Advertisement
10
Maximum duct length:
For satisfactory air movement, the total duct length of a 3 1/4" x 10"
rectangular, 6" or 7" diameter round duct should not exceed 65
equivalent feet. See the WORKSHEET–CALCULATE TOTAL
EQUIVALENT DUCTWORK LENGTH section.
NOTE: It is important that ducting be installed using the most direct
route and with as few elbows as possible. This ensures clear venting
of exhaust and helps prevent blockages. Also, make sure dampers
swing freely and nothing is blocking the ducts. When applicable, in-
stall any makeup (replacement) air system in accordance with local
building code requirements.
Elbows, transitions, wall and roofcaps, etc., present additional
resistance to airflow and are equivalent to a section of straight duct
longer than their actual physical size. When calculating the total
duct length, add the equivalent lengths of all transitions and adap-
tors plus the length of all straight duct sections. The charts on the
following pages show you how to calculate total equivalent
ductwork length using the approximate feet of equivalent length of
some typical ducts.
* IMPORTANT: If a rectangular-to-round transition
adaptor is used, the bottom corners of the damper
will have to be cut to fit using tin snips, in order to
allow free movement of the damper. Equivalent
lengths of duct pieces are based on actual tests and
reflect requirements for good venting performance
with any hood.
Advertisement
