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Radiant Heating

Radiant heating systems supply heat directly to the floor or  to panels in the wall or ceiling of a house. The systems depend largely on  radiant heat transfer -- the delivery of heat directly from the hot surface to  the people and objects in the room via infrared radiation. Radiant heating is  the effect you feel when you can feel the warmth of a hot stovetop element from  across the room. When radiant heating is located in the floor, it is often  called radiant floor heating or simply floor heating.

Radiant heating has a number of advantages. It is more  efficient than baseboard heating and usually more efficient than forced-air  heating because it eliminates duct losses. People with allergies often prefer  radiant heat because it doesn’t distribute allergens like forced air systems  can. Hydronic (liquid-based) systems use little electricity, a benefit for  homes off the power grid or in areas with high electricity prices. Hydronic  systems can use a wide variety of energy sources to heat the liquid, including  standard gas- or oil-fired boilers, wood-fired boilers, solar water heaters, or  a combination of these sources.

Despite its name, radiant floor heating depends heavily on  convection, the natural circulation of heat within a room as air warmed by the  floor rises. Radiant floor heating systems are significantly different from the  radiant panels used in walls and ceilings. For this reason, the following  sections discuss radiant floor heat and radiant panels separately.

RADIANT FLOOR HEAT

There are three types of radiant floor heat -- radiant air  floors (air is the heat-carrying medium), electric radiant floors, and hot  water (hydronic) radiant floors. You can further categorize these types by  installation. Those that make use of the large thermal mass of a concrete slab  floor or lightweight concrete over a wooden subfloor are called "wet  installations,” and those in which the installer "sandwiches" the  radiant floor tubing between two layers of plywood or attaches the tubing under  the finished floor or subfloor are called "dry installations."

TYPES OF RADIANT FLOOR HEAT

ELECTRIC RADIANT FLOORS

Electric radiant floors typically consist of electric cables  built into the floor. Systems that feature mats of electrically conductive  plastic mounted on the subfloor below a floor covering such as tile are also  available.

Because of the relatively high cost of electricity, electric  radiant floors are usually only cost-effective if they include a significant  thermal mass such as a thick concrete floor and your electric utility company  offers time-of-use rates. Time-of-use rates allow you to "charge" the  concrete floor with heat during off-peak hours (approximately 9 p.m. to 6  a.m.). If the floor's thermal mass is large enough, the heat stored in it will  keep the house comfortable for eight to ten hours without any further  electrical input, particularly when daytime temperatures are significantly  warmer than nighttime temperatures. This saves a considerable number of energy  dollars compared to heating at peak electric rates during the day.

Electric radiant floors may also make sense for home  additions if it would be impractical to extend the heating system into the new  space. However, homeowners should examine other options, such as mini-split  heat pumps, which operate more efficiently and have the added advantage of  providing cooling.

HYDRONIC RADIANT FLOORS

Hydronic (liquid) systems are the most popular and  cost-effective radiant heating systems for heating-dominated climates. Hydronic  radiant floor systems pump heated water from a boiler through tubing laid in a  pattern under the floor. In some systems, controlling the flow of hot water  through each tubing loop by using zoning valves or pumps and thermostats  regulates room temperatures. The cost of installing a hydronic radiant floor  varies by location and depends on the size of the home, the type of  installation, the floor covering, remoteness of the site, and the cost of  labor.

TYPES OF FLOOR INSTALLATIONS

Whether you use cables or tubing, the methods of installing  electric and hydronic radiant systems in floors are similar.

So-called "wet" installations embed the cables or  tubing in a solid floor and are the oldest form of modern radiant floor  systems. The tubing or cable can be embedded in a thick concrete foundation slab  (commonly used in "slab" ranch houses that don't have basements) or  in a thin layer of concrete, gypsum, or other material installed on top of a  subfloor. If concrete is used and the new floor is not on solid earth,  additional floor support may be necessary because of the added weight. You  should consult a professional engineer to determine the floor's carrying  capacity.

Thick concrete slabs are ideal for storing heat from solar  energy systems, which have a fluctuating heat output. The downside of thick  slabs is their slow thermal response time, which makes strategies such as night  or daytime setbacks difficult if not impossible. Most experts recommend  maintaining a constant temperature in homes with these heating systems.

Due to recent innovations in floor technology, so-called  "dry" floors, in which the cables or tubing run in an air space  beneath the floor, have been gaining in popularity, mainly because a dry floor  is faster and less expensive to build. But because dry floors involve heating  an air space, the radiant heating system needs to operate at a higher  temperature.

Some dry installations involve suspending the tubing or  cables under the subfloor between the joists. This method usually requires  drilling through the floor joists to install the tubing. Reflective insulation  must also be installed under the tubes to direct the heat upward. Tubing or  cables may also be installed from above the floor, between two layers of  subfloor. In these instances, liquid tubing is often fitted into aluminum diffusers  that spread the water's heat across the floor in order to heat the floor more  evenly. The tubing and heat diffusers are secured between furring strips  (sleepers), which carry the weight of the new subfloor and finished floor  surface.

At least one company has improved on this idea by making a  plywood subfloor material manufactured with tubing grooves and aluminum heat  diffuser plates built into them. The manufacturer claims that this product  makes a radiant floor system (for new construction) considerably less expensive  to install and faster to react to room temperature changes. Such products also  allow for the use of half as much tubing or cabling, because the heat transfer  of the floor is greatly improved compared with more traditional dry or wet floors.

FLOOR COVERINGS

Ceramic tile is the most common and effective floor covering  for radiant floor heating, because it conducts heat well and adds thermal  storage. Common floor coverings like vinyl and linoleum sheet goods, carpeting,  or wood can also be used, but any covering that insulates the floor from the  room will decrease the efficiency of the system.

If you want carpeting, use a thin carpet with dense padding  and install as little carpeting as possible. If some rooms, but not all, will  have a floor covering, then those rooms should have a separate tubing loop to  make the system heat these spaces more efficiently. This is because the water  flowing under the covered floor will need to be hotter to compensate for the  floor covering. Wood flooring should be laminated wood flooring instead of  solid wood to reduce the possibility of the wood shrinking and cracking from  the drying effects of the heat.