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How to calculate the effective
duct length
The overall duct length will determine
the duct diameter.
^ Measure the actual duct length
needed to go from the dryer's
exhaust connection to the vent outlet:
1. Establish the number of elbows or
bends necessary, noting the type of
bend, angle and radius involved
(see Table).
2. Add the additional duct length (as
derived by using the values
specified in the table) to the actual
duct length to get the effective
length.
Note:
Maximum effective duct lengths:
4" (100 mm) diameter
5" (125 mm) diameter
6" (150 mm) diameter
Effective duct lengths over 66 ft (20 m)
require a larger duct diameter.
Longer ducts may lead to slight
decreases in dryer efficiency if the
necessary duct diameter is not
employed.
66 ft (20 m)
132 ft (40 m)
328 ft (100 m)
Tips about the ductwork
– Check with your local building code
to determine the maximum allowable
duct length.
– Use flexible metal pipe or rigid metal
pipe for venting. Non-metallic flexible
hose should not be used since it is a
potential fire hazard.
– For best drying results avoid extra
long ducting with too many bends,
angles or tight corners.
– The shorter the exhaust ductwork,
the more efficient the dryer will be.
Expelled air is slowed down in the
ducts due to friction against the inner
walls. The shorter and smoother the
inner walls are, the less friction will
be created.
– An elbow or bend creates more
friction than a straight duct. An
additional duct length factor must be
added for each elbow or bend (see
Table). This factor expresses the
resistance of the bend in terms of a
straight length of duct.
The additional length factor is then
added to the existing duct to give the
effective duct length.
A larger duct diameter helps to
reduce friction. Therefore the duct
diameter and length should be
increased simultaneously.
Installation
31
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