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The main duct may be any shape or cross sectional area, so long as the minimum cross section area is
provided. The illustrations on
page 21 and page 22 show the minimum cross section area for multiple
dryer round or square venting. These figures must be increased 10 square inches (64.52 square centi-
meters) when rectangular main ducting is used, and the ratio of duct width to depth should not be greater
than 3-1/2 to 1. These figures must be increased in proportion if the main duct run to the last dryer to
where it exhausts to the outdoors is unusually long (over 20 feet [6.1 meters]) or has numerous elbows
(more than one [1]) in it. In calculating duct work size, the cross section area of a square or rectangular
duct must be increased twenty-percent (20%) for each additional 20 feet (6.1 meters). The diameter of
a round exhaust must be increased ten-percent (10%) for each additional 20 feet (6.1 meters). Each 90°
elbow is equivalent to an additional 30 feet (9.14 meters) and each 45° elbow is equivalent to an additional
15 feet (4.57 meters).
IMPORTANT: For extended duct work runs, the cross section area of the duct work can only be
increased to an extent. When the duct work approaches the maximum limits as noted
in this manual, a professional HVAC firm should be consulted for proper venting
information.
IMPORTANT: Exhaust back pressure measured by a manometer at each dryer exhaust duct area
should not exceed 0.3 inches of water column (0.74 millibars).
The duct should be smooth inside with no projections from sheet metal screws or other obstructions
which will collect lint. When adding ducts, the duct to be added should overlap the duct to which it is to be
connected. ALL duct work joints must be taped to prevent moisture and lint from escaping into the
building. Inspection doors should be installed at strategic points in the exhaust duct work for periodic
inspection and clean-out of lint from the duct work.
NOTE: Where the exhaust passes through a wall, ceiling, or roof made of combustible materials, the
opening must be 2-inches (5.08 cm) larger (all the way around) than the duct. The duct must
be centered within this opening.
b. Outside Duct Work Protection
1) To protect the outside end of horizontal duct work from the weather, a 90° elbow bent downward
should be installed where the exhaust exits the building. If the exhaust duct work travels vertically
up through the roof, it should be protected from the weather by using a 180° turn to point the opening
downward. In either case, allow at least twice the diameter of the duct between the duct opening and
nearest obstruction.
IMPORTANT: DO NOT use screens, louvers, or caps on the outside of opening of exhaust duct
work.