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Radiant Floor
Heating and Cooling 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.
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.
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 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.
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.
Radiant Floor Cooling
Radiant floor tubing can also be used to
cool a house, but presently it is only appropriate for dry climates. The floor
temperature is held at 68°F (20°C) by using either a small cooling machine
(chiller) connected to the floor tubing or the steady 55°F (13°C)
temperature of the ground by means of an earth loop. In most climates, the
cool floor can be used to supplement or replace standard ducted air systems,
however problems with over-cooling the floor could lead to wet slippery
surfaces and fungus growth in more humid climates. Radiant floor cooling
technology is still in the experimental stages in most areas, but is rapidly
gaining popularity in Europe.
Cost of Radiant Floor Heating
The cost of installing a hydronic radiant
floor is approximately $4.00 to $6.00 per square foot ($40-$60 per square
meter). This 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
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.
EREC is operated by NCI Information
Systems, Inc. for the National Renewable Energy Laboratory/U.S. Department of
Energy. The statements contained herein are based on information known to EREC
at the time of printing. No recommendations or endorsement of any product or
service is implied if mentioned by EREC.
Energy Efficiency and Renewable Energy
Clearinghouse (EREC)
P.O. Box 3048 Merrifield, VA 22116
Voice: 1-800-DOE-EREC
E-mail: doe.erec@nciinc.com
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