BACKDRAFTING YOUR LAST GASP

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In general, it's unwise to run a furnace blower constantly for any reason, though it won't be a problem as long as neither the furnace burner nor any other combustion appliance operates while the fan is on.

7. Leaks in the wrong parts of a house:

Wind blowing against a house creates high pressure on the windward side and low pressure on the lee. So, even leaky houses can become seriously depressurized if the predominant leaks are on the downwind side. These leaks could be intentional. Fresh-air inlets for a fireplace or furnace or even a dryer exhaust can allow wind to pull air from a house. And any house can become seriously depressurized if there's a window open on the downwind side.

Leaks at the wall-ceiling junction are also likely to encourage depressurization, since they help the house to act like a good chimney. This effect is more pronounced in a multistory house, simply because it's taller. Likewise, open windows on the second story (and particularly downwind) may further increase depressurization.

Gary Nelson has also noticed an interesting phenomenon in three Minnesota houses he's investigated. All three were fitted with power attic ventilators in attempts to solve moisture problems in insulation. According to Nelson, the moisture was coming from the house through holes in the ceiling, so exhausting air from the attic only increased the leakage rate. Not surprisingly, increasing the leakage rate increased depressurization, and all three houses had backdrafting problems.

8. Unsafe practices:

There aren't, and aren't likely to be, regulations prohibiting a homeowner from a foolhardy practice such as heating the kitchen with a gas range turned on high. Likewise, heating systems about which many scientists are skeptical, such as unvented space heaters, are quite common.

(Bruce Eugene Davis, Housing Director of the Economic Opportunity Agency of Washington County, Arkansas, reports that 12% of the houses in his state have unvented heaters, and he has found CO in the exhaust streams of 19% of the units he's tested.)

As for misusing an appliance such as a gas range to heat a space, we can adamantly say don't! But whether you want to have an unvented combustion appliance in your house at all (be it a range, an oven or a gas, oil or kerosene heater) is another matter entirely. You should be aware, however, that even in the absence of carbon monoxide, such an appliance could be contributing to chronic health problems your family members may have. (Canadian researchers are finding out alarming things about certain fungi that flourish in the damp environment often produced by unvented space heaters.) And should an unvented heater malfunction, it could be life-threatening.

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