Step 1: Overview
This in-the-floor heating system consists of one thin continuous cable heating element woven into a mat that you install under the tile. This makes it a project best done when overhauling or changing the floor covering of an existing room or when adding a new room. It can be installed as supplemental heat to take the chill out of the floor or as space heat to warm the entire bathroom. It's also a great project for warming entryway and kitchen floors.
- It's easy to install. You embed a cable-laced mat in the mortar when you lay the tile. If you're not comfortable with the wiring portion, hire an electrician.
- It's safe. Once the heating system is installed, it's nearly impossible to damage. The GFCI-protected thermostat instantly cuts power in the event of a short or other problem.
- It's inexpensive to operate. At 12 watts per square foot, our 30-sq.-ft. mat drew 360 watts of power— about the equivalent of an electric blanket or large TV.
- It takes up zero space. Got a big, clunky radiator? Remove it and gain valuable square footage by installing this stuff.
- It's versatile. If your existing furnace or boiler doesn't have enough oomph to heat a newly remodeled or added space, floor heat can do the job.
- It's really, really comfortable. When your feet are warm, your entire body feels warm. You'll find yourself reading and playing games with your kids on the bathroom floor.
The downside? It can't be retrofitted under existing tile floors, the total initial cost of materials is high, and you'll most likely need to run new wiring from the main circuit panel to the bathroom.
Step 2: Find electrical power
For a heated floor area less than 20 sq. ft., you could (in most cases) draw power from an adjacent GFCI-protected outlet without overloading the circuit. (If the thermostat you purchase is already GFCI protected like ours, you can use any outlet. In any case, the mat must be GFCI protected.) But a larger mat on an existing circuit— a circuit that might also accommodate a 2,000- watt hair dryer—can cause overloads and nuisance circuit breaker trips. For our larger mat, we elected to install a dedicated circuit with its own wiring and circuit breaker. Both 120-volt and 240-volt mats are available.
A programmable thermostat that turns the mat on during busy times, then off when you're sleeping or away, costs more initially but will save energy and money in the long run.
Step 3: Special-order your custom-size mat
A number of companies offer electric resistance floor warming systems. Standard sizes are available at home centers and tile stores. You can also special order custom sizes and shapes by sending a detailed drawing of the bathroom floor plan and location of fixtures. The mats come in 12-, 24- and 30- in. widths and increments of 5 ft. in length (10 sq. ft. minimum). When in doubt, the supplier will specify a mat smaller than you need since the mat cable can't be cut.
Before installing the mat, use a volt-ohm meter (Photo 1) to obtain a resistance reading to make sure it wasn't damaged during manufacturing or shipping. Prep your floor as you would for any tiling job. Install 1/2-in. cement backer board, securing it to the existing subfloor with mortar and cement board screws (Photo 2). Make sure no screw- or nailheads protrude above the cement board. A sharp edge can damage the cable. Tape and mortar the seams to create a solid, continuous surface. Snap tile layout lines on the floor once the mortar has dried.
Step 4: Test-fit the mat to avoid glitches
Before proceeding with the actual installation, do a test layout (Photo 3). Follow these basic guidelines:
- Install the mat up to the area where the vanity cabinet or pedestal sink will sit, but not under it; that can cause excessive heat buildup.
- Keep the mat 4 in. away from walls, showers and tubs.
- Keep the mat at least 4 in. away from the toilet wax ring.
- Keep the blue heating cable at least 2 in. away from itself (Photo 5). Never overlap the cable.
- Don't leave large gaps between the mats. Your feet will be able to tell!
- If your mat is undersized, give priority to the areas where you'll be standing barefoot most often!
Following your preliminary layout, mark the path of the thick “power lead” between the mat and wall cavity (Photo 4) and chisel a shallow trench into the floor. Notch the bottom plate to accommodate the two conduits that will contain the power lead and the wires for the thermostat-sensing bulb.
Step 5: Glue and tape the mat in place
Install the mat, securing it lightly to the cement board with double-face tape (Photo 5). To make turns, cut the mat between two loops in the cable, then flip the mat and run it the opposite direction. Never, ever cut, nick or stress the cable itself. Where the full-width mat won't fit, or where you encounter angles or jogs, carefully cut the mat from around the cable, and hot-melt glue the cable to the floor (Photo 6). Continue using the full mat again when you can.
Install the entire mat complete with cuts, flips and turns to make sure it fits the space right, make any final adjustments, then press the mat firmly into the tape. Use hot-melt glue to additionally secure the mat. Don't leave any humps or loose edges; you'll snag them with your notched trowel when you're applying the thin-set mortar.
If you're not going to tile right away, lay thick corrugated cardboard over the mat to protect the cable. You'll be glad you did when your kid walks in wearing baseball cleats.
Step 6: Wiring setup
Install conduit connectors to both ends of two pieces of 58-in. long 1/2-in. electrical metal tubing (EMT). Fish the power lead cable through one length of conduit. Hot-melt glue the power lead into the groove. Fish the thermostat wires through a second piece of conduit, then weave it 12 in. into the mat, keeping it equidistant from the cable on each side (Photo 7).
Secure the two lengths of conduit to a 4 x 4-in. metal electrical box. Secure this box to the studs so the lower ends of the conduits nestle into the notches you made in the bottom plate (Photo 7). Secure metal protective plates over the notches in the bottom plate to protect the wires and cable where they pass through.
Install the wiring from the area of the main circuit breaker panel (or nearby outlet) to the area of the wall cavity where the thermostat will be located. Don't do any actual wiring in the main panel yet.
Step 7: Install the tile
Select tile that's at least 6 in. square so each tile will span two or more sections of cable. Smaller tiles are more likely to conform to the minor hills and valleys of the cable when you tamp them in place, creating a wavy surface.
Spread the mortar over a 5- to 10-sq.-ft. area of floor. Use the flat side of the trowel to press the mortar firmly through the mat and into contact with the cement board. You can establish a flat, uniform layer by lightly floating the trowel across the tops of the cable. Then use the notched side to comb the mortar to create ridges (Photo 8); a 3/8 x 1/4-in. trowel works well for most tiles. Again, lightly skim your trowel over the cable. The sheathing on the cable is tough, but you still need to avoid any “sawing” type action or jabs with the trowel. It takes a little trial and error to get a flat layer. The No. 1 goof that people make is slamming the edge of their trowel on the floor to knock excess thin-set loose—cutting or nicking the cable.
Place the tile, then tap it firmly into place with a rubber mallet. Do two resistance tests (Photo 1) while installing the tile to ensure you haven't damaged the cable. (If the resistance test fails, see the manufacturer's instructions to find the problem.) Once the mortar has dried, grout the joints.
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Step 8: Final steps
The instructions that came with the mat and thermostat were so darn good we felt comfortable completing the wiring of the thermostat and mat. We left installing the new circuit breaker and final connections in the main circuit panel to a local electrician. We suggest you do the same. Once the wiring is complete, energize the system for a few minutes to make certain the controls work and resistance cable heats. Don't put the system into full operation until the thin-set and grout have had time to properly cure and harden—usually two to four weeks. Then call the family together and play a game of Scrabble on your cozy, warm bathroom floor.