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ALL 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.
IMPORTANT: Exhaust back pressure measured by a Magnehelic in the exhaust duct should not
exceed 0.3 inches of water column (0.74 mb).
NOTE: When the exhaust duct work 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 Duct Work Protection
(1) To protect the outside end of horizontal duct work from the weather, a 90º elbow turned
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 opening of the exhaust duct work.