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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.
However, due to recent innovations in floor
technology, "dry" floors have been gaining a lot of popularity over
wet floors. Much of this is because a dry floor is faster and less expensive to
build. There are several ways to make a dry radiant floor. 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 two layers of subfloor. In these instances, the tubes are often
in 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 since the heat transfer characteristics
of the floor is greatly improved over more traditional dry or wet floors.
Floor Coverings
Although ceramic tile is the most common floor
covering for radiant floor heating, almost any floor covering can be used.
However, some perform better than others. Common floor coverings like vinyl and
linoleum sheet goods, carpeting, wood or bare concrete is often specified.
However, it is wise to always remember that anything that can insulate the floor
also reduces or slows the heat entering the space from the floor system. This in
turn increases fuel consumption.
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.
Most radiant floor references also recommend
using laminated wood flooring instead of solid wood. This reduces the
possibility of the wood shrinking and cracking from the drying effects of the
heat. While solid wood flooring can be used, the installer is strongly advised
to be very familiar with radiant floor systems before attempting to
install natural wood flooring over a radiant floor system. Most manufacturers
and manuals relating to radiant floors offer guidelines to help you resolve
these issues.
Types of Tubing
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 often 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. These
materials have proven themselves to be more reliable than the older choices in
tubing. Fluid additives also help protect the system 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.
It is still not clear what causes this problem, but theoretically excessively
high water temperatures may be to blame. Tightening connections and clamps only
temporarily fixes the leaks. Remember this problem only concerns a specific
brand of rubber tubing. It does not have anything to do with the PEX
tubing, which has performed very reliably for many decades.
Since the price of copper tubing is considerably
lower now than several years ago, it is again gaining some popularity because of
it's superior heat transfer abilities over plastic-based tubing.
Controlling the System
A radiant floor that uses a concrete slab takes
many hours to heat up if it is allowed to become cold. This can be very
inconvenient while waiting for the slab heat up so it can heat the space.
Because of this, most radiant floor systems are not permitted to go into a very
deep night setback. Depending on how the floor is constructed, the time it takes
to re-heat the floor is sometimes longer than the occupant's sleep period.
Many floor systems are also controlled by a floor
thermostat instead of a wall thermostat. The system is also often designed to
keep the circulation pump(s) running while the thermostat only controls the
boiler's burner. Other, more sophisticated, types of controls sense the floor
temperature, outdoor temperature, and room temperature to keep the home
comfortable. Such a system may also use less fuel.
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 in climates where the heating and cooling
loads are similar in size. Another alternative for small houses, or those with
small heating loads, is to use an ordinary gas water heater to supply the
radiant floor system.
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 68o F (20o C) by using either a
small cooling machine (chiller) connected to the floor tubing or the steady 55o
F (13 o C) temperature of the ground by means of an earth loop. In
arid climates, the cool floor can be used to supplement or replace standard
ducted air systems. However, in humid climates, problems with over-cooling the
floor could lead to wet slippery surfaces and fungus growth. Radiant floor
cooling technology is still in the experimental stages in most areas, but is
rapidly gaining popularity in Europe where cooling needs are generally small.
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, the
floor covering, remoteness of the site, and the cost of labor.
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