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STEP 2:
PREPARE THE VENTING SYSYTEM
STANDARD FITTINGS
DUCT LENGTH
The total length of the duct system, including straight duct, elbows, transitions, wall or roof caps must not
exceed the equivalent of 140 feet.
For best performance, do not use more than three 90 degree elbows.
Below are the standard fittings and their equivalent length in feet.
Figure 9
Examples
For 3 1/4"x10" SYSTEMS
1-3 1/4" x 10" 90
o
elbow
1-Wall Cap
8 feet straight duct
TOTAL LENGTH
1-transition
2-90
o
elbows
1-Wall Cap
8 feet straight
TOTAL LENGTH
For 6" ROUND SYSTEMS
1
4567
23
3 1/4"x10"
to 6"=5ft.
90
o
elbow
=10ft.
45
o
elbow
=5ft.
3 1/4"x10"
wall cap
=40ft.
3 1/4"x10"
flat elbow
=10ft.
3 1/4"x10" roof
cap=24ft.
3 1/4"x10" 90˚
elbow=25ft.
6ft.
2ft.
2ft.
3 1/4"x10"
90
o
elbow
wall cap
6ft.
90
o
elbows
transition
wall cap
= 25 ft.
= 40 ft.
= 8 ft.
= 73 ft.
= 5 ft.
= 20 ft.
= 40 ft.
= 8 ft.
= 73 ft.
NOTE: If the existing duct is round, you must use a rectangular-to-round adapter, with a rectangular 3" extension
duct installed between the damper assembly and the adapter to prevent the exhaust damper’s sticking.
To calculate the equivalent length of each ductpiece used, see the examples below.