Installing a Radiant Heat System
By Rob Robillard
Consistent performance, easy installation and simple climate control—Electric radiant heat warms the home from the floor up.
New England mornings can tempt some of the toughest to roll over, turn off the alarm clock and go back to sleep. Getting out of bed and stepping onto a cold wood or tile floor can send chills up your spine and set the mood for the day.
Research is showing that more and more people are remodeling their homes instead of moving or building new. In addition, 60 percent of homeowners 50 years and older intend to stay in their homes through retirement, which also means aging-in-place remodeling.
Using electric radiant heat as a remodeling solution occurred to me several months ago while working on a bathroom that was located above my client’s garage. It was February and the floor of the tile bathroom was frigid.
In these situations using electric radiant heat to supplement the existing heating system is a no-brainer and can instantly create a warm, toasty, supplemental heat source in any room—especially in bathrooms.
Why Radiant Heat?
As technology continues to advance, so has the concept of floor heating. Although floor-heating dates back to ancient Roman bathhouses where slaves tended fires sending hot air through channels under elevated floors, it has not until recently become popular within households.
Many people are familiar with hydraulic under-floor heating systems that require tubing to run a hot liquid beneath the floor, along baseboard heaters or through radiators to heat your home. Yet an alternative to hydraulic heating that is easier and more cost-effective to install is electric under-floor heating.
The science behind electric radiant floor heating is quite simple: The entire floor surface effectively becomes a low-temperature radiator.
These electric systems transfer the heat directly to the floor surface via infrared radiation. The floor in turn warms the people and other objects in the room.
Floor heat is much more efficient and comfortable than heating the air, which does not retain that heat for very long. Unlike forced-air heating systems, under-floor heating warms your body and the objects surrounding you rather than heating the air, ultimately eliminating inconsistent room temperatures and reducing the percentage of dust mites.
When you’re purchasing electric floor heating, you want to make sure that it’s as “hot” as possible—for this, pay close attention to the watts-per-square-foot ratio.
The optimal wattage is 15 watts per square foot. The bottom line is: The hotter, the better! With 15 watts per square foot, you will be able to utilize full heating technology for a faster heat-up and a warmer floor, especially valuable in a bathroom installation where time is spent morning and night for our daily routines.
Where can Electric Radiant Heat Be Used?
Electric floor heating can be used indoors or outdoors. It can be installed under many types of flooring, including laminates, hardwood and tile. The system consists of electric cables built into the floor, or it features mats of electrically conductive plastic that are mounted on the subfloor.
The system can be installed on a mortar bed or gypsum board with only a thin layer of thin-set cement between it and the floor tile.
Retrofitting during a remodel is easy and usually does not significantly raise the floor height.
Electric floor heating systems come in several different forms, such as a mat or cable to accommodate the various size and shape of the area. For complicated areas, a single heating cable can be used under the floor. Heating mats are more likely to be installed in simple areas with more square footage. Using the mats allows retrofitting over existing slabs without significantly raising the floor height.
These systems also offer “smart” technology, controlled by programmable thermostats that can be set to accommodate one’s busy lifestyle. For example you can set your heat to turn on an hour prior to your getting out of bed; to turn down or off when you leave for work; and to turn back on just before you arrive home. In this way you are only using electricity when you need it.
When you consider the many benefits and the non-existent maintenance fees associated with electric radiant heat, these systems provide a realistic remodeling solution to add floor heating. As a carpenter and remodeler I’m always looking for easier, more efficient ways to improve an existing space. The concept of electric radiant heat meets the criteria. It’s easy to install, energy efficient and can be easily programmed.
General Installation Guidelines
The following are very basic guidelines for ordering and installing electric radiant heat. In every case you should follow the manufacturer’s recommendations for specific material installations and applications. A licensed electrician should make all electrical connections.
Electric radiant heat is appropriate for many finished flooring options, but consider the following when planning your project.
Ceramic Tile. Ceramic tile is the most common and effective floor covering for radiant floor heating because it conducts heat well from the floor and adds thermal storage due to its high heat capacity.
Vinyl. Vinyl, linoleum, carpeting or wood can also be used, but any covering intended to help insulate the floor from the room will decrease the efficiency of the heating system.
Wood. For real wood applications, quarter-sawn wood flooring is recommended because it’s always more stable under conditions of extreme moisture or heat, and is always the better choice when placed over radiant heat. Plain-sawn wood should be avoided because it will expand and contract more.
Carpet. If you decide on using carpet, choose a thin carpet with dense padding and install as little carpeting as possible. If some but not all rooms will have a carpet covering, then those rooms should have a separate thermostat loop so the system will heat those spaces more efficiently.
Plan in Advance
There are many great electric radiant heat companies that offer quality products. I suggest that you look at companies that provide assistance with custom designs and offer quality customer service.
Do-it-yourselfers and professionals alike can run into some snags along the way when installing a radiant heating system, so you want to ensure that the company you’ve chosen offers 24/7 installation and technical support.
Be careful because some may say they offer round-the-clock support, but very few actually offer support from an actual person employed by the company. Often times it’s simply a recording or a call center. You want a live person who knows their stuff to be on the other end of the phone when you’re working on your project—day or night.
A quality radiant heating manufacturer will assist you in designing your system and, if needed, provide a detailed printed diagram for you by the next business day.
Measuring. Carefully measure your room, typically to the walls. Use graph paper to draw your floor plan with your measurements.
When measuring, make sure you accurately include the location of vanities, floor vents, tubs and toilet drains in your drawing. Heat mats or wires should not be positioned below permanent fixtures or zero-clearance furniture. Include in the drawing the placement of your thermostat to ensure that when the floor system arrives, the cold lead wires will reach your thermostat location.
Ordering. Determine whether you can use standard size (i.e., off the shelf) heating mats or if you need a custom design. Custom designs always cover the floor area better, but many small square or rectangular rooms can be covered reasonably well with standard mats. The better quality floor heat companies will ship the same day you place the order.
Preparing the Subfloor
Make sure the subfloor is clean and free of debris. With an existing subfloor you may want to install screws in the floor joists to prevent squeaks and ensure that it’s adequately fastened.
Avoid the temptation to tear open the box and start installing your mat without first reading through the instructions and dry-fitting your electric heating system.
All manufacturers have specific guidelines for their systems that must be followed. Reading your instructions will save you time, mistakes, unnecessary steps and money.
When dry-fitting the mats make careful note of the heat-mat power leads and orient them to your thermostat location, making sure you have enough wire to reach the thermostat.
Heated Tile Floor
I usually install a tile backer board as an underlayment if installing tile. Follow the manufacturer’s instructions for the installation of the backer.
Self-leveling method. The self-leveling method to secure the heating mats to the floor can be used for all types of flooring—not just tile. Begin by preparing the self-leveling compound as per manufacturer’s instructions. Priming the subfloor may be recommended by the manufacturer. Using a scraper or a flat trowel spread a layer of the self-leveling compound until the heating cable and guides are completely covered. Allow the compound to set according to the product instructions. Conduct insulation and resistance tests of the electrical system, then proceed with laying the finished floor covering.
Thin-set mortar method. Depending on the product (solid mat or mesh mat), you may have to apply thin-set mortar first. For solid mats, apply the mortar with a 1 ⁄ 4-by-1 ⁄ 4-in. square notch trowel and then set the solid mat into the thin-set. Use a rubber float to press the entire mat surface to firmly create 100-percent contact between the heating mat, the mortar and the subfloor.
If the product is a “mesh mat” you can secure the mesh mat to the sub-floor and then apply thin-set over the heat cables using the flat edge of the trowel. Mesh mats can be stapled, hot-glued or taped to the subfloor. In the installations I’ve done I’ve used a combination of the three to accomplish the install. Note: Never staple through or over the heating wires or floor sensor wire. When applying the mortar, the cable must be completely covered and only the very top of the guides should be visible.
Electric radiant heat systems come with a floor-sensing thermostat with GFCI (Ground Fault Circuit Interrupter). The floor-sensor must be installed into the thin-set as well, following the manufacturer’s recommendations. Most manufactures require this sensor to be installed on top of the heat mat a minimum of 6 to 12 in. within the edge of the heated area and in-between two heating wires.
A 1/4-in. bed of thin-set is then applied over the mat and supply leads using the flat side of a trowel, to ensure the thin-set is level and smooth. Conduct insulation and resistance tests.
Tile Installation. Allow the first layer of thin-set to cure before installing the tile with a second layer of thin-set. This layer is applied with a 1/4-in. square notched trowel.
(Pro Tip: When installing the mesh-style mats, experienced installers may wish to cover the heating system with thinset and install the tile in one step. A 3/8-in. square notched trowel is recommended for a one-step installation.)
Clean the thin-set from between the grout joints with a wet rag while installing tile. Never use a utility knife to remove cured thin-set from grout joints.
Throughout this entire process you should be repeatedly performing an Ohms test.
Electrical Connections. Allow proper thin-set cure time as recommended by the manufacturer (typically 7-10 days) before powering up the electric radiant heat system. The system can then be wired to a Class “A” GFCI or a GFCI circuit breaker, and the thermostat installed in a double-gang wall box with a single mud ring by a licensed, certified electrician.
Thermostat Connections. Refer to the thermostat installation instructions (included with thermostat) for proper wiring. Thermostats should be installed at an appropriate height and in an accessible location in the same room that the thermostat is controlling.
Heated Wood Floor
A wood floor with electric heat requires a different installation method, but you can avoid the use of thin-set.
First, install wood sleepers. Fasten strips of plywood 1- to 2-in. wide by 3/8- to 1/2-in. high, spaced 19 inches apart across the entire plywood subfloor. Leave space for any planned cuts and turns. The sleepers are installed to create lanes into which the heating system roll will be placed.
Unroll the heating mats between the sleepers. Install the floor heating system as recommended into the lanes created by the sleepers, adhering the mesh every 6 to 8 in. with hot-glue or staples (never staple over the wires).
Carefully route the power leads alongside the system within the lane and back to the power supply, making sure that the leads do not pass over the sleepers or the heating wire. Install the thermostat sensor probe (if applicable) 6 in. into the heating mat, centered directly between the heating wires. The sensor wire should not cross over heating wires. Perform an ohm test.
Cover the heating rolls or cables with 3/8-in. self-leveling cement up to and even with the top of the wood sleepers. Use a float trowel to spread the self-leveling cement. Follow the manufacturer’s recommendations for the self-leveling cement and allow proper cure time before moving to the next step.
Once cured, install the hardwood or engineered wood flooring as per the manufacturer’s recommendations. Be careful not to place nails or staples near the system’s heating cable or power leads.
Side Note 1
Radiant Floor Heat Advantages
1. Consistent and easy to control temperature
2. Radiant floor heating is the most efficient mode of heat delivery
3. Comfort can be achieved at lower temperature settings
4. Savings of 20 to 40 percent over forced air systems
5. It’s clean and health; radiant floor heating does not rely on circulating air and so dust particles and allergens are less likely to spread throughout the house
6. It’s also quiet; radiant floor heating operates silently
7. Floor heating saves space because there are no bulky radiators or baseboard units
8. Humidification is unnecessary because radiant heat does not alter residential air moisture content.