A smooth surface
duct allows for optimum airflow. For best results, use galvanized sheet
metal or possibly PVC. Flexible aluminum duct is durable, easy to
install and often used. However, the ridges in aluminum flexible duct
increase static pressure and can reduce air flow and fan performance.
This results in lower CFMs, higher noise levels and greater energy
consumption. The degree to which performance is affected depends on the
length of duct, number and degree of elbows.
Sagging or weaving a
fan duct will also increase static pressure and reduce a fans
performance. When using a flexible aluminum duct, support the entire
length of the duct with braces or hangers to keep it as straight as
possible for the entire run. If the duct lies across the attic, do not
allow it to sag between each joist. Also, avoid weaving serpentine
Using dryer duct
connectors made of nylon or vinyl is not recommended due to high static
pressure caused by its ridges and curvature. Insulated flexible duct
must be fully extended to avoid added resistance.
First, allow a 2-3
foot straight run out of the fan before the first elbow. This allows
airflow to be uniform before passing through the first elbow. An
installation that has a 90-degree elbow immediately after the fan
exhaust port will cause air to flow back into the fan. This will reduce
fan performance and increase noise.
Second, use a long
radius angle to help ensure optimum airflow and minimum airflow noise.
The shortest smooth, inner surface duct with the least number of elbows
will provide optimum fan performance.