Radiant heating has been around for at least 2,000 years, since wealthy Romans warmed their homes with open fires under tile and concrete floors. That long-ago innovation was simple but revolutionary because it eliminated interior fireplace smoke, which was channeled through chambers under the floors and into chimneys behind the walls. It was also a far more comfortable way to keep warm; instead of huddling around a fireplace in the main room, homeowners and guests could walk on a toasty floor anywhere in the house.
Today’s radiant heating systems have come a long way from Roman times, though they’re still “hidden” behind walls and floors. Now homeowners can heat their homes with electrically charged panels or by pumping heated water through a system of flexible rubber tubes. Radiant heat can be installed in walls or ceilings, although radiant-floor heating is the most popular form. Builders embed panels or tubing in concrete slabs or staple them to wooden subfloors in the same amount of time it takes to nail down a piece of plywood.
These innovations have made radiant heating systems a popular alternative to forced-air systems. According to the Radiant Panel Association (RPA), a trade group supporting radiant heating manufacturers and installers, the technology has been installed in nearly 60,000 homes annually for the past ten or twelve years.
How it works
To understand radiant heating, imagine standing next to a large window on a cold day. You feel chilly there because the window surface is colder than you are, not because the temperature in the room is lower near the window. “Radiant heat is heat transfer from a warm surface to a cold surface,” says RPA executive director Lawrence Drake. “When you stand next to a large, cold surface—whether it’s a window, a wall, or a floor—that surface is drawing heat right out of your body.” A radiant heating system creates more of a balance between you and cold surfaces in your home. By heating them, you decrease heat loss from your body, which makes you feel warmer.
Another helpful distinction is the difference between radiant heat, which is all around you, and warm air, which blows across your body. As long as warm air is blowing, you feel warm, but when it stops, your body begins to lose heat to colder surfaces around you. Radiant heat is more like constant sun. “Imagine standing in the shade of a tree on a cold day and then standing in the sunlight,” says David Johnston, president of What’s Working, an international consulting firm on green building and design. “The temperature outside is the same, but your body feels warmer under the sun.”
Also, it’s a common misconception that heat rises, when in fact warm air is what rises. “Heat can travel in any direction,” says Drake. With a forced-air system, much of the warm air blown into a room gets trapped at the ceiling, not near the floor where it would keep you warm. Radiant heat, however, provides a constant source of heat no matter where it’s installed—wall, ceiling, or floor. That can mean decreased energy consumption and lower energy costs.
Another advantage of radiant heat is that it’s quieter than the loud compressors and sounds of rushing air through ductwork of a forced-air system. In addition, it won’t compromise indoor air quality. “When you blow air, you move everything contained in that air, including dust, pollen, and dander,” says Johnston. “People who are sensitive to airborne particles should consider radiant heat to reduce their exposure to allergens.”
Because of the high cost of electricity, water-based radiant heating systems—also known as hydronic heating—are the most common. They can be retrofitted into older homes or planned into new ones. Water systems can also be divided into different zones by using valves and separate thermostats, so homeowners can have control over which rooms are heated. “My house has four zones,” says Johnston. “When we’re not in the bedrooms during the day, we keep those zones cooler. We save energy by only heating the zones we’re occupying at any given time.”
The components of a hydronic system are fairly basic: You need a boiler to heat the water, a pump to push the water through the house, and a system of plumbing pipes to carry the water.
Depending on the size of the house and its heating and water demands, it’s possible to tie the heating system into the main water heater. For larger homes a dedicated water heater is necessary, and even then the type of heater can vary drastically from a standard unit that costs just several hundred dollars to a super-efficient, $6,000 to $8,000 European boiler. It’s also feasible to hook up a radiant heating system to solar-heated water tanks or to tankless water heaters, which heat water on demand rather than wasting the energy to store hot water.
The piping used to carry the hot water can be made of copper, a material that was the only option ten years ago but that’s gradually being replaced in favor of tubing called cross-linked polyethylene, better known as PEX. This rubberized material is extremely flexible and strong and can be stapled underneath plywood subfloors or covered in several inches of concrete. To make a retrofit installation of PEX easier, several new engineered wood products between 1/2 and 11/4 inches thick are available containing premanufactured grooves that accept PEX tubing. The board containing the PEX tubing is nailed directly on top of subfloors and can be covered by hardwood or carpet.
For all its advantages, there are a few drawbacks to using radiant heat.
First of all, it’s expensive. Though advances have been made to make radiant systems less time-consuming to install, they can still cost three times as much as forced-air systems, depending on labor costs, the complexity of the house, and the size of the system. “While it’s usually more expensive, price depends on the application,” cautions Johnston. “It may actually be easier and less expensive to run PEX than duct work.”
Also, radiant heating may not be the best choice for warm climates where houses need to be air-conditioned more often than they need to be heated. In those cases, combine radiant heat with a forced-air system and duct work for the AC. “In Southern regions, radiant heating is more often installed to warm cold-to-the-touch tile floors than for room heating,” says Drake.
Another cautionary note is related to the energy-savings claims made by some radiant heating companies. Many leaders in the green building industry question how much more efficient a radiant floor system is over a forced-air system when it’s installed in a well-constructed, heavily insulated home with good passive solar qualities. According to an article published in the January 2002 issue of Environmental Building News, someone living in a well-designed green home with radiant heating may be able to tell a difference in comfort levels, but that same owner may not be able to tell a difference in heating bills. Even Drake and the RPA concede this last point. “In a tightly built house, you’ll probably find very little difference in energy savings between radiant and forced air,” he says.
Nevertheless, those who’ve experienced radiant heating through a cold winter won’t ever go back to forced air. “I find it much more comfortable,” says Johnston.