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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 45º in the direction of airflow. The main duct should be tapered, with the
diameter increasing before each individual 10-inch (25.4 cm) duct (when dryer common exhaust is used) is
added. If single 6-inch (15.24 cm) basket (tumbler) exhaust ducts are used, the diameter should be
increased after every second 6-inch (15.24 cm) duct.
IMPORTANT: No more than four (4) dryers or eight (8) baskets (tumblers) total should be
connected to one (1) main common duct.
NOTE: Refer to the illustrations on page 18 for examples of multiple dryer (common) venting.
The main duct may be any shape or cross-sectional area so long as the minimum cross-sectional area is
provided. The illustrations on page 18 show the minimum cross-sectional area for multiple dryer venting.
These figures must be increased in proportion if the main duct run from the last dryer to where it exhausts
to the outdoors is unusually long (over 15 feet [4.57 meters]) or has numerous elbows (more than one [1])
in it. In calculating ductwork size, the cross-sectional area of a square or rectangular duct must be
increased twenty percent (20%) for each additional 15 feet (4.57 meters). The diameter of a round
exhaust must be increased ten percent (10%) for each additional 15 feet (4.57 meters).
IMPORTANT: For extended ductwork runs, the cross-sectional area of the ductwork can only be
increased to an extent. Maximum proportional ductwork run cannot exceed 15 feet
(4.57 meters) more than the original limitations of 15 feet (4.57 meters) with two (2)
elbows. 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 basket (tumbler) exhaust
duct area must be no less than 0 and must not exceed 0.3 inches (0.74 mb) water
column (W.C.).
The ductwork 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. Also, inspection doors should be installed at strategic points in the exhaust ductwork for periodic
inspection and cleaning.
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.