Installing a chimney affects its “draw” -- the ability to pull smoke away from the fire in the firebox and smoke shelf below. A minimum standard for a chimney height has an additional important function. A properly installed chimney not only aids draw, but it also gives burning embers time to rise and die before they emerge from the chimney and land on your roof.
How Chimneys Work
Chimneys carry smoke away from fires, not only in attractive fireplaces, but in wood stoves, cooking appliance hoods, furnace flues and even outside barbecues, where smoke is actually drawn back away from the fire into the chimney. Chimneys function as part of a system that, once started, creates its own inertia. As the heat rises from the fire -- or travels toward the cool brick in the case of the barbecue -- it carries smoke, ashes, sparks and noxious gases that are lighter than air along with it. As this collection of combustion products reaches the top of the chimney, it is carried away by prevailing currents of air, creating a partial vacuum in the top of the chimney that pulls more smoke up the chimney and draws more air into the firebox below, aiding combustion.
As the chimney rises above the roof, it dissipates in the winds of the surrounding weather conditions. Of more immediate concern, however, are the “local” swirls and eddies of current caused by the barriers caused by the roof ridge, gable end or parapet edges of the roof and by the building itself. The reason for building the chimney up above the roof -- 3 feet is the norm for a flat roof -- is to draft smoke high enough to ride the winds away from the roof.
Peaked roofs, particularly those with several ridges, present complex local air current problems. Chimney tops that rise within 10 feet of the crest of the roof should rise 2 feet above the peak. Chimneys that rise away from the peak are built to rise 2 feet above the highest point on the roof within 10 feet of the chimney. The top of the chimney, no matter where it is placed, must rise 3 feet above the lowest point where it enters the roof surface. This is the final step of the 2 foot, 10 foot, 3 foot federal rule for masonry chimney height used for furnaces and fireplaces. Additional state or local regulations may also apply to masonry chimney heights.
Separate regulations apply to metal chimneys that lack the insulation qualities of brick and stone. Metal flues must rise from 5 to 15 feet above their furnace or fireplace, depending on appliance and local regulations. Additionally, type “B” vents, commonly used on manufactured homes, require special terminal caps and, depending on size, must rise from 1 to 4 feet above the roof surface. The draft from one metal chimney can also affect others, making separation of the flues by at least 16 inches advisable.
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