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
66 ft (20 m)
5" (125 mm) diameter
131 ft (40 m)
6" (150 mm) diameter
328 ft (100 m)
Effective duct lengths over 66 ft (20 m)
require a larger duct diameter.
Longer ducts may lead to decreases in
dryer efficiency if the next larger duct
diameter is not followed.
Tips about the ductwork
–
Check with your local building code
to determine the maximum allowable
duct length.
–
Rigid metal or flexible aluminum
ducting is preferred. Flexible foil
ducting may move and bounce due
to the high airflow of the dryer. This
could lead to a thumping or grinding
noise.
–
Non-metallic flexible hose should not
be used. 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 duct
diameter should be increased if
longer duct lengths are required.
Ducting
44
