DUCT FITTINGS
Use this chart to compute
maximum permissible lengths for
duct runs to outdoors.
NOTE: Do not exceed maximum
permissible equivalent lengths!
Maximum duct length:
100 foot for 8″ round duct
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.
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.
This Hood Must Use an
8″ Round Duct.
5
Installation Preparation
Total
Equivalent Quantity Equivalent
Duct Piece Dimensions Length* Used Length
Round, 1 ft.
straight (per foot
length)
3-1/4″ x 12″ 1 ft.
3-1/4″ x 24″ (per foot
straight length)
8″ round
90° elbow 15 ft.
8″ round
45° elbow 8 ft.
3-1⁄4″ x 12″
90° elbow 11 ft.
3-1/4″ x 12″ or
45° elbow 6 ft.
3-1/4″ x 12″
90° flat elbow 24 ft.
8″ round to
3-1/4″ x 12″
or 3-1/4″ x 24″ transition 1 ft.
3-1/4″ x 12″ to 8″
round transition 8 ft.
8″ round
to 3-1/4″ x 12″ or
transition 90° elbow 16 ft.
3-1/4″ x 12″ to 8″ round
round transition 90° elbow 17 ft.
8″ round
wall cap
with damper 30 ft.
3-1/4″ x 12″ wall cap
with damper 30 ft.
8″ round
roof cap 26 ft.
8″ round
roof vent 26 ft.
*Actual length of straight duct plus duct fitting
equivalent. Equivalent length of duct pieces are
based on actual tests conducted by GE Evaluation
Engineering and reflect requirements for good
venting performance with any ventilation hood.
Total Duct Run
