The Health House program's builder guidelines, which Building System Magazine described as “the most stringent in the nation for building a healthier, more energy and resource efficient home,” recommend that only direct-vent, sealed combustion fireplaces be installed in new and existing homes. The guidelines have been field tested and proven in a number of climates and regions, from Alaska and Minnesota to California and south Florida.
“Unvented gas fireplaces dump hazardous carbon monoxide and other pollutants directly into our homes' air, creating unnecessary health risks to the people living there,” said Robert Moffitt, Communications Director for the American Lung Association Health House program. “That is why we recommend consumers and builders install direct vent, sealed combustion fireplaces, the only type we allow in homes that earn the right to be called registered Health Houses.”
Direct vent gas fireplaces use pipes to draw in oxygen from outside the home to help fuel the fire, while expelling soot particles and potentially dangerous combustion gases, such as carbon monoxide, to the outside. The entire system is sealed off from the living area of the home, offering the warmth, and appearance of a fireplace without the indoor air pollution risks. To protect occupants' health, the American Lung Association Health House program recommends installing direct vent, sealed combustion gas fireplaces since they send 100% of the fire's exhaust outdoors and use 100% of the outside air for the operation.
Unvented gas fireplaces operate without a chimney, flue, or vent. They dump 100% of the fire's carbon monoxide, odors, and soot directly into the room – essentially making your living space a chimney.
Byproducts of combustion appliances, such as gas fireplaces, include hazardous pollutants like carbon monoxide, particulates, and water vapor which are introduced into the living space if they are not vented directly out of the home. Each of these pollutants poses a serious threat to the health and well being of the home's residents. Excess water vapor can contribute to structural degradation of the home.
-excerpt from Hearth and Home Technologies Documentation