American Dryer Corp. Gas/HSI & Steam ID-120 - Manual (Page 23)

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IMPORTANT: Exhaust back pressure measured by a manometer at each dryer exhaust duct area
must be no less than 0 and must not exceed 0.3 inches (0.74 mb) of water column
(W.C.).
The duct 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.
b. Outside Ductwork Protection
1) 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 nearest obstruction.
IMPORTANT: DO NOT use screens, louvers, or caps on the outside opening of the exhaust
ductwork.