Radiant floor heating has been used for centuries. The Romans channeled hot air under the floors of their villas. The Koreans channeled hot flue gases under their floors before venting them up the chimney. In the 1930s, architect Frank Lloyd Wright piped hot water through the floors of many of his buildings. Some home builders' surveys have shown that, if given a choice, most new home owners prefer radiant floor heat over other types of systems.
Advantages of Radiant Floor Heating Most people who own radiant floor heating feel that the most important advantages are comfort and quiet operation. Radiant floor systems allow even heating throughout the whole floor, not just in localized spots as with wood stoves, hot air systems, and other types of radiators. The room heats from the bottom up, warming the feet and body first. Radiant floor heating also eliminates the draft and dust problems associated with forced-air heating systems.
Even heat distribution may result in lower heating bills. With radiant floor heating, you may be able to set the thermostat several degrees lower, relative to other types of central heating systems. This is because the entire surface of the floor radiates about the same amount of heat that the human body does, making the occupant feel warm even though the air temperature might be only 65ºF (18ºC). It also radiates this heat for a long period of time. Radiant systems may result in less infiltration of outside air into the house compared to houses with forced-air heating. Radiant floor heating proponents claim that fuel savings of 15% to 20% over forced air systems are possible. However, recent reports suggest that this may not be the case, since occupants may not be comfortable with a "low" thermostat setting and thus not set it lower.
Radiant floor heating also allows for lower boiler temperatures, which may result in the boiler lasting longer (a 45 year life is not unusual). Radiant floors operate between 85-140ºF (29-60ºC), compared to other hydronic heating systems' range of 130-160ºF (54-71ºC).
To some, the greatest advantage of radiant floor heating is aesthetic. The system is "invisible." There are no heat registers or radiators to obstruct furniture arrangements and interior design plans. Radiant floor systems also eliminate the fan noise of forced hot air systems.
Types of Radiant Floor Heating 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. All three types can be further subdivided by the type of installation: those that make use of the large thermal mass of a concrete slab floor or lightweight concrete over a wooden subfloor (these 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 or subfloor (dry installations).
Because air cannot hold large amounts of heat, radiant air floors are not cost-effective in residential applications, and are seldom installed.
Electric radiant floors are usually only cost-effective if 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 am). 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. This saves a considerable number of energy dollars compared to heating at peak electric rates during the day.
Hydronic (liquid) systems are the most popular and cost-effective systems for heating-dominated climates. They have been in extensive use in Europe for decades. Hydronic radiant floor systems pump heated water from a boiler through tubing laid in a pattern underneath the floor. 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.
Wet installations are the oldest form of modern radiant floor systems. In a "wet" installation, the tubing is embedded in the concrete foundation slab, or in a lightweight concrete slab on top of a subfloor, or over a previously poured slab. If 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.