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6
Installation Preparation
Professional Vent Hoods
Duct
fittings
Total
Equivalent Quantity Equivalent
Duct Piece Dimensions Length* Used Length
Round, 1 ft.
straight (per foot
length)
3-1⁄4" x 12" or 1 ft.
3-1/4" x 24" (per foot
straight length)
90° elbow 15 ft.
45° elbow 9 ft.
3-1⁄4" x 12" or
3-1/4" x 24"
90° elbow 15 ft.
3-1⁄4" x 12" or
3-1/4" x 24"
45° elbow 9 ft.
3-1⁄4" x 12" or
3-1/4" x 24"
90° flat elbow 20 ft.
7" round to
3-1⁄4" x 12" or 10" round
to 3-1/4" x 24" transition 1 ft.
3-1⁄4" x 12" to 7" round
or 3-1/4" x 24" to 10"
round transition 5 ft.
7" round to
3-1/4" x 12" or 10" round
to 3-1/4" x 24"
transition 90° elbow 5 ft.
3-1⁄4" x 12" to 7" round
or 3-1/4" x 24" to 10"
round transition 90° elbow 15 ft.
Round
wall cap
with damper 30 ft.
3-1⁄4" x 12" or
3-1/4" x 24" wall cap
with damper 30 ft.
Round
roof cap 26 ft.
Round
roof vent 24 ft.
Use this chart to
compute maximum
permissable lengths
for duct runs to
outdoors.
Note:
Do not exceed maximum
permissable equivalent lengths!
Maximum duct length:
100 foot for range hoods.
Flexible ducting:
If flexible metal
ducting is used, all
the equivalent feet
values in the table
should be doubled.
The flexible metal
duct should be
straight and smooth
and extended as
much as possible.
Do NOT use flexible
plastic ducting.
Total Duct Run
Note:
Any home ventilation
system, such as a ventilation
hood, 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.
*Actual length of straight duct plus duct fitting
equivalent. Equivalent length of duct pieces are based
on actual tests conducted by GE Evaluation Engineer-
ing and reflect requirements for good venting
performance with any ventilation hood.
48" Hoods Must Use
10" Round Duct
or 3-1/4" x 24"
Note:
Outlet on top of 48" hood
is 4-1/2" x 18-1/2".
30" and 36" Hoods
May Use 7" to 10"
Round, or 3-1/4" x 12"
to 3-1/4" x 24"
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