The furnace flue isn't exactly a subject with a lot of sex appeal, I'm afraid. Not like the ozone layer, or salmon spawning streams. But it is October, and furnaces are being fired up all over town. That means your furnace flue, if you have one, needs your attention. There are at least three good reasons: carbon monoxide, nitrogen dioxide and sulfur dioxide.
These three compounds can be found in the fumes from your gas or oil furnace - especially if it hasn't been serviced in a while. The fancy name for them is combustion byproducts. If, instead of being whisked safely up your chimney, these pollutants are allowed to accumulate inside your house, they can cause some very nasty health effects.
Here's the short list: headaches, dizziness, nausea, fatigue, excessive yawning (don't laugh), heart palpitations and chronic bronchitis. In extreme cases, collapse, unconsciousness and death.
According to the Centers for Disease Control in Atlanta, carbon monoxide is the leading cause of poison-induced death in this country, killing between 3,500 and 4,000 people every year. (Not all of these people are killed by furnace exhaust, of course. Unvented area heaters on the blink, faulty car exhaust systems and cars left running in attached garages account for many of them.)
A far greater number of people suffer from chronic, low level exposures to combustion byproducts because their furnaces are venting, at least some of the time, into the house. Indoor air and energy experts estimate that 40 percent to 50 percent of houses have furnaces that are inadequately vented. People living in those houses may feel unwell when they are at home in the winter, experiencing persistent "flu-like" symptoms. Their symptoms subside when they are out of the house for extended periods. Unfortunately, those most susceptible to the ill-effects of these gases are those most likely to be housebound - infants, the elderly and the chronically ill.
Your combustion gases might not be drafting safely up your flue because of backdraft or blockage.
Backdraft occurs when your furnace is improperly ventilated - that is, not getting enough air. Combustion, of course, requires oxygen and your furnace is no exception. If it doesn't get the air it needs, it will suck air down the flue. If air is coming down the flue, exhaust fumes certainly aren't going up it.
How much air your furnace needs depends on the Btu input of your furnace. When your furnace was installed, the technician should have calculated the ventilation requirement and made sure that your furnace met it.
This doesn't always happen, of course, and, also, things change. Perhaps you've weatherized the basement since then. Perhaps you've installed more powerful fans upstairs and they're drawing air up from the basement and sucking furnace gases out of the flue.
Backdrafting can be a sporadic thing, occurring, for instance, when you have all your fans, your dryer and your gas hot water heater going. Or it can be almost constant, if the furnace room is simply inadequately ventilated.
A blocked chimney is usually caused by decaying mortar or tile inside your chimney tumbling down and filling it up past the point where the furnace vent comes into it. According to brickmason Bob Martin, the decay is accelerated by the following process: Sulfur in oil-burning furnace fumes is deposited inside your chimney. If the chimney isn't adequately capped, rainwater seeps into it and combines with the sulfur to form sulfuric acid, corroding the chimney.
Many older houses in Seattle had oil burners for years, then converted to natural gas. Exhaust from a natural gas-burning furnace is moist. The moisture soaks the sulfur-covered inside of the chimney, causing it to deteriorate further.
Fortunately, King County code now requires homeowners to have chimneys lined with metal when converting from oil to gas - with some exceptions. And most furnaces installed within the last five years or so come equipped with sensors that shut off the furnace automatically if the furnace backdrafts.
How can you tell if your furnace is venting properly? Jeff Portteus, owner of Affordable Gas Service, says the best indicator is simply sweat on your windows. Remember, gas furnace fumes are moist. After the furnace comes on in the morning, if it is venting into the house, moisture will condense on the cold glass of your windows.
Another clue, Portteus continues, is rust on the grillwork on the front of your furnace. If moist fumes are backdrafting, the moisture will rust your metal there.
And, in many, but not necessarily all, cases, your house will develop a characteristic sour smell, especially noticeable when you first enter the house.
If you suspect you might have a problem, call a furnace service person right away. An experienced technician can tell pretty easily whether you have a problem. And if you don't see warning signs, but haven't had your furnace serviced in a couple of years, have it done now. Both furnace and indoor-air experts recommend you have your furnace serviced every fall.
Lastly, carbon monoxide alarms are a good tool. These devices plug into ordinary electrical sockets and, just like smoke detectors, give off a piercing wail when they sense anything over a threshold amount of carbon monoxide. Find them at hardware and safety supply stores.
To learn more about this sexier-than-you-anticipated indoor-air pollution problem, call the Washington branch of the American Lung Association (800) 586-4872 and ask for a copy of their free pamphlet, "What You Should Know About Combustion Appliances and Indoor Air Pollution." If you get the answering machine, leave your name and address along with your request. And while you're at it, ask about the conference "Indoor Air: Homes, Offices, Schools, Health Effectsm Justice" on Nov. 2 in Seattle.
Susan McGrath's column runs every two weeks in the Home/Real Estate section. Send questions and comments to: The Household Environmentalist, P.O. Box 70, Seattle, WA, 98111.