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Terry68 06-08-2010 02:41 PM

radiant floor heat over slab subfloor
We are remodeling our kitchen and would like to install radiant floor heat under tile to provide supplemental heat in the winter. Our kitchen is on an elevated slab foundation. I'm getting mixed advice regarding the best install method. The system I plan to use is 240V wire to cover 200 sq. ft of heated space. Because of the slab, the manufacturer recommends installing cork underlayment as a thermal break to reduce the heat loss into the foundation, but gives no specific instructions in the install manual. A local tile company told me in our area, the underlayment would provide minimal benefits (we live in South Carolina), and not to use it. A representative of the radiant heat company suggested using double-sided tape to adhere 3 mm cork sheets to the slab (I was concerned about the thickness of the new floor), and proceeding with the wire, mortar, and tile install. He said using a full floor adhesive may release fumes if the radiant heat system warms it. This doesn't seem stable, and I'm worried about future cracking. Any suggestions?

Bud Cline 06-08-2010 05:35 PM

The cork is a bad idea. Installing the cork with double sided tape is ludicrous. I know people want to believe what they hear from a manufacturer but the suggestion to use the tape is asking for trouble. Those guys want to sell their heating product and anything they can do or say to make it sound easy and inexpensive they will do or say to you.

Here's your best bet:

Use a product called DITRA from Schluter Systems. It is suggested the DITRA be used over the heat system but this is not totally necessary. DITRA can be used under the heat system if you think you require a thermal barrier, which you don't. Here's Schluter's suggestion:

Schlüter-DITRA is also recommended for uncoupling of radiant heated floors consisting of thin electric heating mats. Schlüter-DITRA may be installed either above or below the heating mat. However, the uncoupling function is more effective if Schlüter-DITRA is installed above the heating mat.

You should understand that the electric heat mat style floor warmers are not intended to supplement the heat of the room. They are merely intended to heat the floor covering (tile) and provide you with a warm-to-the-touch feeling.

I have installed countless electric floor heat systems and I can promise you the fact that they are installed over concrete is not at all a major concern. The systems work and work well.

I personally think the only way to install the heat system is to bury it in Self Levelling Compound to create the need thermal-mass. If you think you need to separate the heat from the concrete then DITRA is the thing to do but DITRA isn't really necessary either in this case.:)

Scuba_Dave 06-08-2010 06:23 PM

My bathroom is only heated with radiant electric heat
I can heat the bathroom to 80 if I want
I used "raw" heating wire instead of a heat mat - more work, but not too bad
Spaced at 2" apart it provides 15w of heat per sq ft

So you need to be careful of what you buy if you intend to heat the kitchen with radiant electric - as Bud stated
The pre-made matts are spaced further apart
And around here it costs more to heat with electric VS Oil etc

warmsmeallup 06-08-2010 07:00 PM

We do nothing else but install low voltage and line voltage electric radiant hfloor warming and primary heating systems. I agree that using Ditra will not give you the thermal break you are looking for. Nor should you use taped down cork as a thermal break.

If you want to make the best of the heat generated by not heating the uninsulated slab below, you're going to need to use up about 1" of the floor height to build an insulator below the tile. If you have the height to play with then use a 3/8" polystyrene insulator/vapor barrier (Northeastern Ohio Foam Products, NOFP.COM, makes a great one), then use 1/2" backer board (cement board) over the top of the insulator and then your mortar and tile. This design will help to direct all the heat generated upward.

We don't install tile so I can only parrot what I am told there but the word is that you need a minimum of 1-1/4" of solid material below the tile for rigidity. So, again, if you have the floor height to play with, build it whatever way you need.

It's correct that to cover 200sf you need a 240v/30a dedicated circuit to run the mat/cable system however, most GFCI thermostats are only good up to 16a. Your system will draw 21+ FLA. So, be sure you don't also need to have a (30 or 40 amp) contactor that activates the mat(s) via the tstat. This is if you get a decent mat that puts out at least 12 watts psf.

It is also not advisable to use line voltage as primary heat. Though it may meet the load requirements (I'll repeat, may meet), I wouldn't install an element that only comes with a 10 year warranty as the sole source for heat. That's not to say that there aren't any 15 year old line voltage systems still working. But I wouldn't want to find out at 11 years I don't have any heat and the element needs replacement.

On the other hand, low voltage can be use as primary, if that's what your're looking for, because 1) you can adjust the output to meet the load requirement and C) it comes with a 25 year warranty. It is more expensive than the line voltage mats or cable but you get what you pay for.

Kat13 06-12-2010 10:00 PM

Stupid Question
What exactly does the non programmable thermostat do? ( or does not do )
Are you able to adjust the temp. or is it just an on/off switch.

warmsmeallup 06-13-2010 07:43 AM

The better programmable thermostats allow you to set a desired temperature for different times of the day, usually 4 weekday set points and 4 separate set points for weekends. (i.e. 72 when you wake up, 68 when you leave for work, 72 when you arrive home from work and 65 at bedtime.)

Non-programmable just set to a single temperature that you change manually when you want something different. If you've ever had one of those old style dial thermostats where you turned the dial on the face to a temperature and walked away, that was a non-programmable thermostat. Some just have a 1 - 5 setting on the face, no temps at all.

Daniel Holzman 06-13-2010 09:20 AM

The amperage requirements presented by Warmsme appear to be incorrect. The OPS is building a 200 sf floor system, which AT MOST will draw 15 watts per square foot (my kitchen floor heat system draws 10 watts per square foot). The maximum draw will be 3000 watts, which is less than 15A at 240 volts. I think the number presented (30A) was for a 120 V system, which would produce 3600 watts of heat at 30A, well above the maximum draw of 3000 watts.

warmsmeallup 06-13-2010 10:47 AM


Originally Posted by Daniel Holzman (Post 455531)
The amperage requirements presented by Warmsme appear to be incorrect.

Dan: your right! Thanks for correcting me. 200sf using 240v (we use 15 wpsf product) is about 10a. I used the 'wrong' calculator, the batteries were dying, the key's were sticking...;)

Kat13 06-13-2010 05:52 PM

More stupid questions:

Is there just a simple on/off switch that can be installed. (Do they make one for the floor heating system?)
Or do you have to use the thermostat?
As long as the tiles are warm, I wont need to adjust the temp. I just want to be able to turn it on & off, as needed.

What about a dimmer switch. LOL :whistling2:

Scuba_Dave 06-13-2010 05:58 PM

I've yet to see one where you do not use a thermostat
Left on 100% power will greatly shorten the life I'd think
I went with the programmable thermostat

It has a power bar indicator...5 bars is full power...down to 1
Most cases it stays at between 1-3 unless I really turn it up

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