A healthy human being can tolerate small exposures to many indoor pollutants, for example a short-term exposure (say, a month) to a moderate level of radon isn’t nearly as serious as several years of exposure to the same level.
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One key to preventing combustion pollutants from entering your home in the first place is by understanding how to avoid backdrafting. Backdrafting occurs when so much air flows down a chimney that the combustion by-products can’t be properly expelled from the house.
First, “combustion by-products” are the gases, such as carbon monoxide or carbon dioxide, that result from burning fuels such as oil, natural gas or wood. In most houses, these combustion by-products are pushed out of the home through a chimney having a natural draft. The draft is called “natural” because there is no fan—the only pressure involved is naturally occurring.
In today’s houses, a natural draft doesn’t always work well, especially when a house is depressurized (a condition when the air pressure in the home is less than the air pressure outside the home). When a house becomes depressurized for any reason (natural, accidental or controlled pressures), make-up air can enter the house from the outdoors by coming down the chimney; after all, a chimney is just a hole in a house that air can move through in either direction.
And that is when backdrafting occurs—when so much air from the outside flows down a chimney that the combustion by-products can’t escape from the house, thus spilling into the home via the chimney.
Fortunately, there are ways to prevent backdrafting from occurring:
1. If a house has any type of fuel-burning appliance connected to a chimney - a water heater, furnace, wood stove, etc.- it is important to make sure that the chimney is operating correctly.
2. There are fuel-burning appliances that are immune to backdrafting. Some don’t rely on a natural draft because they expel their combustion by-products outdoors directly without using a conventional chimney.
3. If anything is done to a house that can affect the house tightness (e.g. weatherization or insulation) or the amount of pressurization/depressurization (e.g., operating a central vacuum), chimney operation should be evaluated and corrected if needed.
The bottom line is to not use unvented combustion appliances; always ensure that combustion by-products are vented outdoors. If backdrafting has occurred in the home it important to: 1) know what to look for, like flu-like symptoms such as headache, dizziness, nausea, vomiting, diarrhea, weakness, general malaise, or shortness of breath, that can be caused by CO, and 2) contact the fire department immediately to ensure family safety. Be sure to use ventilation devices and methods that enable enough fresh air to enter the home to help prevent backdrafting.
This article was adapted with permission from the book, Understanding Ventilation by John Bower, and by permission of the Home Ventilating Institute (HVI).
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