Installing Electric Radiant Heat

Intro

Electric radiant heat, installed directly over cement board, plywood, a mortar bed, or a concrete slab, is a plastic mat with interwoven heater cable. The mat is imbedded in thinset before the final flooring material is installed. A wall-mount thermostat or timer controls the heat.

The 120-volt circuit or power source for the radiant heat mats must be GFCI-protected. Mats are available in a variety of lengths. Check the manufacturer's specifications for the wattage your situation requires.

Prestart Checklist

Time
About 8 hours to install mat, wire, and tile for an average bathroom

Tools
Digital ohmmeter, drill, 1/2-inch bit, drywall saw, fish tape, chisel, hot-glue gun, 3/8-inch trowel, screwdriver, stripper, long-nose pliers, lineman's pliers, tools for laying the flooring, jeweler's screwdriver

Skills
Stripping, splicing, and connecting wires to terminals; installing boxes; running cable into boxes; setting tile

Prep
Rough-in the plumbing; install the subfloor.

Materials
12/2 cable, mat, box, armored power cable, thermostat and/or timer, thinset, flooring, mortar or grout

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Installing Underfloor Electric Radiant Heating

The heating element is only a layer of thinset away from the tile, brick, or stone flooring surface. Power cables supply electricity; a sensor bulb relays the temperature back to the thermostat. Insulation beneath the substrate and subfloor increases heating efficiency.

Step 1

Install a large-capacity box 60 inches above the floor for the thermostat. Using 12/2 cable, add a new circuit or extend an existing circuit but do not connect the power source. Pull cable provided with the mat to the box.

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Step 2

Unpack the mat. Check the resistance using a digital ohmmeter. The reading should be within 10 percent of the rating shown on the UL label. This is your benchmark for confirming that the heat cable is not nicked during installation. Write the reading down.

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Step 3

Clean the floor of debris and tighten any protruding screw or nailheads. Roll out the mat and position it. Check that it is no closer than 3-6 inches to walls and fixtures. Attach double-sided tape to the floor, but do not attach the mat to the tape yet.

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Layout Alternatives

While this type of electric radiant heating can be used only indoors, it can be installed wherever a tile, brick, or stone surface makes sense: bathroom, mudroom, or kitchen.

Step 4

The armored power lead connection is thicker than the mat and must be sunk into the substrate. (Cement board is shown here.) Trace around the connection; use a cold chisel to cut a channel for it.

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What If... You're working in tight quarters?

Mats can run under a toilet but must be at least 3 inches from the toilet ring. In addition a mat can run beneath the kick plate of a vanity. Never overlap mats, cut a mat to fit, nor attempt to repair a cut or nicked heating wire. If the wire is damaged you must replace the entire mat.

Step 5

Glue the power cable connection into the channel with hot glue. Mark along the power cable and slide it to one side. Working a few feet at a time, run a continuous bead of hot glue. Press the lead into the hot glue.

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Step 6

Press down on the tape to adhere it to the cement board, then pull off the backing and firmly press the mat onto the tape. Weave the sensor bulb between two heating elements. Adhere the sensor bulb wire with dots of hot glue. Check resistance with an ohmmeter. Record the reading.

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Step 7

With the flat side of a 3/8-inch notched trowel, apply thinset over an area of the mat. Then turn the trowel over and rake the thinset to 1/4-inch uniform depth. Be careful not to snag the mat. Do not clean the trowel by banging it on the mat. Tile the area of the floor covered with thinset.

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Step 8

Check mat resistance once again using the ohmmeter. If the ohm reading is either 0 or infinity, the heating element has been damaged. The tile must be removed and the mat replaced.

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Safety First: Don't overload circuit or components

Checking the amperage a new installation demands tells you whether an existing circuit can carry the load or you have to install a new circuit. Each square foot of electric radiant heating mat draws 0.1 amp. That means adding radiant heating mats to the work area of an average-size kitchen adds only 5 or 6 amps to the circuit (50-60 square feet of mat). Each component in the system, such as a dimmer or timer, must be rated to take the amperage.

Step 9

The control shown above has built-in GFCI protection. Using a jeweler's screwdriver, attach the two sensor wires to the screw terminals on one side of the control. Connect the ground from the mat power lead directly to the house ground.

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Step 10

Attach the black control lead marked LINE to the incoming black wire. Connect the white control lead LINE to the incoming white wire. Attach the control leads marked LOAD to the wires connecting to the mat, black to black and white to white. Fold the wires into the box.

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Step 11

Attach the faceplate. Connect to the power source or connect the line to a new breaker. Turn on the power and follow the manufacturer's instructions for setting the thermostat.

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