Install Electric Radiant Heat Mat Under a Tile Floor
Electric radiant heat mats with wall-mounted thermostats are easy to install under new tile floors and provide comfortable, invisible heat at a reasonable cost. They work well as supplemental heat in bathrooms, kitchens and entryways.
Steps
- Prepare your floor for tile by installing tile backer board on the floor, securing it to the existing sub-floor with thin-set mortar and cement board screws or nails.
- Make a scale drawing of the bathroom floor, including toilet, tub and vanity locations, and bring it to the tile store or home center so you can buy the proper size mat or combination of mats. Points to keep in mind when sizing the mat are: 1) 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; 2) Keep the mat 4 in. away from walls, showers and tubs; 3) Keep the mat at least 4 in. away from the toilet wax ring; 4) Keep the heating cable at least 2 in. away from itself. Never overlap the cable; 5) Don’t leave large gaps between the mats. Your feet will be able to tell!; 6) If your mat is undersized, give priority to the areas where you’ll be standing barefoot most often.
- Check the wiring with a continuity tester, after purchasing the mat, to make sure it wasn’t damaged during manufacturing or shipping.
- Install an electrical outlet box {{safesubst:#invoke:convert|convert}}. up in the wall near where the wiring leads from the mat will terminate. Run new wiring to the box from the circuit breaker panel or - if the mat is small - from a nearby outlet (consult manufacturer's instructions for more information about wiring requirements).
- Note: following your preliminary layout, you should mark the path of the thick “power lead” between the mat and wall cavity 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.
- Draw the layout lines for the tile on the floor. 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.
- Install the mat, securing it lightly to the cement board with double-face 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. [NOTE: This installation shows an open type of mat that is mortared and tiled in one step. Other types of radiant heat mats must be mortared in place first, allowed to dry, and then tiled. Follow the manufacturer's instructions.]
- Check the mat wiring again with the continuity tester.
- Install conduit connectors to both ends of two pieces of 1/2-in. electrical metal tubing (EMT) that are long enough to run from the floor to the electrical box for the thermostat. 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. Nail metal protective plates over the notches in the bottom plate to protect the wires and cable where they pass through.
- 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; 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.
- Lay the tile, then tap it firmly into place. Do two resistance tests 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.
- Connect the power lead and thermostat wire to the thermostat, following manufacturer's instructions. Use a programmable thermostat so the mat will only come on when it's needed. It costs more initially but will save energy and money in the long run.
Warnings
- Don't slam the trowel against the floor to knock the mortar off - you could accidentally cut the heating cable.
