First, let's talk about chimney sizes. Chimneys can be broken down into two main categories; metal and masonry. Metal chimneys, also called prefab, class A and Double or triple wall are usually found in three diameters as show below. Larger diameters which may apply to prefab fireplaces. round flue tiles or single wall stainless chimney liner are also shown. Each diameter has a specific useable area, which is also called it's cross section. The capacity of a chimney to remove smoke and/or BTU's from an appliance is directly related to this area.
|Chimney Size||Square Inches||Common Venting Uses|
|6 Inch Inside Diameter||28.3||Stoves, Central Heaters|
|7 Inch Inside Diameter||38.5||Stoves, Central Heaters|
|8 Inch Inside Diameter||50.3||Stoves, Small Fireplaces, Central Heaters|
|10 Inch Inside Dia.||78.6||Stoves*, Fireplaces, Central Heaters|
|12 Inch Inside Dia.||113||Fireplaces, Central Heaters|
One thing that we notice from this table is that a 6" flue is almost 1/2 the size of an 8" flue. The natural tendency might be to think that a 6" flue is "only" two inches smaller that an 8" one, but in truth you can see the difference is much larger.
|Flue Size (O.D.)||Square Inches||Common Venting Uses|
|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|
Since smoke rises in a circular fashion, only a certain area of a square flue tile is really being put to effective use. The listings in the table above refer to the usable area.
Venting a Freestanding StoveIn general, your should use the same size chimney diameter as the flue collar on your stove, i.e. a 6" stove should use a 6" chimney or a 7 1/2 X 7 1/2 flue tile. In most cases, you can also step up a size or two..for instance, a 6" stove could vent into an 8" insulated chimney. If your flue is over 3 times the size of your stove outlet ( i.e. a 6" stove into a 13 X 13 Flue tile), then you may have problems with excess cooling of the smoke, resulting in poor draft and/or excess creosote formation. A stainless steel flue lining of the same size as the stove outlet will solve this problem.
Venting a Masonry Fireplace
Masonry fireplaces are built to varying sizes. The most popular range of sizes: 32" to 36" Wide 25" to 29" High It is important for the size of the chimney to be correctly calculated to match the fireplace opening. The rule of thumb is a 1/10 ratio, meaning that the fireplace opening can be 10 times as large as the flue tile cross section.
As an example: Fireplace 27 High" by 32" Wide = 864 Square inches of opening means the flue size must be 86.4 square inches or larger - A 13" X 13" tile will meet this criteria. In certain cases, the 1/10 rule can be expanded to as much as 1/12, that meaning that the opening could be 12 times the cross section of the flue. However, other factors such as chimney height, smoke chamber construction and location of then chimney must be taken into account. Flues for double sided fireplaces should be calculated by adding together the openings on both sides. This may require very large flue tiles such as 18" x 18".
More information on chimneys and draft can be found here.
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