American Dryer Corp. ADH-120 - Manual (Page 21)

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IMPORTANT: For extended ductwork runs, the cross section area of the duct can only be increased
to an extent. Maximum proportional ductwork runs cannot exceed 20 feet (6.1
meters) more than the original limitations of 40 feet (12.2 meters) with three (3)
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.
ALL 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 (5.08 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 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.