Chimney Downdraft
Sometimes the air will not rise properly in a chimney and will fall back into the fireplace or stove. This downdraft (or backdraft) condition results in poor combustion, smoke, and odors. It is generally caused by either deflected air currents or chilled flue gases.
Air currents can be deflected down into a chimney by higher nearby objects, such as a portion of the structure, another building, a tree, or a hill. It is therefore important to build the chimney higher than any other part of the structure or any nearby objects. Because the deflected air entering the chimney has not passed through a hot fire, it will cool the air in the flue and weaken the draft. When the air becomes cooler, it also becomes heavier and falls back down into the chimney.
Chimneys built on the outside of a structure, particularly those exposed on three sides, must have walls at least 8 inches thick in order to prevent chilling of the flue gases. Remember that flue gases must not be allowed to cool. The cooler the gas, the heavier it becomes.
Prefabricated Metal Chimneys Prefabricated (factory-built) metal chimneys are commonly made of 24-gauge galvanized steel or 16-ounce copper (see Figures 3-27 and 3-28). These chimneys normally can be used with any type of gas-, oil-, wood-, or coal-burn- ing appliance; however, check the Underwriters Laboratories (UL) certification for the intended fuel use because some are restricted to gas fuels.
Freestanding fireplaces, such as those illustrated in Figures 3-17, 3-18, and 3-19, are equipped with lengths of pipe designed to extend upward from the top of the unit for approximately 8 feet (see Figure 3-29). These lengths of pipe are available with porcelain enamel surfaces that match the color of the fireplace.
Prefabricated metal chimneys are designed for ceiling support installation, through-the-wall installation, and cathedral ceiling or open-beam installation (see Figures 3-30 and 3-31). Check the local codes and regulations before installing a metal chimney. Closely follow the chimney manufacturer’s installation instructions.
Note
Both double-wall and single-wall metal chimney pipes are available. A single-wall pipe is considered an extreme fire hazard.