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IMPORTANT: Exhaust back pressure measured by a Magnehelic in the exhaust duct must be no
less than 0 and must not exceed 1.0 inch (2.48 mb) of water column (W.C.).
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 the horizontal ductwork from the weather, a 90º elbow turned downward
should be installed where the exhaust exits the building. If the ductwork 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, louvers, or caps on the outside opening of the exhaust
ductwork.