BACKDRAFTING YOUR LAST GASP

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1. Weatherization: Because of efforts to reduce air leakage, it takes less to depressurize a modern house than a turn-of-the-century residence. Homes built between about 1940 and 1975 in northern climates are about onethird as leaky as the previous housing stock. According to Gory Nelson of the Minneapolis Blower Door Company (which builds equipment for testing house leakage), the average new Minneapolis home can reach a serious level of depressurization under the influence of a fireplace or a large exhaust fan. And a tight, energy-efficient home may be depressurized by a single bathroom exhaust fan.

2.Fireplaces: When a fireplace is burning actively, it draws as much as 600 cubic feet per minute (cfm) of air from a home—easily enough to backdraft other combustion appliances. Even more serious, the fireplace's natural draft drops as the fire burns down. It may win the battle of depressurization early in the burn, backdrafting other appliances, and then begin backdrafting itself later, during the charcoal phase, when the carbon monoxide concentrations are highest.

Fireplaces are one thing that all the experts dealing with backdrafting agree on: If a modern house is to have one (and if it is to be used), a fireplace should have an ample outside air source and should be equipped with tight-fitting doors that are closed as soon as the flames begin to die.

3.Indoor barbecues and kitchen-island exhaust fans: Modern houses have many exhaust fans, some of which are very powerful. Jim White says that the two most potent inhalers of house air are indoor barbecues and kitchen-island exhaust fans. Typically, neither type of device is fitted with an outside air source, so they suck 400 to 600 cfm of air out of the house. No natural-draft burner can overcome this level of depressurization in even an average home. When the exhaust fan is operating, the furnace, gas water heater, woodstove, and the fireplace, too, will work backwards. White cites examples where the suction has bowed picture windows in to the extent that images distort.

Attempts to fit indoor barbecues and island exhaust fans with their own air inlets are impractical unless the retrofit incorporates an intake fan comparable to the exhaust fan. To provide natural make-up air to the average indoor barbecue fan, you'd need a hole about 32 inches in diameter. When 23 Canadian building code and standard committees convened in Ottawa in October to consider regulations to control backdrafting and spillage, indoor barbecues and other large exhaust fans were high up on the list for discussion.

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