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Electric and Hydronic Radiant Heat

While most of the press about radiant heat has focused on hydronics, makers of electric cable systems also report a healthy growth in their business. Electric radiant floors aren't appropriate for every home, but they can be an excellent solution to certain design problems. Choosing the right system means knowing what you want it to do, and looking past manufacturers' claims to the system's real costs and benefits. Here are some guidelines.

Hydronic Tubes vs. Electric Cable

In a hydronic system, water is heated by a boiler or water heater and circulated through flexible tubes buried in the floor. The floor absorbs this energy, then gives it off as radiant heat, which warms people and objects in the room.

An electric system works the same way, but instead of tubes, electric heating elements warm the floor. Electric systems are easier and less expensive to install than their hydronic counterparts. They're also less expensive to zone. They can be used to heat a whole house or to provide spot comfort in kitchens and baths.

System Types

Electric elements can consist of cables coated with electrical insulation, or of fabric mats with the cables woven into them. Like hydronic tubes, electric elements are embedded in the floor system. Depending on their purpose, insulated cables use anywhere from 8 to 15 watts of electricity per square foot of floor area (the upper limit permitted by code). Free-form cables are usually embedded in a 1-1/2-in.-thick slab of gypsum underlayment or lightweight concrete. As with hydronic tubing, you need to consider the ability of the framing to support the slab's weight and make adjustments to window and door heights for the slab's extra thickness. (For more details see June 1996.)

Mats resemble electric blankets and consist of loops of cable embedded in a substrate, which is often a mesh fabric. The loops use approximately 12 watts per square foot and are spaced closer together than embedded cables.

According to David Flinn at DK heating systems, which makes the Warm Touch product, the closer the cables are to the surface, the closer together they must be spaced to distribute heat uniformly over a given surface area. This makes mats more expensive per square foot than free-form insulated cables.

However, mats are less expensive to install, because the installation is a lot quicker and they generally require less floor thickness than cables. Mats can often be placed in a mortar bed beneath floor tiles. This adds as little as 1/8 in. to the floor height. Some can be rolled out on the subfloor beneath a carpet and pad. Mats come in a range of standard sizes. Most manufacturers will also make custom sizes or shapes.

Performance Issues

Makers of floor-warming mats like to cite their products' fast response time. One company points out that when a radiant heat source is buried 1 in. deep in a concrete slab, it takes several hours to fully heat the slab. By comparison, it takes only 65 minutes to heat up a tile floor that has been laid over a mat. What this comparison leaves out, though, is the effect of thermal mass. Like everything else, thermal mass has its pros and cons. The thick-slab system will take longer to heat up, but the slab's thermal mass will keep the home warm for a longer period of time. If electric rates vary by time of day, you can save money by setting the clock thermostat to heat the slab during off-peak hours when electric rates are lowest. The preheated slab then keeps the house warm through the peak-demand periods, maintaining comfortable temperatures without using much additional energy.

On the other hand, if you need quick temperature changes, thermal mass tends to work against you. For example, quick warm-ups are needed in areas used only occasionally, such as a workshop that's used a couple of hours per night, or a bathroom where the home owner wants to warm up the floor before shower time. Be aware, however, that fast systems may not be best for all floor coverings. For example, quick temperature changes will put a lot of thermal stress on wood flooring. Wood floors tend to fare much better with constant or slow-changing temperatures, which is what you get with a thick-slab system.

Cost Effectiveness

Of course, the deciding factor in whether to choose an electric system is usually cost. How do you determine when an electric radiant floor is cost-effective? "That's a difficult question," says Larry Drake of the Radiant Panel Association. The variables, he says, are the cost of the 10-15 watts of electricity per square foot of floor area the system uses, how often the system will be running, the efficiency of the boiler you would be using if you had opted for a hydronic system and the cost of fuel for that boiler.

A big advantage of hydronic systems is the flexibility of the fuel source. You can use gas, oil, electricity and even solar energy. Plus, you can change fuels for the price of a new boiler. With electric cables, you'll save the cost of a boiler, but you can't change fuel sources. Special heating or off-peak rates available in some areas can make electricity equivalent to, or even less expensive than, fossil fuels.

Another factor affecting cost is the size of the installation. Hydronic systems have high start-up costs because you have to buy a boiler. The installed cost of an electric floor can more or less be figured by the square foot. As a result, electric radiant heat tends to be more cost-effective on small floors. The larger the area you need to heat, the better a hydronic system will look. For instance, in a home with forced-air heating, it would probably cost too much to run hydronic tubes into a small bathroom. In this case, an electric floor-heating system would make much more sense.

According to Mike Breedlove of Easy Heat, you should expect to spend $300 to $400 to install an electric system for warming a small bathroom. An equivalent hydronic system would run $4,000 to $5,000. Floor-warming systems, says Breedlove, can operate on less than a penny per square foot per day if electricity costs around 6 cents per kilowatt hour. "If you're using it as a personal comfort product," he says, "then the cost of energy is less significant."

Good Applications

Drake suggests that most of the time an electric floor will be used to enhance comfort rather than to provide primary heating. And while it's most often used as a supplement in homes with forced-air heating systems, an electric floor can even be used to complement a hydronic radiant floor in areas where you need quick response time. That's what Drake did in his own house. While most of his home sits over a hydronic slab, the tiled entryway is warmed by an electric system. It uses very little electricity and can be turned on only when it's needed. The electric floor doesn't do space heating: the entry is tempered with heat from the rest of the house. But temperature sensors in the floor make sure that everybody's feet stay comfortable when they take their shoes off. Could Drake have extended his hydronic system to the floor in the entryway? Sure, but he would have had to embed it in a slab, and would have had to leave it on most of the time in order for the tiles to be warm when they needed to be. He would have also needed a separate zone and relatively expensive controls. Another good candidate for electric radiant floors may be highly efficient homes with thick insulation, airtight construction and passive solar features. The lower installed cost of electric radiant heat helps offset potentially higher fuel costs. Lower overall heating load reduces the cost of supplemental energy. Compared to forced air systems, there are additional energy savings from not having leaky ductwork.

Electric radiant floors offer an option for certain applications where the smaller spaces or low heating loads make hydronic systems impractical or too expensive.

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