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We make air handling technology. Better.
Air Handling Unit
Installation, Operation, and Maintenance Manual
April 2021
13.1 Surge
Surge is caused when the pressure on the fan is
high relative to the airflow at the speed it is running.
Increasing the flow and keeping the pressure constant
will move a fan out of its surge area. Reducing the
pressure and keeping the flow constant will also move
a fan out of its surge area. Setting the VFD to a lower
setting will not keep a fan out of surge. It will reduce
both the flow and the pressure and is more likely to
make the problem worse than better.
Surge can be identified by observing the fan. If the fan
is shaking visibly it is most likely in surge. This shaking
is often mistaken for fan vibration due to fan balance.
It usually takes instrumentation to determine whether
or not a fan is out of balance, so if you can see the fan
shaking, it is in surge. A fan in surge also produces
pressure pulsations which can often be identified by
feeling the ductwork or the outside of the air handler
cabinet. If pulsations can be felt with your hand on the
outside of the cabinet, the fans could be in surge. The
pulsations will be more noticeable on the inside of the
cabinet. It is more difficult to feel the pressure pulsations
on low pressure fans than it is on fans running at high
pressure (>5” WC).
Fans should not be allowed to run in surge as damage
to the motors and bearings can occur from long term
operation in surge. Damage to the motors and bearings
of your ClimateCraft air handling unit resulting from
operation in surge it not covered by the ClimateCraft
Limited Warranty. The fans also produce excessive noise
and the pressure pulsations can produce objectionable
noise and vibrations in other building elements.
Variable air volume systems will usually go into surge
under low airflow operation. Depending on the design of
the air distribution system, the duct static pressure set
point and the selection of the fan, the minimum airflow
that the fans can deliver before going into surge will be
30% to 50% of the design airflow.
Surge is often noticed when a building is under
commissioning and not occupied. Under those
conditions the building demand for air is low and the
system may want to operate below the minimum which
will result in fan surge. When the building is occupied
and has internal loads the problem often goes away.
If these conditions exist, ClimateCraft recommends
reducing the duct static pressure set point. This will
change the relationship between flow and pressure in
your system and will often keep the fans out of surge.
As the building becomes occupied you can increase the
duct pressure set point to make sure you have enough
air in the extremes of your duct runs. It is a good idea
to keep this set point as low as possible as your fan
energy consumption will be lower the lower you keep
it. If you have a FanMatrix™ fan system installed in your
air handler you can also shut one or more of the fans off
until the building become occupied.
If the problem persists after the building is occupied you
may have a design or construction problem with the
system. Consult your building design professional and
ClimateCraft for solutions to the problem.
