INSTALLATION PROCEDURES
3-8
the duct work must be increased in proportion to the length and number of elbows in it. In
calculating duct size, the cross section area of a square or rectangular duct must be increased by
twenty (20) percent for each additional twenty (20) feet. The diameter of a round exhaust duct
should be increased ten (10) percent for each additional fifteen (15) feet. Each 90° elbow is
equivalent to an additional thirty (30) feet, and each 45° elbow is equivalent to an additional fifteen
(15) feet.
Figure.3-7 Single Dryer Venting
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
twenty (20) feet more than the original limitations of twenty (20) feet with two
(2) elbows. When the duct work approaches the maximum limits as noted in this
manual, a professional Heating Venting Air Conditioning (HVAC) firm should
be consulted for proper venting information.
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.
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.
Outside Duct Work Protection
To protect the outside end of horizontal duct work from the weather, a 90° elbow bent
downward should be installed where the exhaust exits the building. If the exhaust duct work travels
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