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NOTE: Where the exhaust duct passes through a wall, ceiling, or roof made of combustible materials,
the opening must be 2 inches larger (all the way around) than the duct. 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 bent download
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 of the exhaust duct work.
IMPORTANT: For extended duct work runs, the cross section area of the duct work can only be
increased to an extent. Maximum proportional duct work runs cannot exceed fifteen (15)
feet with two (2) elbows. When the duct work approaches the maximum limits as
noted in this manual, a professional HVAC firm should be consulted for proper venting.