Field Controls SWG-3 - Choosing the Right Size

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DRAFT
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.
CHOOSING THE RIGHT SIZE
TABLE 1
CONTROL
SIZE
NOMINAL
CROSSSECTIONAL AREA
(SQ. IN.)
CONTROL
SIZE
NOMINAL
CROSSSECTIONAL AREA
(SQ. IN.)
6"
28
16"
201
7"
38
18"
255
8"
50
20"
314
9"
63
24"
452
10"
78
28"
616
12"
113
32"
804
14"
154
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"
SIZING THE CONTROL
DRAFT CONTROL