TL;DR

Reduced airflow from a CUE fan is most commonly a slipping belt losing RPM, a dirty impeller, or a closed inlet or discharge damper. Check dampers first, then fan RPM with a tachometer, then impeller condition.

What you might see

measured airflow below design CFM
downstream process temperatures rising due to insufficient cooling
fan motor current lower than normal (belt slip) or higher than normal (system resistance)
damper position indicator shows partially closed

Likely causes

Tap to mark as ruled out

Belt slipping causing the fan to run below design RPM
Partially closed inlet or discharge damper restricting flow
Dirty impeller blades with material buildup reducing blade area and flow efficiency
System resistance increased from a blocked duct, filter, or heat exchanger

Required tools

Non-contact tachometer
Manometer (for static pressure measurement)
Anemometer or pitot tube for airflow measurement
LOTO kit

Safety first

RPM measurement with a non-contact tachometer requires the fan to be running. Keep clear of the rotating shaft and belt drive. Do not remove the belt guard.
Lock out before entering the fan or duct system for impeller inspection.

Procedure

7 steps · ~30 min

1
Check all inlet and discharge dampers for full-open position. Confirm that control actuators have received the correct open command from the BAS or control system.
2
Measure fan RPM with a non-contact tachometer at the shaft or sheave while the fan is running. Compare to the design RPM from the Greenheck fan schedule or nameplate.[1]
3
If RPM is below design, check belt tension and slip (see the high-noise procedure for this machine). Correct tension and re-measure RPM.
4
If RPM is at design but airflow is still low, lock out the fan and inspect the impeller for material buildup. Clean if dirty.
5
Inspect the duct system and filter housings downstream for blockage. A blocked filter bank on the discharge side increases system resistance and reduces flow.[1]
6
Measure static pressure at the fan inlet and discharge with a manometer. Compare to the design static pressure. Higher than design pressure indicates system resistance has increased. Lower than design inlet pressure indicates an upstream blockage.
7
If all mechanical checks are good and airflow is still low, the system resistance curve may have changed due to a process modification. Consult the original fan selection chart with the new resistance to determine if a sheave change is needed.[1]

Sources

Greenheck CUE Centrifugal Upblast Roof Exhaust Fan Installation and Maintenance Manual, Greenheck Fan Corporation
Greenheck CUE Utility Fan installation, operation and maintenance instructions, airflow measurement and system resistance considerations (general)
View source

More guides for Greenheck CUE

How to fix a slipping belt on a Greenheck CUE fan

A slipping belt loses fan speed and generates heat at the sheave. Check belt condition, tension, and sheave groove wear. A glazed or cracked belt must be replaced, not just re-tensioned.

~30 min7 steps

How to clean dust buildup on Greenheck CUE fan impeller blades

Dust buildup on fan blades causes imbalance and reduces airflow. Clean the impeller with a wire brush, restore uniform blade geometry, and schedule more frequent cleanings if the source is upstream filtration inadequacy.

~45 min8 steps

How to diagnose excessive vibration on a Greenheck CUE centrifugal fan

Vibration above 0.3 in/s on an industrial fan is usually an imbalanced impeller or worn bearings. Take a vibration spectrum reading, clean the impeller, and re-balance if needed. Replace bearings if the spectrum shows subsynchronous noise.