University of Wisconsin - Extension

Madison - In recent years a type of heating system dating back thousands of years has been making a come-back, according to John Merrill, University of Wisconsin-Extension housing specialist. This system uses air, electricity or, most often water to heat surfaces, which then radiate heat into the adjacent room. Most of the systems are designed to heat floors. According to the Radiant Panel Association, a trade association for the radiant heating industry, the industry has experienced a 700 percent increase in sales since the early 1990s.

"If you think about the way you are warmed by sunlight," Merrill says, "you can appreciate the advantage of being warmed directly by radiation from one surface, the sun, to another surface, your body. Just as the sun doesn't have to warm the air to make you feel warm so radiant heating systems don't have to heat the room air, as other heating systems do, to make you warm."

"On the other hand," Merrill says, "if the surfaces in a room are cold your body radiates heat to these colder surfaces. This means that the higher the temperature of surfaces in the room the warmer you feel."

An article in the March 2002 issue of the "Rural Builder" describes some of the advantages of radiant heating. Comfort seems to be a clear advantage. You can have warm floors in the midst of winter. You also avoid cool drafts that are common with forced air or baseboard radiator units. Furthermore, the stratification that occurs as warm air rises to the ceiling is less of a problem if you aren't depending primarily on air temperature for comfort.

Another advantage claimed for radiant heating is energy savings. In theory, you should be able to achieve the same level of comfort at a significantly lower thermostat setting. However, research reported in a Canada Housing and Mortgage Corporation technical report released in 2001 raises questions about whether home owners actual lower their thermostat setting and save energy. Thermostat settings in homes with radiant heating systems were actually set slightly higher than those in homes with forced air systems.

There are other potential energy savings, Merrill says. Boilers can be run at lower temperatures so there is less standby and line loss. There is likely to be less wasted heat as warm air leaks out through cracks and other openings. Radiant systems are often zoned room by room so that heat can be saved by only heating rooms where needed. Because of the low temperature hot water needed, radiant heating works well with solar energy systems allowing additional energy savings.

Other advantages include quieter operation. There are no noisy furnace blowers or clanking and gurgling radiators. Without radiators or registers there is much more flexibility in furniture placement. Indoor air quality may be improved, Merrill says, since there is less dust flowing around and dust particles are not heated to the point that they can produce possibly toxic gases.

On the other hand, radiant heating does have some drawbacks, as described in an article in the January 2002 edition of "Environmental Building News." In the first place, radiant heating is usually much more expensive to install than conventional heating systems. A radiant system for a typically sized house can cost $10,000 installed, an estimated 50 percent more than a forced air system. Building an exceptionally well insulated and airtight house can provide more energy savings and some of the same comfort advantages at the same or a lower cost. The surfaces in this energy efficient house will be warm enough that a warm floor may not be as noticeable as a comfort feature, Merrill points out.

"Radiant systems need insulation on the cool side to assure that the heat goes to the room and not the ground or basement," says Merrill. "Insulating the floor by itself can have a big impact on comfort, perhaps as much as the radiant heating."

Most radiant heating systems have a long lag time before the floor is warm; similarly, it takes a long time to cool, according to Merrill. This means the control system must be much more sophisticated and other heat sources such as extended use of an oven or solar gains can cause over heating.

At this point radiant heating systems provide heat only. Separate systems are required to provide cooling, air cleaning and ventilation. A forced air system can do all four. Radiant systems can provide cooling but this is not recommended in climates with high summer humidity since condensation and mold can occur.

There was a time when radiant heating systems were prone to failure due to broken pipes or other problems. These system reliability problems have been largely overcome with one important qualification. Systems are reliable if trained and experienced installers install them. This is in spite of the fact that the supplies and installation instructions make it sound like a do-it-yourself job. According to the article in "Rural Builder" the industry is concerned about the negative image created by systems installed by folks lacking the training and knowledge to do the job properly. Many contractors report a major part of their business is correcting mistakes made by other less experienced installers.