Ask a typical hearth retailer what’s the hottest product in the showroom these days, and you’ll get a two-word answer: "Vent free." Sales are booming for gas-fired appliances that release combustion byproducts directly into the living space instead of to the outdoors through a vent or chimney. Four million unvented appliances are already in homes, and the industry expects to sell half a million more in the coming year. Vent-free appliances are available in a variety of styles. The most popular products these days are the inexpensive gas logs, which homeowners can buy off the shelf at home centers for a few hundred dollars. The logs are placed in a typical wood-burning fireplace, but with the damper closed. The vent-free category also includes stoves, wall-mounted heaters, and factory-built fireplaces. Sizes range from 10,000-Btu/hr. space heaters to big units that look like high-end wood stoves and produce up to 40,000 Btu/hr. Price and convenience are key selling points: Because the units vent directly into the living space, homeowners save the cost of a vent or chimney and don’t have to put the units near an outside wall. The main advantage of vent-free heaters, however, is also the chief drawback: With no vent, the byproducts of combustion may degrade indoor air quality and increase moisture problems. Although gas is a relatively clean-burning fuel, it does cause some pollution. Natural gas (methane) and bottled gas (propane) are compounds of hydrogen and carbon, which when burned combine with oxygen to produce water vapor (H2O), carbon dioxide (CO2), and, if combustion is incomplete, carbon monoxide (CO). In addition, hot gas flames also convert atmospheric nitrogen and oxygen to nitrogen dioxide (NO2). In general, cooler gas flames produce more CO2, while hotter flames produce more NO2; either is harmful in high concentrations (See ""). The pollution levels in particular houses will depend on how the appliances are sized and operated. Officially, the vent-free industry recommends that the units be sized very small and operated only for short periods. Reports indicate, however, that some hearth retailers are still applying the "bigger is better" mentality in their sales and marketing efforts. But the hearth industry itself does not unanimously endorse vent-free appliances, regardless of how they are sized or operated. Fireplace maker Heatilator, Inc., has taken a stand against the devices: In a letter to retail dealers, the company’s president said that pollution and safety concerns outweigh the technology’s value. (In addition to air quality concerns, Heatilator officials have warned that using gas logs in factory-built fireplaces designed to be operated with the damper open may create dangerous overheating). However, Heatilator’s position puts the company in the minority. With scores of other manufacturers taking the opposite view, it is clear that builders and remodelers will be asked with increasing frequency to recommend or install vent-free heaters in the years to come. When making that decision, it’s important to weigh the advantages of low price and design flexibility against the potential risk of adverse health effects caused by higher-than-normal concentrations of CO, CO2, and NO2, and the likelihood of moisture damage to building structure and finishes from excessive amounts of water vapor. At a minimum, builders must follow the industry’s latest conservative sizing guidelines, advises John Crouch of the Hearth Products Association: "This technology should not be used just because it’s cheap. There are sizing guidelines that show the size of the appliance that is appropriate for the DOE heating zone and the size of the space. In small rooms in some climate zones, you may not be able to find an appliance small enough to be used for more than two hours at

a time." Crouch also says builders must warn homeowners about the risks and limitations of the appliances. "Homeowners have to understand that they cannot mess with that fire, they cannot disturb those logs," he explains. "And the family should understand that the appliance is to be used for four hours, and not in lieu of...

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