TL;DR
Fan vibration and squealing almost always come from a slipping or worn belt. Check belt tension and condition first. If the belt is good but vibration continues, check the fan wheel for accumulated dust or debris.
What you might see
- vibration felt in the unit cabinet or nearby ductwork
- squealing or chirping from the fan section
- belt debris visible in the fan compartment
- reduced airflow at the supply diffusers with fan running at normal speed
Likely causes
Belt slippage from too-low tension or a glazed belt surface
Worn or cracked belt nearing the end of service life
Misaligned sheaves placing uneven load on the belt and causing oscillating vibration
Fan wheel imbalance from accumulated dirt or debris on the blade surfaces
Required tools
- Belt tension gauge
- Straightedge for sheave alignment
- Replacement V-belt (correct cross-section and length from unit data plate)
- LOTO kit
Safety first
- Lock out the MAU electrical supply before entering the fan section. The fan wheel can freewheel from draft even with power off.
- Follow fall protection procedures when working on a rooftop MAU. Secure all tools to prevent them from falling from height.
Procedure
- 1
Lock out the MAU disconnect before opening the fan access panel.
Warning: Lock out the main electrical supply to the MAU before opening the fan section. MAU units on rooftops also require fall protection compliance at the access point. - 2
Inspect the belt for glazing, cracking, fraying, or hard spots. A belt with any visible cracking should be replaced.[1]
- 3
Check belt tension using a belt tension gauge. Compare the measured deflection at mid-span to the specification on the unit's drive label. A belt that deflects more than the specified amount is under-tensioned.
- 4
Check sheave alignment with a straightedge across both sheave faces. Misalignment greater than 1/16 inch per foot of shaft distance requires sheave repositioning.
- 5
Inspect the fan wheel blades for accumulated dust or debris. Clean with a brush if needed. Even a small amount of debris on one blade causes measurable vibration.
- 6
Replace the belt if it is worn, glazed, or cracked. Adjust tension per the unit specification after replacement.
- 7
Restore power, run the unit, and verify the vibration is resolved and the belt runs true on the sheaves.
Sources
Greenheck Greenheck RV / RVE / IGX Make-Up Air Unit (MAU) general technical documentation, Greenheck
Make-up air unit fan drive system maintenance, belt tensioning and sheave alignment, general HVAC references (general)
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