Radiant Floor Heating

What makes this still-emerging technology a good fit with the homes you build?

Chuck Green
Professional Builder
January 1, 2004

Unlike forced-air heating (left), radiant doesn't heat the air, so the warmest temperatures are at the first 5 or 6 feet above the floor — where the people are. In addition, radiant floor heat doesn't rely on air circulation, so it doesn't stir up dust particles, allergens and other indoor air pollutants.

Radiant floor heating commands only a percentage point or two of the North American heating industry. Nonetheless, RFH is the fastest-growing segment of that market, expanding at an annual rate of 20% to 25%. Although many builders remain uncertain or even unaware of RFH, those who install these systems regularly have no qualms about the technology from a performance or marketability standpoint. The technology not only works well across numerous and varied applications, but it's also what increasing numbers of buyers want, as Americans become better versed in RFH's distinctive benefits.

"People buy radiant floor heating for the matchless personal comfort as well as the documented energy savings the technology can provide," says Jan Andersson, heating brand manager at Uponor Wirsbo, an international maker of RFH systems, whose North American headquarters are in Apple Valley, Minn.

Wirsbo pioneered the use of cross-linked polyethylene, or PEX, tubing for radiant heating systems in major European markets in the late 1960s and early '70s, bringing the technology to North America in 1984. Wirsbo became the first company to manufacture PEX tubing in the United States eight years later.

Because of PEX's strong, cross-linked molecular structure, this highly engineered plastic offers superior strength and durability over other materials, making it the preferred tubing material on both sides of the Atlantic for hydronic (warm-water) floor heating applications. (Electric RFH systems are also available.)

Andersson says PEX-based RFH offers other advantages as well:

High flexibility: RFH works with any type of floor surface, including tile, wood, marble, slate and linoleum.

High efficiency: By design, RFH is a low-temperature system that can be controlled precisely on a room-by-room basis through zoning. It also outperforms conventional heating systems at controlling natural heat loss in a home's inhabitants, so comfort can be achieved at a lower thermostat setting than is necessary with a forced-air system. "Floor heating systems can provide energy savings of 20% to 40% over comparable forced-air systems for similar applications," Andersson says, depending on building use, occupancy, design and construction. Even greater savings likely will be achieved in homes with high ceilings, large windows, high infiltration or any combination of these factors.

Minimal maintenance: For example, the system involves no ductwork, eliminating annual duct cleaning.

Fewer pollutants: RFH technology is quieter and cleaner than other forms of home heating because it does not involve circulating air. "Radiant doesn't stir up dust particles, allergens and other pollutants, as typically happens with a forced-air system," Andersson says. "This is a critical consumer concern because of the super-tight manner in which homes are built today and how that can trigger and aggravate respiratory ailments."

Wide acceptance: Manufactured to ASTM Standards F876 and F877, PEX-based RFH is code-approved throughout North America and can accommodate just about any type of structure and floor plan. "Some options may prove more complicated than others, but the industry has spent the past two decades steadily refining its offerings to make sure we can serve any type of residence whose builder wants to use radiant," says Andersson.

Because the feet are the body's primary thermostat, a warm floor means a person feels more comfortable overall with a lower thermostat setting.

How It Works
As Andersson notes, "Radiant is the most efficient means of transferring heat energy from one surface to another because it warms people directly." Here is a simple explanation of how it works:
Other systems concentrate the heat energy they provide through various delivery outlets. Conventional hydronic systems pump hot water through metal piping to convective baseboards, which warm the air in a room. Forced-air systems blast hot air through ducts connected to grilles or registers. An RFH system turns the entire floor into a primary, low-temperature heat source that delivers radiant energy evenly across a room to its occupants and surrounding objects, but not the ambient air.

RFH warms the floor surface to 75 to 80 degrees by running warm water through a PEX tubing grid — a series of uniform, carefully installed tubing loops — that can be buried in a concrete slab or poured floor underlayment or hung from joists below the floor. Newer systems embed the PEX tubing in prefabricated wooden panels installed atop an existing subfloor and beneath the floor covering.

Whatever the approach, the system's warmth radiates through the floor covering, traveling at the speed of light and releasing energy only after contact with a cooler object — a person or furniture.

"Since radiant doesn't heat air, the warmest temperatures are not up high in a room, but at the first 5 or 6 feet above the floor, where the people are," Andersson explains. This phenomenon, along with the fact that radiant does not rely on air circulation, means that people experience natural heat loss at a lower rate than they do with a forced-air system.

"Our feet serve as our body's primary thermostat," says Andersson. "When our feet are cold, we are cold. That's why we feel uncomfortable standing on a cold tile floor in the bathroom on a cold winter morning, even when the ambient air temperature is 72 degrees. The colder floor surface draws heat from our bodies faster than it can be replaced by the heat in the air."

The "natural" response is to crank up the wall thermostat, which only burns more fuel without making us that much more comfortable. Radiant, on the other hand, targets the body's "real" thermostat. "Toasty feet mean you'll feel pretty comfortable all over, and with a lower thermostatic setting than with a forced-air or baseboard system," Andersson says. "Experience shows that perfect thermal comfort may be achieved at thermostat settings of 65 to 68 degrees."

Zoning a radiant system can save even more energy. Each room on an RFH system can have its own tubing loop with its own thermostat, although it's more common to assign a thermostat to a group of rooms with similar functions and use — for example, the bedrooms on one loop; the kitchen, dining room and great room on another; and perhaps the bathrooms on a third loop.

"People prefer to keep their bedrooms cool at night, but they want that bathroom nice and warm at 6 a.m. on a winter morning," Andersson notes. "Radiant allows you to do that, boosting conservation as well as comfort, because you're not burning energy where it's not needed."

In a radiant floor heating installation, a series of uniform tubing loops are installed in a grid that then can be buried in a concrete slab or poured floor underlayment.

Marketing Hurdles
Although RFH represents only a small slice of the home heating pie, that's not because home buyers, after lengthy deliberation, reject the technology. "People often get frustrated, wondering after the fact why their builders didn't tell them about this technology," says John Fantauzzi, technical director of the Radiant Panel Association. "Had they known it was an option, they would have seriously considered it." Several likely reasons cause builders' reluctance to market RFH actively:

Cost concerns: The No. 1 negative is RFH 's larger upfront installation cost versus that of a conventional HVAC system. "The higher front-end costs can make builders, especially production companies, reluctant," says Fantauzzi. "They don't want to raise the prices of their homes."

Couple these cost fears with a poor grasp of the technology and how to sell it to consumers, and it's no wonder many builders shy away from the radiant opportunity. But Andersson and Fantauzzi suggest taking the longer view.

"Comparing radiant and forced-air systems is an apples-to-oranges proposition," Andersson says. "You can make radiant more competitive by making the systems smaller and more simplified, but these systems are designed to be a primary heating source for the entire house. The real benefit is the payback radiant offers over the long run in reduced fuel costs."

Fantauzzi seconds the notion that RFH brings additional value to the consumer, not only in immediate and superior comfort, but also as a long-term investment. "Incorporating radiant into their marketing helps builders set themselves apart from local competitors ."

Mechanical concerns: An RFH mechanical room looks radically different than that for a forced-air system. "A custom radiant job with all the piping loops, wires, thermostats and controls can cover a lot of wall space and leave a novice feeling a bit intimidated," Andersson says.

Installer concerns: The strange-looking mechanical room most visibly expresses typical builders' misgivings about RFH and therefore their reluctance to promote the technology. The severity of this marketing hurdle is directly proportional to a builder's size.

"A lot of builders, especially volume builders, are a little more hesitant to embrace radiant because it changes their practices a bit," Fantauzzi explains. "They have to account for something different in terms of their installation methods."

Custom builders have been more receptive, he says. "They're more attuned to doing things of a one-at-a-time nature."

But if builders, regardless of size or market orientation, find a quality subcontractor with experience and expertise in radiant, they're much more likely to at least consider offering RFH. Traditionally, this sub most often has been a plumbing and (hydronic) heating contractor because a boiler serves as a hydronic radiant system's principal heating source. (Local codes permitting, storage-tank-type water heaters are suitable for smaller applications.) However, as Andersson points out, "An increasing number of forced-air heating and air-conditioning contractors have begun to explore the radiant option by way of offering a complete comfort package. The builder, in turn, seems to appreciate having only one sub do all the heating and air-conditioning work."

Many of these subs are relatively small operators doing only three or four RFH jobs annually. "They may not be as up on the radiant game as they'd like to be," Andersson says. Wirsbo works to accelerate their professional development with an ongoing commitment to technical support and product education. For example, the company employs eight technical specialists in the United States to consult with installers daily. In addition, the Wirsbo Mini-Camp, an interactive, hands-on experience in radiant product application and installation, has trained more than 6,000 professional trades people.

"Even if an installer doesn't have extensive experience with radiant floor heating, the builder should feel comfortable knowing his sub will get all the support he needs to get the job done," Andersson says. "Wirsbo is committed to building a future not only for ourselves, but also for radiant heating and the builders and installers involved in it."

Getting Started
So which first steps should a builder take to get involved with radiant floor heating? Andersson urges a careful check of all supplier and installer options. Research not just the track record of the system and its components, but also the quality of the support: skilled personnel in the field and factory, product and applications training, ongoing technical support, etc.

This research should lead the builder to a qualified local installer. "Unfortunately, it is not uncommon for an installer to go his own way to cut costs, perhaps by installing cheaper plastic pipe that isn't cross-linked," Andersson says. "The consumer winds up with a system that falls short of expectations, the builder is embarrassed, and the entire radiant industry takes a bad hit.

"In an emerging market like this, you're bound to have people trying to make a quick buck by taking shortcuts. Good, careful research will preclude those problems and locate the right supplier and installer."