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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 and should be spaced at least 38-1/4 inches (97.2
cm) apart. The main duct should be tapered, with the diameter increasing before each individual duct (10-
inch [25.4 cm] minimum) is added.
IMPORTANT: No more than four (4) dryers should be connected to main common duct.
The main duct may be any shape so 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 (64.52 square centimeters) when rectangular main ducting is used, and
the ration 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 from the last dryer to where it exhausts to the outdoors is unusually long
(over 15-feet [4.6 meters]) or has numerous (more than two [2] elbows 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 15-feet (4.6 meters). The diameter of a round exhaust must be increased ten percent (10%) for
each additional 15-feet (4.6 meters). Each 90º elbow is equivalent to an additional 30-feet (9.2 meters) and
each 45º elbow is equivalent to an additional 15-feet (4.6 meters).
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 15-feet
(4.6 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.
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. 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 (2.54 cm) larger than the duct (all the way around).
The duct must be centered within this opening.
a. Outside Ductwork Protection
1) To protect the outside end of the horizontal ductwork from weather, a 90º elbow bent downward
should be installed where the exhaust exits the building. If the ductwork travels vertically up
through the roof, it should be protected from the weather by using a 180º turn to point the opening
downward. In either case, allow at least twice the diameter of the duct between the duct opening
and the nearest obstruction.
IMPORTANT: DO NOT use screens, louvers, or caps on the outside opening of the exhaust
ductwork.