Underfloor heating calculations

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I have an interesting 'situation' with one of my clients, and wondering if anyone one here has some experience of underfloor heating in screed.

The job is a new build outbuilding/office in a garden, only about 22sqm.

The client sourced and purchased underfloor heating cable to install in the screed during the build (the supplier did the calcuations and specced a suitable product) and I was asked in to test and connect up.

The tests were all fine and the heating was connected up temporarily earlier this week, but has proved completely unable to produce any real heat on the surface of the floor at all - the most that can be detected is minimal warmth in some areas, and a lack of cold in the rest. The clients (not calibrated!) thermometer registers about 15C when left on the floor.

Now I've looked at all the paperwork, the supplier specified a 2100W cable, equating to just over 96W/sqm. This is installed as required, with insulation below and 70mm screed on top.

Given that most mats (which are only installed in 5-10mm adhesive/compound) seem to run at 150-200sqm, it looks to me as if the cable is just not up to the job of primary room heating, which is what the client wanted.

Having said that, I have little experience with under floor heating, so anyone out there have a more educated opinion?

The original supplier are going to come and 'inspect' the job, but it now appears that 99% of their work is wet, so have they just got their calculations wrong?
 
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I am the same as you & do not have much experience of u/f heating but I have yet to see one that the user is happy with(wet or dry), they seem to me to be being very oversold.
 
I've 'installed' underfloor electric heating several times (ie I've worked with the builder and completed the wiring). I also have it installed in my bathroom. Properly specced and installed it works just fine. Typical recommended thermal density is around 100w/m for background heat and 200w/m for foreground. Background level is usually just to take the chill of tiles for bare feet.

Measuring the temperature of the floor isn't a very reliable way of testing because even at a room temperature of 21 degrees it will feel cool (try sticking your hand into water at 21 degrees). At these low power levels the temperature differential between the floor and the air is negligable. Has it been left long enough to stabilise? It will take a l-o-o-o-o-ng time to stabilise through 3 inches of concrete! Has the control been set to respond to air temperature (usually there's a choice of whether to use air or floor temperature and the latter doesn't work very well except for an overtemperature trip).

The main issues are to do with losses through the floor (type of materials and depth of installation. I think a garden office (4 exterior walls) of around 4x5m would take at least 3kW to warm it up in a decent time from cold and maybe 1-2kW to keep it warm. This installation sound marginal with a very long stabilisation time especially given the recent low temperatures.
 
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robertw63 said:
It will take a l-o-o-o-o-ng time to stabilise through 3 inches of concrete!

It certainly will. The volumetric specific heat of concrete is about 8MJ/cu.m/°C and you have 1.5cu.m of the stuff in that floor (22sq.m x 70mm). A quick calculation shows that, from a cold start, your 2.1kW heater will take over one and a half hours to raise its temperature by one degree - and that's with perfect insulation underneath! :eek: :eek: :eek: Underfloor heating is not a good choice for rooms that don't need constant heat.

Are you sure that 2.1kW is enough for a 22 sq.m room with four outside walls? The correct way to do this is to calculate the heat loss through the boundaries of the room. It's not that hard to do. Here's an example: A brick cavity wall with insulation has a thermal conductivity of 0.5 W/sq.m/°C (at least that's what my old book says). Multiply this by the total wall area and then by the worst case temperature difference. If your wall area is 50 sq.m and you want to stay warm when it's five degrees below outside, you'll lose 0.5 x 50 x 25 = 625W. The hardest part is finding the correct thermal conductivities for the materials in that room. Maybe somebody on the building forum can help. In my experience, windows often lose more heat than all the other surfaces put together.
 

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