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chimneys
for fireplaces and wood stoves

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.

If you are purchasing a new stove or fireplace insert, you may wish to ask that the person doing the installation has been certified by, for example, the National Fireplace Institute:

https://www.nficertified.org/public/.

Chimney inspections

The National Fire Protection Association defines three categories of chimney inspection, probing increasingly inaccessible areas¹. In most parts of the United States, inspections are required by law.

Level I: When a chimney is cleaned, or an appliance replaced with an identical one.

Level II: When a property changes hands, when a new appliance is connected, the chimney lined or relined, after any malfunction or other event that might have damaged the chimney. Includes all the checks of Level I, adding tasks like entering the crawlspace.

Level III: When a Level I or II inspection raises suspicions of a hazard that cannot be confirmed at that level of access. After an incident damaging the chimney or building. All the checks of Levels I and II, but rising to removal of parts of the building or chimney, if that is necessary to get access to parts that must be inspected.

1. NFPA.
NFPA 211: Standard for Chimneys, Fireplaces, Vents, and Solid Fuel-Burning Appliances. 2019 edition.
Page 211-48. It is greatly to the credit of the NFPA that, unlike many similar standards organizations, they make an important standard freely accessible on their website: NFPA 211

for further reading

Sobering statistics on the results of poorly maintained chimneys, from the Chimney Safety Institute of America:

https://www.csia.org/statistics.html

Reuben Saltzman is a professional home inspector. His blogs are always informative; here's his take on chimney inspections.

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: 5 March 2019.