At this time of year I get a lot of questions about heating systems and most recently several questions about Radiant Floor Heating retrofits. While I’m the first to tell you that I’m not a heating guy, efficient heating has long been an interest of mine. So I went to the Department of Energy website and others find some information for you.
There are radiant heating systems can supply heat directly to the floor or to panels in the wall or ceiling of a house. The delivery of heat directly from the hot surface to the people and objects in the room via the radiation of heat is known as infrared radiation. If you ever walked along a brick wall just after the sun sets, the stored-up warmth you feel is infrared radiation.
Radiant heating is more efficient than baseboard heating and usually much more efficient than forced-air heating because no energy is lost through the ductwork. The lack of moving air can also be advantageous to people with severe allergies as dust and pet dander or germs are not lofted into every room. Forced air systems also tend to dehumidify the air more than radiant systems and dry out our skin and noses. Hydronic (liquid-based) systems generally use high-efficiency boilers that burn less gas than forced air furnaces and steam boilers. The hydronic systems can also be heated with a variety of energy sources, including gas-or-oil-fired boilers, wood-fired boilers, solar water heaters, or a combination of these sources.
There are radiant air floors (air is the heat-carrying medium); electric radiant floors; and hot water (hydronic) radiant floors. Air does not hold large amounts of heat, radiant air floors are not cost-effective in most applications, and are rarely installed. Because of the relatively high cost of electricity, electric radiant floors are usually found locally in bathrooms, systems that feature mats of electrically conductive plastic and are mounted onto the subfloor below a tile floor covering. The majority of radiant systems I have seen in this region are floors heated by gas-fired hydronic (hot water) boiler setups.
Hydronic (liquid) systems use less gas than furnaces or steam to warm the house. Hydronic radiant floor systems pump heated water from a boiler through tubing laid in a pattern underneath the floor. Systems installed in the 1940’s to the ‘70’s used copper piping, in the 1980’s plastic piping such as PEX became commonly used. In some systems, the temperature in each room is controlled by regulating the flow of hot water through each tubing loop. This is done by a system of zoning valves or pumps and thermostats. The cost of installing a hydronic radiant floor varies by geographical location and also depends on the size of the home, the type of installation, the floor covering, remoteness of the site, and the cost of labor.
A "wet" installation embeds the tubing within a concrete floor slab (commonly used in "slab" ranch houses that don't have basements) and is the oldest form of modern radiant floor systems. The tubing can be embedded in a thin layer of lightweight gypsum concrete, or other material installed on top of a wood subfloor. If concrete is used and the new floor is not on solid earth, additional floor structure may be necessary because of the added weight. A structural engineer should be consulted to determine the floor's load-carrying capacity.
"Dry" floors, in which the tubing run beneath a wood floor, have been gaining in popularity, mainly because a dry floor is faster and less expensive to build. But because dry floors involve heating some air space, the radiant heating system may need to operate at a higher temperature. A common retrofit installation involves suspending the tubing underneath the subfloor between the joists. This method usually requires the basement ceiling removed and drilling holes through the floor joists in order to install the tubing. Reflective insulation is installed under the tubes to direct the heat upward. In new construction, the tubing can be installed from above the subfloor, between the subfloor and finish flooring. In these instances, the 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.
Several companies now make a plywood subfloor material manufactured with tubing tracks and aluminum heat diffuser plates built into them. The manufacturers state that this product makes a radiant floor system (for new construction) considerably less expensive to install and faster to react to room temperature changes.
Ceramic and stone tiles are common and effective floor coverings for a concrete slab radiant floor, as it conducts heat well from the floor and adds thermal storage because of its high heat capacity. One building I’ve seen recently had a color-stained, polished concrete floor, which I found the thought of being barefoot on warm concrete in February as intriguing. Common floor coverings like vinyl and linoleum sheet goods, carpeting, or wood can also be used, but any covering that helps to insulate the floor from the room will decrease the efficiency of the system.
Most of the sites that I’ve read make these statements, if you want carpeting, use a thin carpet with dense padding and install as little carpeting as possible. Wood flooring should be laminated wood flooring instead of solid wood. This reduces the possibility of the wood shrinking and cracking from the drying effects of the heat.