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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
(6.1 meters) more than the original limitations of 20 feet (6.1 meters) with two (2)
elbows. When the ductwork approaches the maximum limits as noted in this manual,
a professional heating, ventilating, and air-conditioning (HVAC) firm should be
consulted for proper venting information.
IMPORTANT: Exhaust back pressure measured by a manometer in the exhaust duct must be no less
than 0 and must not exceed 0.3 inches (0.74 mb) of 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. 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 duct.
Outside Ductwork Protection
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 the
nearest obstruction.
IMPORTANT: DO NOT use screens, louvers, or caps on the outside opening of the exhaust
ductwork.