BACKDRAFTING YOUR LAST GASP

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Because there are so many devices in urban areas that burn carbon fuels, though, CO usually lingers there in the 5- to 10-ppm range and may exceed 10 times that level near busy highways. CO begins to affect hu mans after we've been exposed to as little as 15 ppm over eight hours—causing minor confusion and loss of sense of time—and at 1070 (that's 10,000 ppm) it kills in 10 to 20 minutes. Combine carbon monoxide's extreme toxicity with the difficulty of detecting it (the gas is colorless, odorless and tasteless at all but the highest concentrations) and the subtlety of its symptoms (victims of acute CO poisoning typically lose consciousness without recognizing the seriousness of their distress), and you have a prescription for accidental death. But even comparatively small concentrations of carbon monoxide present serious long-term health hazards.

TESTING FOR BACKDRAFTING

Though sophisticated instruments are required to accurately measure the potential for backdrafting in a house, you can perform a "worst-case" test of your own with nothing more than a finger for a test instrument. There are two stages to the test, the procedures for which we've adapted from recommendations by CMHC, Gary Nelson and Joseph Lstiburek.

Furnace Return Test

Close all outside doors and windows. Shut the door between the furnace room and the rest of the building, and turn on the furnace fan (no need for the burner). Using a portable smoke source (an incense stick or cigarette) inside the furnace room, look for air movement under or around doors. If the smoke trails toward the furnace, you've got return ductwork leaks that need repair.

Basic Depressurization Test

On a mild, still day, close all exterior doors and windows. At a time when the furnace hasn't run for several hours, turn on every exhaust device in the house (kitchen and bath fans, dryer, water heater, attic

and wholehouse fans) and open all possible doors between the furnace and the rooms where the exhausts are located. Keep all other interior doors closed. Go to the furnace room and have someone else turn on the furnace at the thermostat. Feel at the draft control on an oil furnace or the draft hood on a gas furnace for the heat of a backward flow in the chimney. Redo the test on gas water heaters and gas dryers after they've had a chance to cool thoroughly. If spillage continues for longer than about 30 seconds on any combustion appliance, you could have a problem.

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