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113386-14 www.amdry.com 17
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.
3. Multiple Dryer (Common) Venting
If it
is not feasible to provide separate exhaust ducts for each dryer, ducts from individual dryers may be
channeled into a “common main duct.” The individual ducts should enter the bottom or side of the main
duct at an angle not more than 45° in the direction of airflow and should be spaced at least 48-3/4”
(123.83 cm) apart. The main duct should be tapered, with the diameter increasing before each individual
14-inch (35.56 cm) duct is added.
IMPORTANT: No more than four (4) dryers should be connected to one (1) main common duct.
The main duct may be any shape or cross-sectional area, as long as the minimum cross section area is
provided. The illustration on
page 18 shows the minimum cross section area for multiple dryer round or
square venting. These figures must be increased 10 square inches (64.51 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 [6.09 meters]) or has numerous elbows (more than two [2])
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 (6.09 meters). The diameter of a round exhaust must be
increased ten percent (10%) for each additional 20 feet (6.09 meters). Each 90° elbow is equivalent to an
additional 15 feet (4.57 meters).
IMPORTANT: Exhaust back pressure measured by a manometer at each dryer exhaust duct area
must be no less than 0 and must not exceed 0.3 in WC (0.75 mb, 75 Pa).
IMPORTANT: For extended ductwork runs, the cross section area of the ductwork can only be
increased to an extent. Maximum proportional ductwork runs cannot exceed 20 feet (6.09 meters)
more than the original limitations of 20 feet (6.09 meters) with two (2) elbows. When the ductwork
approaches the maximum limits as noted in this manual, a professional HVAC firm should be consulted for
proper venting information.
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.
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