Radiant-floor heating offers a number of significant benefits:
By far, the biggest selling point for radiant-floor heating is comfort. The large radiant surface means that most of the heat will be delivered by radiation-heating occupants directly-rather than by convection (the primary mechanism of heat delivery from conventional hydronic baseboard “radiators”). Warmer surfaces in a living space result in a higher mean radiant temperature, a measure of surface temperatures in a space that influences the rate of radiant heat loss from occupants). With higher mean radiant temperatures, most people are comfortable even at lower air temperatures. Delivery of the heat at floor level with a warm floor surface also allows occupants to walk around barefoot even in winter-a very popular feature. Enhanced comfort should be a big selling point in any green home, so a strong case can be made for this heating approach.
“Until you’re lived with this form of heat,” says Radiant Panel Association executive director Larry Drake (who got involved with radiant heating after years of working with solar houses), “it’s hard to understand how comfortable it is.” He argues that with green homes in particular, after going to all the effort and expense to incorporate healthy and sustainable materials, ensuring high levels of comfort with radiant heat should be a top priority.
There is potential for saving energy with radiant-floor heating through several mechanisms, including lower thermostat settings, lower-temperature boiler settings, and reduced infiltration. Homeowners with radiant-floor heating are likely to be comfortable at lower air temperatures because of the elevated mean radiant temperature in such homes, the lack of significant airflow (as occurs with convective hydronic heating and forced-air heating systems), and the delivery of heat at floor level. Proponents of radiant-floor heating argue that someone normally comfortable at 72°F (22°C) will be comfortable in a building with radiant-floor heating kept at 68°F (20°C). If this is true, one would expect people with radiant-floor heating to keep their thermostats lower and thus realize significant energy savings.
The second opportunity for energy savings with radiant-floor heating is through keeping the boiler temperature lower than is necessary with conventional baseboard hot water distribution. The typical European approach with radiant-floor heating is to circulate fairly low-temperature water on an almost-continuous basis, varying the water temperature as needed to satisfy the load. This practice might reduce heat loss into unconditioned space if boiler and piping are located in an unheated basement, but experts EBN spoke with suggest that the savings would be very small at best-especially because of the additional electricity consumption to operate pumps for long hours. Green building consultant Marc Rosenbaum, P.E., of Meriden, New Hampshire, suggests using a low-mass boiler that is fired on-demand, rather than a high-mass boiler operated almost continuously.
The third opportunity for energy savings (over forced-air heat) is that radiant-floor systems do not increase the rate of air infiltration. Standard forced-air heating systems can significantly increase or decrease air pressure in different parts of a building, which in turn can increase air infiltration/exfiltration rates-at least in a conventional, leaky building. With radiant-floor heating, as with baseboard hydronic heating, this will not happen. (A well-designed, properly balanced forced-air system should not increase infiltration.)
Potential for use of solar energy. The relatively low temperature required for circulation water in a radiant-floor heating system provides an opportunity to utilize solar hot water. This approach works best with concrete-slab systems; higher-temperature water is generally required when the tubing is attached to the underside of wooden floors. While such systems are fairly complex and expensive, radiant slabs offer one of the best ways to make use of solar energy for heating portions of a building without direct access to sunlight. Most practical are systems in which solar energy heats water in a storage tank that can then be circulated through the slab. According to an EREN Consumer Energy Information Brief (www.eren.doe.gov) titled “Solar Radiant Floor Heating,” such systems typically cost at least $14,000. Backup heat is still required and can be provided with a wood stove, through-the-wall-vented gas heater, electric resistance heat, or backup heating element in the solar storage tank.
By operating a boiler at a lower temperature, its life can be extended. Radiant-floor heating systems typically use water temperatures of 85-140°F (30-60°C), compared with baseboard hydronic systems that typically operate at 130-160°F (55-70°C). At these operating temperatures, boiler life can exceed 45 years, according to information from DOE. (Shapiro is skeptical of this claim, however, pointing out that newer boilers are made for cold-start operation and should hold up well with this temperature cycling.)
Radiant hydronic floor heating is extremely quiet. Unlike forced-air heat, there is no noise from a fan or airflow through ducts; and unlike hydronic baseboard heat, there is usually no gurgle of water through baseboard radiators or creaking from expansion and contraction. The primary noise will be the sound of circulating pumps and the fan used in power-venting the boiler. With radiant-floor systems that have tubing attached to the underside of wood flooring, there may also be some creaking from expansion and contraction.
Because there are no baseboard radiators or air registers with radiant-floor heating, there is much greater freedom as to where furniture can be placed. Radiant-floor heating systems are “invisible.”
An argument can be made for improved indoor air quality in houses with radiant-floor heat. Compared with a conventional forced-air distribution system, there is likely to be less dust circulated around the house. And unlike electric baseboard or forced-air heat, there will be no surfaces hot enough to burn dust particles-which could introduce volatile chemicals or toxic particulates into house air (even passing through filters). This concern would be greatest for people with acute chemical sensitivities. In fact, veteran builder Max Strickland, of Burkholder Construction in Travers City, Michigan, first became interested in radiant-floor heating several years ago after his wife became chemically sensitive. He’s worried about “frying the air” with conventional heating systems and feels that conventional filters on forced-air systems are not effective. Strickland went on to build an American Lung Association (ALA) Health House in Travers City three years ago, and he now incorporates radiant-floor heating into all of his homes (typically 4 to 6 high-end custom houses per year).