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Radiant Floor Heating Systems

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 builder's surveys have shown that, if given a choice, most new home owners prefer radiant floor heat over other types of systems. Check out our general page radiant heating page for more information about radiant heating.

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 equates with lower heating bills. With radiant floor heating, you can set the thermostat several degrees lower. 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 65°F (18°C). It also radiates this heat for a longer period of time than a forced air system. Another advantage is that radiant systems do not increase the infiltration of outside air into the house structure as forced air systems generally do. Radiant floor heating also allows lower boiler temperatures allowing them to last longer (a 45 year life is not unusual). Radiant floors operate between 85-140°F (29-60°C), compared to other heating systems' range of 130°-160°F (54°-71°C). Fuel saving of 15% to 20% over a forced air system is common.

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.

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Types of Radiant Floor Heating

Air-Heated Radiant Floors

Because air cannot hold large amounts of heat, radiant air floors are not cost-effective in residential applications, and are seldom installed. Although they can be combined with solar air heating systems, those systems suffer from the obvious drawback of only being available in the daytime, when heating loads are generally lower. Because of the inefficiency of trying to heat a home with a conventional furnace by pumping air through the floors, the benefits of using solar heat during the day are outweighed by the disadvantages of using the conventional system at night. Although some early solar air heating systems used rocks as a heat-storage medium, this approach is not recommended.

Electric Radiant Floors

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

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 additions onto homes for which it would be impractical to extend the heating system into the addition. However, homeowners should examine other options, such as mini-split heat pumps, which operate more efficiently and have the advantage of also 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 underneath the floor. 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 is approximately $4–$6 per square foot ($40–$60 per square meter), depending 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 cables or tubing, the methods of installing electric and hydronic radiant systems in floors is about the same.

So-called "wet" installations embed the cables or tubing within 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 slab systems have high heat capacity and are ideal for storing heat from solar energy systems, which have a fluctuating heat output. The downside of the 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 underneath the subfloor between the joists. This method usually requires drilling through the floor joists in order 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 since the heat transfer of the floor is greatly improved over more traditional dry or wet floors.

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 subfloor (dry installations).

Because air cannot hold large amounts of heat energy, 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 pm 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 kilowatt-hours compared to heating at peak electric rates during the day.

Hydronic 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.

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.

Some "dry" installations involve suspending the tubing underneath the subfloor between the joists. This method usually requires drilling through the floor joists in order to install the tubing. Reflective insulation must also be installed under the tubes to direct the heat upward. Tubing may also be installed from above the floor, between an old and new subfloor. In these instances, the tubes are often in reflective aluminum sleeves that spread the heat to the sides, away from the tubing, and direct it upwards. The tubing and its reflectors are secured between furring strips (sleepers) which carry the weight of the new subfloor and finished floor surface.

Although ceramic tile is the most common floor covering for radiant floor heating, a variety of finished floor surfaces can be used. The choices include vinyl flooring, carpeting, and wood. Carpeting and padding, however, insulate the floor and reduces some of the benefits of radiant floor systems. If you want carpeting, use a lower nap carpet and thin, denser padding. You will also need to increase the system water temperature to compensate for the insulating properties of the floor covering. Most installers and some wood floor manufacturers also recommend using laminated wood flooring instead of solid wood. This reduces the possibility of the floor shrinking and cracking from the drying effects of the heat.

Older radiant floor systems used either copper or steel tubing embedded in the concrete floors. Unless the builder coated the tubing with a protective compound, a chemical reaction between the metal and the concrete led to corrosion of the tubing, and to eventual leaks. Major manufacturers of hydronic radiant floor systems now use cross-linked polyethylene (PEX) or rubber tubing with an oxygen diffusion barrier. This material is much more durable and slows the effects of corrosion in the system. Additives and filtration systems also help protect hydronic heating systems from corrosion.

There have been recent reports of problems with rubber tubing produced by one chemical manufacturer. Leaks develop at the metal connections or fittings, and in some cases the tubing becomes rigid and brittle. Tightening connections and clamps only temporarily fixes the leaks. This situation is currently in litigation. Remember, this problem only concerns this specific brand of rubber tubing. It does not have anything to do with the PEX tubing, which has performed very reliably for many decades.

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Controlling the System

A radiant floor that uses a concrete slab may take several hours to heat up if it is allowed to become cold. This can make the home uncomfortable as the slab heats up. Because of this, most radiant floor systems are controlled by a floor thermostat instead of a wall thermostat as in a forced air heating system. The floor thermostat usually allows the circulation pump(s) to run continuously and only controls the burner. Other, more sophisticated, types of controls sense the floor temperature, outdoor temperature, and room temperature to keep the home comfortable. Such a system can use less fuel because it adjusts the water temperature to meet the needs of the home.

Although radiant floor systems are usually heated by a boiler, they can also be heated with a geothermal heat pump. Such a system offers even greater energy savings compared to other types of heating plants.

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Cost of Radiant Floor Heating

The cost of installing a hydronic radiant floor fluctuates depending on the size of the room, the type of installation (i.e., concrete slab or wood floor), the floor covering, remoteness of the site, and the cost of labor. For additional information contact:

• Hydronic Radiant Heating Division The Hydronics Institute PO Box 218 Berkeley Heights, NJ 07922 Phone: (908) 464-8200 Internet: (World Wide Web) http://www.gamanet.org
• Radiant Panel Association P.O. Box 717 Loveland, CO 80539 Phone: (970) 613-0100 or (800) 660-7187 Internet: (E-mail) info@rpa-info.com; (World Wide Web) http://www.rpa-info.com

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Bibliography

The following articles provide additional information on radiant floor heating. This bibliography was reviewed in November 1998.

Books and Reports

• How Come?: Hydronic Heating Questions We've Been Asking For More Than 100 Years (With Straight Answers), D. Holohan, Dan Holohan Associates, Inc. 1995. Available from Dan Holohan Associates, Inc. , 63 North Oakdale Avenue, Bethpage, NY, 11714; Phone: (800) 853-8882; $20.00.
• Possibilities and Limitations of Radiant Floor Cooling - #4014, B. Olesen and D. Liedelt, 1996. Available from American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE), 1791Tullie Circle NE, Atlanta, GA 30329, (800) 527-4723. $15.00.
• Pumping Away (And Other Really Cool Piping Options for Hydronic Systems), D. Holohan, Dan Holohan Associates, Inc. 1995. Available from Dan Holohan Associates, Inc. , 63 North Oakdale Avenue, Bethpage, NY, 11714; Phone: (800) 853-8882; $20.00.

Articles

• "Adding Radiant-Floor Heating," J. Truini, Home Mechanix, (91:792) pp. 56-61, February 1995.
• "Can Radiant Floor Cooling Succeed Despite Design Limitation?" N. Nisson, Energy Design Update, (17:5) pp. 5-8, May 1997.
• "Electric-Radiant Floors," B. Kaercher, Jr. Fine Home Building, (No. 75) pp. 68-72, June/July 1992.
• "Heat at Your Feet," Home Mechanix, (89:780) November 1993.
• "Heatway, Goodyear Square Off in Court Over Radiant System Failures," Energy Design Update, (18:10) pp. 2-4, October 1998.
• "Hydronic Radiant Heat for Wood-Framed Floors," J. Siegenthaler, The Journal of Light Construction, (11:11) pp. 34-37, August 1993.
• "Mixing Forced-Air and Boiler Heat," R. Groff, Fine Home Building, (No. 85) pp. 86-87, January 1994.
• "Radiant Floor Systems," C. Wardell, Custom Builder, (10:6) pp. 26-30, September/October 1995.
• "Radiant Slab Techniques," J. Siegenthaler, The Journal of Light Construction, (10:11) pp. 21-24, August 1992.
• "Radiant Slab on a Tight Budget," J. Siegenthaler, The Journal of Light Construction, (13:10) pp. 47-50, July 1995.
• "Wet or Dry System? Here's How They Compare, "Air Conditioning,
  Heating & Refrigeration News, (196:4) p. 22, September 25, 1995.