Photo 1: Securing the heat mat with roofing nails. To the right is a groove chiseled into the plywood underlayment. The diamond lath was applied to reinforce a layer of self-leveling compound used to encapsulate the wires.

Floor warming systems are one of the most popular upgrades for ceramic tile floors, but floor warming should never be confused with space heating. Floor warming systems deliver low-output heat, through electrical resistance wires embedded or located below the surface of the floor tile installation, and are designed specifically to warm the surface of the tiles-not the space above the tiles. Radiant heat systems are also located below the tiles, but their high heat output allows these systems to heat the tiles as well as the space above the tiles.

In moderate climates where space heating is infrequently used, floor warming systems may provide comfortable surroundings year-round, but in cold climates, floor warming systems must be used as a supplement to an existing space heating system.

Floor warming and hydronic systems can be used with a variety of materials including sheet goods, hardwood strip flooring, and carpet, but these systems can reach their peak efficiency when the finishing material is ceramic tile. Floor warming systems, like hydronic systems, work best when the heat source-the tubing or the electrical resistance wiring is close to the tile. With hydronic, tubes should be located in the top one third of the installation sandwich. With resistance wiring, the wires can be embedded in the tile bond coat and be positioned immediately below the tile for the most efficiency and heat output. The electrical resistance floor warming system I prefer to use can even be installed, with special construction, below the subfloor on an existing floor tile installation, but, obviously, the heat output and efficiency will be diminished, and it will take longer for this type of application to begin radiating heat.

Photo 2: Using the proper size notch trowel to raise them slightly above the heating wire, tiles can be installed-in one step-directly over the installed heat mat. A slightly larger size notch trowel is used to spread thinset in the areas where the mat is not installed to keep all tiles on a level plane.

Structural support and surface preparation.

Electrical resistance floor warming systems add virtually no weight to an installation, but if the install includes ceramic or stone tiles, all the usual concerns about joist spacing plus subfloor and underlayment thickness required for ceramic tiles over wood or concrete construction apply. For best results, consult the 2006 TCA Handbook*, pages 26 through 31, Methods RH110, RH111, RH115, RH116, RH122, RH130, RH135, and RH140 to find the method that most closely matches the installation you are planning. If possible, ask the manufacturer of the system you intend to use for a tile installation spec for your situation.

Most floor warming systems are relatively low consumers of electricity. Nevertheless, it is good practice to have the electrician connect the system to its own circuit, and locate an outlet box as close as possible to the floor where the system will be installed. Depending on the system, it may be necessary to carve a groove in the underlayment so the power supply cable will not interfere with the tiles. ( Photo 1) If the installation is in a wet area, make certain the system you select is UL® listed for use in wet areas, and ask the manufacturer for advice on where to locate a waterproofing membrane. The system I use can be installed above the membrane; others may require the heating wires to be placed below the membrane. Either way, to get the best performance and to secure any warranty, follow the system manufacturer’s instructions to the letter.

Photo 3: The author presses down a mounting tab to securely space and anchor an insulated heating wire.

Installing the electrical resistance wire

There are two basic formats: single wire and mat systems. Electrical resistance heating wires can be factory-laced into a carrier such as the mat shown in Photos 3 and 4. The mat is held in place with roofing nails: I stake each corner of the mat with a single nail-stretching the mat to keep it flat as I fasten the four corners-and use additional nails, as needed, to keep the mat snug against the floor. The mat is shipped in a single roll: To cover the desired floor area, the mat must be cut and the roll reversed, as shown in the upper right ofPhoto 2.

The other format is the single wire installation. This requires the installer to attach a spacing anchor strip around which a single strand of insulated heating wire is anchored. (Photo 3) I prefer this system because it is less expensive than the mat-mounted wires so the profit for arranging the cables goes into my pocket instead of the manufacturer. When installing tiles using the one-step method, I feel the single strand system is slightly easier to work with than the mat system. Like the mat system, once the wire is strung, tiles can be installed directly over the wire. (Photo 4)

Heating wires must never be installed below built in cabinets or closed-in furniture (Spindle-leg tables and chairs and other, open-structure furniture, art objects, and furnishings that do not block the escape of heat, are OK). Keep the wires at least four inches away from wax rings, never cross one resistance wire with another (the combined heat output will melt the insulation, and render the heating cables useless), and keep the mat six inches away from walls (nobody stands in the space so the energy to warm those tiles is completely wasted).

Regardless of the format, an installer has to have a way to monitor the condition of the wire from the time the wire is installed on the floor until all the installation materials have cured. This will vary from one system to the next, so consult the installation material manufacturers’ instructions for minimum curing times requiredbeforethe warming system is put into operation. As well, make certain you follow minimum cure restraints before exposing one material to another., with the list of materials including self-leveling or featheredge compounds, membrane materials, thinset mortars (I do not recommend water-based organic adhesives), grouts of all types, sealants, sealers and impregnators, and cleaning and maintenance products. Excess moisture, released during the curing of various installation products and unable to evaporate through upper layers, can adversely affect the performance of the warming wire, and reduce the anticipated strength of the entire installation.

In addition to monitoring the cure times for the installation materials, an installer has to ensure that the heating wire is not destroyed during the tile installation. Normally, most system wires will be delivered with a tag certifying the resistance of the wire when it left the factory, and most require that the installer check the resistance again prior to installation. The manufacturer of my preferred system has a device that stays attached to the connecting wires throughout the installation, and emits a loud tone if the heating wire is compromised.

Photo 4: Fresh tiles installed over the single-strand format. An extra anchor strip was installed to help keep the wire uniformly aligned so the finished floor’s heat output is evenly distributed, with no hot or cold spots.

Installing the tiles

Depending on your tile installation skills and the size of the tiles you want to install, there are two ways to approach covering a floor warming system with ceramic tile. The first method is the one-step technique shown inPhotos 2 and 4. This method requires careful notch trowel selection and mortar application to achieve the required 95% industry-recommended uniform adhesive contact, including any areas not covered by the heating wire. It also depends on the size of the tiles. Large size tiles-12, 18, and longer-are relatively easy to install using one-step methods, but as the tile size drops to a point where the tiles I need to install are not long enough to bridge at least two heating wires, I use an alternative method.

Photo 5: The author distributing a self-leveling compound over a floor warming mat. Galvanized diamond lath, installed under the heating mat, is barely visible on the left.

Encapsulating the floor warming mat

Even though most resistance heating wires occupy little space, they do lift the effective “surface” of the floor by the thickness of the wire and that creates a problem during the one-step method: How to maintain 95% contact, minimize joint seep-through, and maintain a smooth surface while the tiles are floating in a sea of thinset created by the wires. But by covering the heating wires first with self-leveling compound (Photo 5), the tile installation can be done quickly without a thick layer of thinset and the associated cleaning it requires, and on a surface that is smooth, stable, and very un-lumpy. In balance, probably worth the small extra cost for the time it may save you regardless of the tile size.

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For information on resistance heating materials, go