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Duct Fittings
Note: Any home ventilation system, such as
a cooktop with a downdraft exhaust
mechanism, may interrupt the proper flow of
combustion air and exhaust required by
fireplaces, gas furnaces, gas water heaters,
and other naturally vented systems. To
minimize the chance of interruption of such
naturally vented systems, follow the heating
equipment manufacturer’s guidelines and
safety standards such as those published by
NFPA and ASHRAE.
Use the chart at the right to compute the
maximum permissible lengths for duct runs
to outdoors.
Note: Do not exceed maximum permissible
equivalent lengths.
Flexible ducting
WARNING: Do not use flexible plastic
ducting.
If flexible metal ducting is used, all the
equivalent length values entered on the
worksheet for flexible ducting should be
doubled. The flexible metal duct should be
straight and smooth, and extended as much
as possible.
Vent installation should not exceed 150 feet
equivalent length.
Total
Equivalent Number Equivalent
Duct Pieces Length* Used Length
1 ft.
6" round, (per foot
straight length) feet
1 ft.
3
1
4
" x 10" (per foot
straight length) feet
6", 90
elbow 15 ft.
6", 45
elbow 9 ft.
3
1
4
" x 10"
90 elbow 16 ft.
3
1
4
" x 10"
45 elbow 5 ft.
3
1
4
" x 10"
90 flat elbow 18 ft.
6" round
to 3
1
4
" x 10"
transition 7 ft.
3
1
4
" x 10"
to 6" round
transition 5 ft.
6" round
to 3
1
4
" x 10"
transition
90 elbow 20 ft.
3
1
4
" x 10"
to 6" round
transition
90 elbow 12 ft.
6 " round wall
cap with damper 21 ft.
3
1
4
" x 10" wall
cap with damper 27 ft.
roof cap 20 ft.
roof vent 24 ft.
GEA00240
Total duct run should not exceed 150 ft.
6" round
6" round
Table of Equivalent Lengths
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