degrees, so not much derating for that sort of system


If you used 90C SWA cables, but applied the 70C SWA current ratings, then the rating will be OK for almost a 50C ambient, and even in the hot side of the slab it should not really exceed that..

But if you can, get your cables on the cold side of the insulation.

I'm going back this afternoon to take more more precise measurement of the cable run, but am thinking that only around 15 metres of the run will actually be under the slab, and the run abuts the wall on one side..

I don't know how close the pipes will be run but am guessing there will be a significant gap between the bottom of the wall and the 1st run of heating pipe.

Best to go under everything else then.