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Separated masonry flues but at same height (C) Daniel FriedmanFireplace Flue Size Specifications
Relation of fireplace size to flue opening dimensions

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Fireplace chimney flue dimensions:

Table of required chimney flue size and flue dimensions for masonry fireplaces. To provide adequate draft and safe venting of combustion products the chimney flue for a fireplace has to be adequate in size or cross-sectional area.

This article series provides information about masonry fireplaces, including inspection for damage/hazards (cracks and gaps that appear at masonry fireplaces due to chimney or fireplace settlement or movement), fireplace chimney sizing requirements, draft problems, chimney safety, creosote problems, inserts, and related fireplace and chimney topics.



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Masonry Fireplace Chimney & Flue Size Requirements

Brick chimney rain cap and cap - San Miguel de Allende, Mexico (C) Daniel FriedmanAccording to the Masonry Institute of America, the required flue size for masonry fireplaces (burning wood) is basically a function of the area of the fireplace opening.

Typical standards require a cross-section area of the fireplace flue or chimney/vent to be equal to 1/10 of the area of the fireplace opening itself, for a conventional wood-burning installation and without considering the effects of a glass fire-door.

[Click to enlarge any image]

The FHA requires using a 1/8 ratio instead of 1/10 for chimneys that are less than 15 feet high and the 1/10 ratio for chimneys that are 15 feet or more tall. [9]

How do we measure fireplace chimney height?

Note that "chimney height" for the purpose of determining fireplace requirements, is measured not from the ground outside nor from the "floor" of the fireplace hearth.

Rather you should use the distance from the fireplace throat to the top of the chimney. Don't include the chimney cap in height measurements - that added distance does not develop draft in the flue.

Chimney Cross-Sectional Flue Shape Effects on Draft

At FLUE SIZE SPECIFICATIONS where we discuss chimney flue sizing for venting heating appliances such as boilers, furnaces and water heaters, we explain that in comparing two flues of exactly the same square inches of cross-sectional area, a round flue will have better draft than a rectangular one.

For this reason, fireplace and chimney guides offer a table of effective chimney vent area (measured as a cross-section of the flue opening) and flue sizes. You can find effective area and flue size tables in the Uniform Mechanical Code and in the MIA's Masonry Fireplace & Chimney Handbook and also in NFPA 211 - Standards for Chimneys & Fireplaces. [9][10][11]

Un-lined chimney flues are also specified as larger than lined installations and use the 1/8 ratio we explained above.

Wood burning Fireplace Openings & Required Chimney Flue Sizes

To use the fireplace chimney flue sizing table below, calculate the area in square inches of the cross section of the inside of the fireplace opening. For a rectangular fireplace opening just multiply its width by its height in inches to calculate the value to look for in column A of the table. The dimensions given in columns B & C of the chimney sizing table present standard clay chimney flue tile dimensions and shapes.

Table of Wood burning Fireplace Opening Sizes vs. Required Chimney Flue Sizes

Type of Fireplace Square Inches of Fireplace Opening 1 Required Flue Size by the 1/10 rule Required flue size by the 1/8 rule
A
B
C
Masonry hearth face surrounding single fireplace opening
 
672 sq .in.
8 x 13 inches
12 x 12 inches
 
720
8 x 13
8 x 17
 
780
12 x 12
12 3/4 round
 
936
8 x 17 oval
13 x 13 square
 
1008
9 x 19 oval
10 x 21
 
1120
10 x 21
13 x 17
 
1536
13 x 20
17 x 17 oval 2
 
1920
17 x 17 oval
17 x 20
 
2085
17 x 20
21 x 20
       
Fireplace hearth open at front and one side
 
1107
10 x 21
12 x 16
 
1223
12 x 16
13 x 20
 
1388
12 x 16
13 x 20
 
1884
17 x 17 oval
17 x 20
 
2085
17 x 20
21 x 20
       
Fireplace hearth open at front only but with two wythe brick wall on one side
 
1344
10 x 21
13 x 20
 
1470
13 x 17
17 x 17 oval
 
1764
13 x 20
17 x 20
 
2016
17 x 17
17 x 21 oval
       
Fireplace hearth open on three sides, one short side enclosed to form fireplace back
 
1638
13 x 20
17 x 17
 
1932
17 x 17 oval
17 x 20 oval
 
2184
17 x 20 oval
21 x 20 oval
       
Fireplace hearth open on three sides, one long side enclosed to form fire-back
 
1782
13 x 21 oval
17 x 20
 
1971
17 x 17 oval
17 x 21 oval
 
2490
17 x 21 oval
21 x 21 oval
 
2850
21 x 21 oval
2/13 x 21 oval 3
       

Notes to the fireplace chimney flue sizing table:

1. These square inch areas were calculated by MIA based on common masonry fireplace widths and heights in inches; the effects of variations in fireplace depth, shape, and extent to which the fire-box follows optimum design principles were not considered.

2. We have not determined what MIA means by "oval" fireplace flues when both dimensions are equal

3. MIA notes that in a masonry wall some masons construct a single lined flue and leave the left flue unlined with 8" of space in the wall for added venting. Fire safety of this design relies on solid masonry construction and may not be code approved in all jurisdictions.

Adapted from Masonry Fireplace and Chimney Handbook, 2nd Ed., James E. Armhein, S.E., M.I.A. Masonry Institute of America, Los Angeles, CA 213-388-0472 prepared to include requirements of the 1994 UBC and other codes. - MIA[9].

Fireplace & Woodstove Articles

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Continue reading at FIREPLACE HEARTH DIMENSIONS & SUPPORT or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.

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