Most old chimneys are not up to current standards. Today’s standards reflect the use of air-starved woodburning stoves that deposit much more creosote than fireplaces do. The resulting chimney fires are hotter and last longer.
Because of the fire danger, building codes regulate chimney construction very closely, guided by the National Fire Protection Assn. Problems may arise when a preexisting chimney is converted to another use; for example, when a chimney that served a fireplace is used for a wood stove.
According to the National Fire Protection Assn., within a building a chimney must be at least 2 inches away from any combustible material. Where the chimney is outside the house, a 1-inch air space must be left between the chimney and the wall of the house. The walls of a masonry chimney must be solid brick or concrete at least 4 inches thick, or, if stone, at least 12 inches thick.
Height promotes draw; chimneys that are too short will not draw properly. Twelve feet is a practical minimum. (The height of a chimney is measured from the outlet of the furnace or stove, not from ground level.) The top of the chimney should rise at least 3 feet higher than the roof and 2 feet higher than any part of the building with 10 feet.
The cross-sectional area of the chimney must be at least as large as the area of the stove's outlet, but not more than three times larger.
Wood Heating Education and Research Foundation, Washington, DC.
R. L. Stone.
Fireplace operation depends on good chimney design.
ASHRAE Journal, February 1969, page 63.
R. L. Stone.
A practical general chimney design method.
ASHRAE Transactions, vol. 77, no. 1, pages 91-100 (1971).
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Last revised: 30 September 2010.