17
IMPORTANT:
For extended ductwork runs, the cross section area of the ductwork can only be
increased to an extent. Maximum proportional ductwork runs
cannot
exceed 15 feet
(4.57 meters) more than the original limitations of 15 feet (4.57 meters) with two (2)
elbows. When the ductwork approaches the maximum limits as noted in this manual,
a professional heating, venting, and air conditioning (HVAC) firm
should be
consulted
for proper venting information.
ALL
ductwork
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.
a. 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 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.
3. Multiple Dryer (Common) Venting
If it is not feasible to provide separate exhaust ducts for each dryer, ducts from individual dryers may be
channeled into a “common main duct.” The individual ducts should enter the bottom or side of the main duct
at an angle not more than 45º in the direction of airflow and
should be
spaced at least 38-1/4” (97.15 cm)
apart. The main duct
should be
tapered, with the diameter increasing before each individual duct (14-inch
[35.56 cm] minimum for gas models and 16-inch [40.64 cm] minimum for steam models) is added.
IMPORTANT:
No more than four (4) dryers
should be
connected to one (1) main common duct.
The main duct may be any shape so long as the minimum cross-sectional area is provided. The
illustration
on
page 19
shows the minimum cross section area for multiple dryer round or square venting. These
figures
must be
increased 10 square inches (64.52 square centimeters) when rectangular main ducting is
used, and the ratio of duct width to depth
should not be
greater than 3-1/2 to 1. These figures
must be
increased in proportion if the main duct run to the last dryer to where it exhausts to the outdoors is unusually
long (over 15 feet [4.57 meters]) or has numerous (more than two [2]) elbows in it. In calculating ductwork
size, the cross section area of a square or rectangular duct
must be
increased twenty percent (20%) for
each additional 15 feet (4.57 meters). The diameter of a round exhaust
must be
increased ten percent
(10%) for each additional 15 feet (4.57 meters). Each 90º elbow is equivalent to an additional 30 feet (9.14
meters) and each 45º elbow is equivalent to an additional 15 feet (4.57 meters).
Summary of Contents for AD-81 II
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