Inside every duct,
there is a constant pressure being exerted at any point from all
directions. When an exhaust fan moves air through the duct, the air
counters resistance from this pressure which is known as static
pressure. Thus, an exhaust fan has to have the ability to overcome the
static pressure in a duct so as to effectively duct stale air to the
outside of your home. This can be done by calculating the equivalent
duct length of any duct.
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