Proper fan installation is necessary to optimize performance. The following
points outline installation techniques to help achieve optimum performance. For
existing construction, depending on the building's structure and the ability to
access the ducts, you may not be able to use all of these techniques, however,
the more techniques used, the better the performance you can expect from your
ventilation fan. In new construction where the walls and ceilings are open, with
proper planning, applying these techniques can be relatively easy.
In order to reduce elbows and optimize fan performance, install the fan with the
exhaust port pointed in the direction of the termination point. Be sure to use
the duct diameter size specified for the selected fan. Reducing the duct
diameter (at any point in the duct run) may create substantial static pressure
and reduce the fan's performance by as much as 90%.
A smooth surface duct allows for optimum airflow. For best results, use
galvanized sheet metal or 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
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 through trusses.
Using a flexible duct made of nylon or vinyl is not recommended due to high
static pressure caused by its ridges and curvature.
Elbows and bends should be avoided as much as possible. However, the fact is
that many installations require at least one elbow, as shown in Figure G. There
are two precautions you can take when installing elbows to achieve optimum
First, allow a 2 - 3 foot straight run out of the fan before the first elbow.
This allows airflow momentum to build 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
use a long radius angle to help ensure optimum airflow and minimum airflow
The shortest smooth, inner surface duct with the least number of elbows will
provide optimum fan performance.