BACKDRAFTING YOUR LAST GASP

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4. Smaller exhaust fans acting in concert:

In order of the volume of air they expel from a building (after indoor barbecues and island fans), clothes dryers (about 200 cfm), furnaces (about 120 cfm), standard range hoods (about 100 cfm), bathroom ex haust fans (50 to 80 cfm), woodstoves (less than 50 cfm) and gas water heaters (about 40 cfm) all can contribute to depressurization.

5. More-efficient and smaller furnaces: Most of the improvements in furnace efficiencies achieved in the last decade have come from more-effective heat extraction. As a result, flue gas temperatures are lower and draft is reduced proportionately. In fact, draft is so threatened by low exhaust temperature that the most-efficient furnaces now have fans to remove the waste products. It's the middle ground where problems can arise. A chimney that was marginal with a 65%-efficient furnace—because it was too large or was leaky or was on a cold outside wall—may not work at all at 80% efficiency.

The problem can be magnified when a house is extensively weatherized, reducing the need for heat, and a smaller and more-eficient furnace is installed. The flue gases will be cooler and their volume will be too little for the chimney's capacity. To cope with lower flue temperatures and smaller furnaces, natural-draft chimneys should be built from insulated stainless steel or, even better, insulating refractory in a size appropriate to the furnace capacity.

In the U.S., assuming that federally mandated minimumefficiency levels are instituted on schedule in 1992, the majority of new furnaces will have induced-draft fans, effectively eliminating the possibility of backdrafting.

6. Leaky forced-air furnace ductwork: Furnace installers and homeowners usually devote a lot of attention to sealing up furnace supply ducts, the tubes through which hot air is pushed by the fan. Typically, though, the return lines, through which cool air is pulled back to the furnace, get much less attention. Consequently, if the return ducts happen to be leaky in the furnace room, which isn't unusual, the furnace's own fan can depressurize the area near the furnace. In this case, you get a double whammy: The chimney backdrafts because of the fan, and then the fan distributes the poisons to the living area.

Well-sealed ductwork is particularly important on gas air-conditioning systems. While in the heating mode, the supply fan won't come on until the plenum has heated to a set level, allowing the chimney time to establish good flow. In air-conditioning mode, however, the distribution fan already may be on when the burner cycles. Bear in mind, too, that natural chimney draft is low in summer because of warmer outside temperatures.

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