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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 show the minimum cross section area for multiple dryer round
or square venting. These figures must be increased 10 square inches (64.52 square centimeters)
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 ductwork 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 ductwork runs, the cross section area of the ductwork can only be
increased to an extent. When the ductwork approaches the maximum limits as noted
in this manual, a professional heating, venting, and air conditioning (HVAC) firm
should be consulted for proper venting information.
IMPORTANT: Exhaust back pressure measured by a manometer at each dryer exhaust duct area
must be no less than must not exceed 0.3 inches (0.74 mb) of water column (W.C.).
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 ductwork 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 ductwork for periodic
inspection and cleaning of lint from the ductwork.
NOTE: When the exhaust ductwork passes through a wall, ceiling, or roof made of combustible
materials, the opening must be 2-inches (5.08 cm) larger than the duct (all the way around).
The duct must be centered within this opening.
b. Outside Ductwork Protection
1) To protect the outside end of the horizontal ductwork from the weather, a 90° elbow bent downward
should be installed where the exhaust exits the building. If the exhaust ductwork 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 opening of the exhaust
ductwork.
IMPORTANT: It is recommended that exhaust or booster fans not be used in the exhaust ductwork
system.
20
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