BACKDRAFTING YOUR LAST GASP

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Fireplaces, if they're used, should have outside air inlets inside the fire chamber and well-sealed glass doors. Woodstoves are difficult to retrofit with outside air supplies, but models built for mobile homes come so equipped. One advantage of utilizing aerodynamic uncoupling over pressure neutralization is that major exhaust devices (like indoor barbecues) are no longer a problem, since there are no pollutants to be drawn in. In addition, in colder climates, infiltration (air moving into the house through the walls) is preferred to exfiltration (air moving out through the walls) because water vapor in the indoor air can cause problems if it condenses inside a wall.

Forensics

By now you should have a pretty good idea what causes backdrafting and how to stop it. But, as a final exam, see if you can figure out what happened to you back in our bedtime story. Don't peek below until you've figured out your explanation. Here's how we stacked the deck against your surviving the first four paragraphs. First, you spent the day messing with the air leakage characteristics of the building. That new furnace might have worked acceptably in the old leaky building, even though it's not cold enough to produce really strong draft, but the addition of caulk and storm windows has thrown it a curve. There's a hearty fire in the fireplace and that chimney is drawing correctly as you go to sleep. The cracked window on the downwind side is fighting against it—but it isn't winning yet. Later in the evening, the balance in the tug of war changes.

The fire burns down some, enough to let the thermostat call for heat from the furnace. Sure enough, it backdrafts, but it's in good tune and there's not much CO being produced. (Later in the heating season, should someone else take up residence, the performance will deteriorate under the adverse conditions.) You're asleep, so you don't notice the odor. Later still, the fire dies to its charcoal phase, producing lots of carbon monoxide and very little draft. It's chilly with that open window encouraging infiltration, though. Soon the flow reverses in the fireplace chimney, spilling CO-laden air into the room. Less than an hour later it's all over.

C0: THE INVISIBLE KILLER

WHEN CARBON-CONTAINING FUELS oxidize (burn) with air, they create hundreds of new compounds. Many of these waste products are dangerous, but only the simplest of them—carbon monoxide (CO)—is likely to occur in concentrations that immediately jeopardize health. Because of the deadly gas's tremendous affinity for blood in the lungs (an affinity more than 200 times that of oxygen), it readily steps in the way of the life-giving gas and denies cells their breath. People who die from exhaust fumes in cars or from smoke inhalation in fires suffocate from carbon monoxide poisoning. Luckily for us, carbon monoxide normally makes up only a very small part of Earth's atmosphere—about 1/10 of a part per million (ppm).

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