Preventing heat loss through a cement slab and other questions

[jadnashua]

As you get below the frost line, the earth temperature tends to be the average temperature of the year - it maintains a fairly constant temperature. Unless the soil is wet under there, or your slab is on ledge, adding heat will act like a big heat sink. It may take awhile to heat it up, but once it is warm, it will stay that way for awhile. The biggest problem is if there is any moisture that is flowing...it can remove the heat faster than it can be built up. that being said, while I like the idea of Wedi, I'm not sure it would end up being cost effective. Nice stuff, though, and I'd use it if cost weren't a factor just because. Depending on how level your slab is, slc might be worth it to both flatten and level it. You'll need a long level or a laser level to determine that. The longest straight edge you can come up with (maybe a nice straight - yeah, right try to find one) 2x6 would work. Wider boards can often end up being straighter than the thinner stuff, but are harder to maneuver.

It might be worth it to hire out the slc - probably not a big enough area to justify a mixing pump, but you'd have to talk to the people. If you have a good crew and a bunch of people mixing, doing a large area is just hard work, not impossible.

[bshockli]

Star you have to be mistaken about 2 kWh a month. In cwp's case of 400 sq-ft @ 12 W/sq-ft. This calculates to 4800W. That is 4.8kW - if it ran for only a half hour a month it would use 2.4kWh.

Star was right about it not running all the time so ragdoll's calculation are off the mark as well.

Another thing to think about is that 4800W at 120V is 40A (need three 20A circuits to not overload). If it is a 220V system it carries 20A.

Concretes thermal conductivity = 0.72 W/m-K
Plywood's thermal conductivity = 0.12 W/m-K
Air' thermal conductivity = 0.0263 W/w-K

q = energy
k = thermal conductivity
A = area
dT = change in temperature

q = -kA(dT) Fourier's Law

If...
the temperature difference between the steady state operating temperature of the floor heater and concrete is equal to the temperature difference between the the floor heater and the air.
Then...
concrete will strip energy (heat) away from floor heater 27 times faster than the air in the room. (unless there is air movement - then you have to factor it as convection not conduction)

Problem is... the chance that your ground temp is the same as the air temp is very slim. More likely you have something like this...

Operating temp of the heater = 90-110°F
Air Temp = 70°F
Ground Temp 50-70°F

If the above happens...
Concrete strips q (heat) 37 times faster than the air in the room.

I am temped to apply convection to the equation and see how things compare - I will get back to on that one.

[cwheel]

Yeah, something's not right with that 2 KWh number. What is more likely is that you are averaging 2 KW/hr for the ENTIRE month. That would put your total monthly electric bill around $140, which is more reasonable for a decent sized house.

bshockli...thanks for that post the thermodynamics nerd in me appreciated it. The one thing to keep in mind is that your results for how much more the concrete absorbs energy versus the air are not entirely valid because you did not include convection (as you had indicated). Once a steady state temperature in the slab (and tile) has been achieved, the convection between the tile and air, along with the radiation from the tile to the room's objects, will vastly outweigh the energy transmitted from the warm concrete to the slightly cooler earth. At least that's what I predict.

In addition, the thickness of the tile compared to the concrete slab means that the energy transmitted from the wire thru the tile should be greater than from the wire thru the slab to the earth. Remember that the heat conduction thru a surface is inversely proportional to the thickness of the barrier.

[Regis @ Danfoss]

Yeah. Wouldn't be too worried about heatloss in SoCal.

However. Heat doesn't rise

Hot air rises (and i'm full of it) but heat will go to the point of least resistance first. So even in SoCal you'll lose efficiency to the slab. I've use cork underlayment before, but WEDI is the best. It's easy to cut and worth the price.

If you're going to tile in one step, do it over a mat system (Nuheat, Danfoss, etc.). The cable systems (Warmwire, Flextherm) are good too, but the strapping is a pain on a cement slab, and because the strapping is thicker than the cable, you have to level first. And you can't self-level on WEDI. At least not the last time i tried...

Your installation will be more efficient with WEDI, so go ahead with that. As for consumption, my kitchen in Chicago runs me $18-something a month in the winter time (I have a little Kw/meter on the circuit since i work for the big guy). At least it gives you an indication...

Regis

[cwp]

Regis -

So, you said, "The cable systems (Warmwire, Flextherm) are good too, but the strapping is a pain on a cement slab, and because the strapping is thicker than the cable, you have to level first. And you can't self-level on WEDI. At least not the last time i tried..."

You're right, WEDI doesn't recommend SLC, so I figured I would have to use thinset to level. I was planning on putting WEDI down (as insulation over the slab) and then using Danfoss LX cable and using thinset over it as a skim coat/leveler and then once it dried, using thinset again to lay my 18/18 tile. Is the strapping too thick for that, or will that work to level?

And when you mentioned cable systems, you didn't mention Danfoss LX, and you work for them. How come that wasn't on your list?

[xxPaulCPxx]

Per my conversation with WEDI... they said they didn't know if it would work, because they had not tested it.

Have you Regis actually done SLC over WEDI?