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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.
6″ round
roof cap 20 ft.
6″ round
roof vent 24 ft.
Total duct run should not exceed 150 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
downdraft cooktop.
Installation instructions.
19
Duct Fittings
Use the chart at the right to compute maximum
permissible lengths for duct runs to outdoors.
NOTE:
Do not exceed maximum permissible
equivalent lengths!
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
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.