How to Install Suntouch WarmWire In-Floor Heating Part 2

by Roger

Installed WarmWire Heating Elements

Figure 8

Now that your floor looks like a can of silly string exploded (figure 8 ) its time to add more layers to it and cover up all your hard work.  If you have not yet done all the hard work then your floor doesn’t look like that. Check out How to install WarmWire Part 1. I’ll wait.

Okay, you may want to check out a speed reading class. Just sayin’. :D My preferred method is Schluter Ditra underlayment installed atop the WarmWire for your tile installation.

The best method of leveling out your floor for your Ditra would be an SLC or Self-Leveling Cement. This product is mixed with water and poured over your WarmWire. When mixed properly (follow the instructions to the letter – really) and poured it will – wait for it – level itself. When cured you will be left with a level, flat floor.

You can actually install your tile directly to this layer if you chose to do so. I do not chose to do so. I prefer to have an additional uncoupling membrane above these layers then my tile. That’s just how I roll.

For purposes of speed (SLC requires 24 hours to cure) I will level the floor above the wires with mortar. If you chose to do this, and you may since it is considerably less expensive, you need to take absolute care with the method I utilize. You risk damaging the wire while leveling the floor so be careful.

To level the floor above the WarmWire with mortar I use a 3/16″ square notch trowel. Mine is metal – yours should not be! You can purchase a plastic trowel make exclusively for use over the wire. These are much safer to use as they would require actually trying to damage the wire with it. It’s difficult to damage it accidentally if you use the plastic version.

The most difficult thing to do over in-floor heating elements is installing tile properly. That means flat, flat, flat. By using the ditra and utilizing my method you will begin your tile installation with a flat substrate over the heating element.

I mix my mortar (thinset) fairly thick for this. It assists in holding the shape and getting absolute coverage with the mortar – no voids in the mortar bed. I will first use the flat side of my trowel and fill all the areas between the wires moving my trowel in the same direction the wires are running.

Leveling over WarmWire for Ditra Installation

Filled and Combed

In the left side of this photo I have the flat troweled the mortar and in the right side it is ‘combed’. Notice everything is running the same direction as the heating wires.

Once that is all filled in I will flip the trowel over to the grooved side and ‘comb’ the thinset in the same direction by placing the trowel so the wires are between the grooves. That is so that the teeth of the trowel are actually touching the floor beneath the wire and the wires are between the notches. This will give you a grooved bed on which to install your Ditra.

Once that is finished place your Ditra over the top and grab your straight edge, level, 2 x 4, or whatever you have that is straight and handy. You want to press the Ditra into the thinset to it fully embeds into the fleece on the back of the Ditra. Place your straight edge on top of the Ditra and run it back and forth in the direction the wires are running or at a slight angle (figure 9).

Leveling Ditra over WarmWire

Figure 9

If you run your straight edge in the other direction you will end up with a roller coaster for a floor. Your straight edge will go from the top of the wires into the spaces between the wires and back up. It will not be flat if you do it in this manner. By keeping it either in the same direction, or a 45 degree angle, as the wires it will always remain on top of the wires for the length of your straight edge. Your floor will come out flat and even.

You need to make sure you put enough pressure on the straight edge to embed the Ditra fleece into the thinset. If you stand back you can see the difference in the color or shade through the top of the Ditra. It will turn a darker shade when embedded correctly and you will be able to see any spots you may have missed.

Leveling Ditra over WarmWire


Installed and Uninstalled Ditra over WarmWire

Embedded and not

The left side of this photo shows the Ditra embedded correctly and the right side the Ditra is simply laid onto the mortar bed without embedding it yet.

Simply keep doing this as you walk (crawl) your way out of the room. Be careful not to step or kneel on any parts you’ve finished. There is a considerable amount of mortar beneath your Ditra so it is easy to put ‘dents’ in it with your foot or knee. As long as you use a fairly decently sized straight edge you should end up with a perfectly flat, stable floor ready for the tile of your dreams.

After 24 hours you can walk on the Ditra to wire your heating element and install your tile. You need to wait this long for it to cure. Stepping or crawling on it prematurely will create dents or otherwise compromise your membrane and substrate. Be patient.

Ensuring Ditra is flat over WarmWire

Contractor Glamour Shot!

Wiring the control box and heating element is fairly straightforward. Read through the directions included with your control box to ensure it is wired the same as I am describing – it may not be. I don’t know, I can’t see it from here.

There are three things you need to attach to the control box: the power from the house, the heating element, and the temperature control probe. The power from the house gets wired to the ‘LINE’ side of the control box and the heating element is wired to the ‘LOAD’ side.

The power from the house consists of three wires. They should be white, black, and ground which is usually green or a bare copper wire. The ground is not wired to the box, only to the heating element. The black is wired to the black and white to white of the control box. Easy enough even if you’re colorblind. The black wire is the hot wire! It’s not like your car radio in high school. The white is neutral. This is the way it should be but I didn’t wire your house so don’t blame me if it isn’t.

WarmWire Control Box Wiring

Figure 10

The probe has two wires – red and black. They are wired into the appropriate clips in the back of the control box.

The heating element has white, black, and silver. White to white, black to black, silver to ground. Please note that in this photo (figure 10) I have two different heating elements running to the control box so there is twice as much electrical spaghetti hanging out of the wall. You should not have that many wires.

After it is all wired up just stuff all the wires straight back into the box and

WarmWire Control Box Pins

Figure 11

attach the control box to the wall. The face of the control box is plugged into the back plate with the pin cluster in the back of it (figure 11). Place it in there carefully. They bend easily if not lined up correctly – just trust me on that one without testing it for yourself.

Now you can turn the breaker back on. Oh, you turned that off, didn’t you? Sorry, if you are currently sporting a Yahoo Serious hairdo, it may be my fault. You want to turn the breaker off before you start wiring.

Now the bad news – after you install your tile you must wait 3 – 6 days before turning on your underfloor heating. You need to give your mortar plenty of time to fully (relatively) cure before zapping it with unnatural heat. Doing it sooner may compromise the strength of the mortar and, in turn, your tile installation. You don’t want to do that now, do you?

WarmWire Control Box Installed

Warm Tootsies!

You’ll just have to wear socks for a couple of more days.

There you go – warm tootsies for those cold mornings. If this has helped you out please send me a photo of your completed tile installation. I would love to see what people do with my good advice and bad humor. I keep tellin’ my wife that it is a great combination but, you know, she doesn’t believe me. Oh well, extreme jackassery is getting to be my tag line so why quit what works?

Send me your photos at [email protected] and lets see what you can do.

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if I got this right I first remove the vynil floor in the bathroom. Then lay down some thinset, put down backerboard and screw it into the floor. Then place down the radiant heat coils, then slc or thinset? Then ditra, then thinset and finally start tiling. 1) is that correct? 2) manufacturer says no sharp objects when laying the coils. Wont there be a problem after all the screws used in the backerboard the coils will find a sharp edge (screw head)



Hi Jeff,

The backer is not required, although you can use it if you want. The coils can go directly to the wooden floor, then coils, then ditra. The screws in the backer, if you use it, should be countersunk.


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