Control Size Nominal Cross-Sectional Area (Sq. In.)
6" 28
7" 38
8" 50
9" 63
10" 78
12" 113
14" 154
16" 201
18" 255
20" 314
24" 452
28" 616
32" 804

Choosing The Right Size

(Sizing the Control)
Simple rules of thumb to guide size selections:
1. For most applications, the draft control should be the same size as the flue pipe. In other words, use a 6" control for a 6" pipe.

2. If the vent pipe or breach is square, use the round equivalent. For example - on a 14" x 14" breach use a 14" control. Little flow occurs in the corners of a square pipe, so its capacity is approximately the same as a round pipe of the same diameter.

3. If the breach is rectangular or oval, compute its cross-sectional area and select a draft control having the same or a greater nominal cross-sectional area. A breach 14" high x 10" wide would have a cross-sectional area of 140 square inches. From Table 1, select a 14" control with a cross-sectional area of 154 square inches.

4. Where a control larger than 32" is required, use more than one draft control with combined cross-sectional areas equal to or greater than that of the breach.

5. When chimneys are of an unusual height, or if the draft to be maintained is either very high or very low, it is advisable to deviate from the rules of thumb outlined here and refer to the sizing chart.


Diameter of Vent or Breach If Chimney Height is Use This Size Control If Chimney Height is Use This Size Control If Chimney Height is Use This Size Control
4" 15' or less 4" 16' or more 5"
5" 15' or less 5" 16' or more 6"
6" 15' or less 6" 16' or more 7"
7" 15' or less 7" 16' or more 8"
8" 15' or less 8" 16' or more 9"
9" 15' or less 9" 16'-30' 10" 31' or more 12"
10" 20' or less 10" 21'-40' 12" 41' or more 14"
11" 20' or less 12" 21'-40' 12" 41' or more 14"
12" 20' or less 12" 21'-40' 14" 41' or more 16"
13" 22' or less 14" 23'-45' 16" 46' or more 18"
14" 22' or less 14" 23'-45' 16" 46' or more 18"
15" 22' or less 16" 23'-45' 16" 46' or more 18"
16" 30' or less 16" 31'-50' 18" 51' or more 20"
17" 30' or less 18" 31'-50' 20" 51' or more 20"
18" 30' or less 18" 31'-50' 20" 51' or more 20"
19" 30' or less 20" 31'-50' 20" 51' or more 24"
20" 30' or less 20" 31'-50' 20" 51' or more 24"
21" 30' or less 20" 31'-50' 24" 51' or more 24"
22" 30' or less 24" 31'-50' 24" 51' or more 24"
23" 35' or less 24" 36'-60' 24" 61' or more 28"
24" 35' or less 24" 36'-60' 24" 61' or more 28"
25" 35' or less 28" 36'-60' 28" 61' or more 28"
26" 40' or less 28" 41'-70' 28" 71' or more 28"
27" 40' or less 28" 41'-70' 28" 71'-100' 28"
28" 50' or less 28" 51'-100' 32" 100' or more 32"
29" 50' or less 28" 51'-100' 32" 100' or more 32"
30" 50' or less 32" 51'-100' 32" 100' or more 32"
31" 50' or less 32" 51'-100' 32" 100' or more Two 24"
32" 50' or less 32" 51'-100' 32" 100' or more One 24" One 28"
33" 50' or less 32" 51'-100' One 32"One 20" 100' or more One 32"One 24"
34" 50' or less 32" 51'-100' One 32"One 24" 100' or more One 32"One 24"
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