BACKDRAFTING YOUR LAST GASP

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What to Do

If you have a woodstove or an oil furnace that backdrafts, you're probably aware of it. Wood smoke is visible and fuel-oil furnaces provide a telltale whiff of sulfur to the nose. Unfortunately, gas appliances don't offer such ready hints. In any event, if backdrafting has been occurring often in a fossil-fuel burner, there should be some signs on the appliance itself. On an oil-burner, look for soot smudges around the draft control (a T in the exhaust from the furnace that has a freeswinging flap).

If they're extensive, you've been having at least some spillage. On a gas furnace or water heater, look at the outlet from the unit. There will be a cone-shaped hood there that admits extra air to the chimney to stabilize draft, and soot will show up around it if the appliance has been backdrafting significantly. Even if you find no signs that your furnace or water heater has been backdrafting, it's a good idea to perform the simple tests described in the sidebar. If one or more of your combustion appli ances is backdrafting for longer than about 30 seconds at start-up, you should fix the problem.

As long as the heater is correctly tuned, the situation isn't life-threatening. But poor air quality may already be causing health problems—sore throats, irritation to nostrils and eyes and asthmalike symptoms—and may be making the indoor environment overly humid. If you ignore the problem, the performance of the burner will deteriorate, and the situation could well become deadly. Experts take two approaches to solving backdrafting problems: One method goes after the mechanism of backdrafting; the other eliminates sources. The potential for backdrafting can be controlled by balancing indoor and outdoor air pressure with a fan that pushes air into the house. Jim White points out that air will get into the building anyway, so it may be best to heat it and push it in intentionally. Source control, as advocated by Lstiburek and Nelson, is done by a procedure called aerodynamic uncoupling. Fortunately, this isn't as complex as it first sounds.

It simply means that anything that burns fuel gets its supply air from the outdoors and dumps its waste back out there. For the purposes of combustion, the furnace, water heater, gas dryer and woodstove or fireplace all operate a "they were outdoors. The first three can s 11 be exhausted with a power venter, which can be plumbed to handle all the burners at once. (Tjernlund Products offers such equipment, as well as inexpensive sensors that will shut off a backdrafting appliance. Call 800/ 255-4208 for information.)

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