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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 air flow and should be spaced at least 38-1/4 inches apart.
The main duct should be tapered, with the diameter increasing before each individual duct (10-inch
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 sect is provided. The illustration on page xx
shows the minimum cross section area for multiple dryer round or square venting. These figures must be
increased 10 square inches 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 fifteen [15] feet) or has
numerous (more than two [2] elbows in it). In calculating duct work size, the cross section area of a square
or rectangular duct must be increased twenty (20) percent for each additional fifteen (15) feet. The
diameter of a round exhaust must be increased ten (10) percent for each additional fifteen (15) feet. Each
90º elbow is equivalent to an additional thirty (30) feet and each 45º elbow is equivalent to an additional
fifteen (15) feet.
IMPORTANT: For extended duct work runs, the cross section area of the duct work can only be
increased to an extent. Maximum proportional duct work runs cannot exceed fifteen
(15) feet with two (2) elbows. When the duck work approaches the maximum limits
as noted in this manual, a professional HVAC firm should be consulted for proper
venting. HVAC should be consulted for proper venting information.
The duct work 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 larger (all the way around) than the duct. The duct must be
centered within this opening.
a. Outside Duct Work Protection
1) To protect the outside end of horizontal duct work from weather, a 90º elbow bent downward should be
installed where the exhaust exits the building. If the duct work 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 or caps on the outside of opening of exhaust duct work.