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Chimneys and Fireplace Sizing

Chimneys and fireplaces for stoves burning wood or coal as fuel.

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Chimneys can be separated in two main categories:

  • metal
  • masonry

chimney sizing

Prefab or Metal Chimneys

Prefab or metal chimneys, class A and double or triple walls are usually found in three diameters. Each diameter has a specific usable area, which is also called it's cross section.

The capacity of a chimney to remove smoke and/or energy (BTU's) from an appliance is directly related to the cross section area.

Chimney SizeCommon Venting Uses
Inside diameter
Inside Area
(square inches)
6 28.3 Stoves, Central Heaters
7 38.5 Stoves, Central Heaters
8 50.3 Stoves, Small Fireplaces, Central Heaters
10 78.6 Stoves, Fireplaces, Central Heaters
12 113 Fireplaces, Central Heaters

Masonry Flues

Flue SizeCommon Venting Uses
Outside Diameter
Inside Area
(square inches)
7 1/2 X 7 1/2 31 Stoves, Central Heaters
8 1/2 x 8 1/2 41 Stoves, Central Heaters
8 1/2 X 13 70 Stoves, Fireplaces, Central Heaters
13 X 13 99 Fireplaces
13 X 18 156 Fireplaces
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Related Topics


Boiler house topics, fuels like oil, gas, coal, wood - chimneys, safety valves, tanks - combustion efficiency.


Heating systems - capacity and design of boilers, pipelines, heat exchangers, expansion systems and more.

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Natural Draft - Air Flow Volume and Velocity

Air flow - volume and velocity - due to stack or flue effect caused by indoor hot and outdoor cold temperature difference.

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