HVAC Fan & AHU Vibration Checker

Per ISO 10816, up to 0.71 mm/s RMS is new-machine condition (zone A) and up to 1.8 mm/s is acceptable for unrestricted long-term operation (zone B). Between 1.8 and 4.5 mm/s the machine should be scheduled for corrective maintenance (zone C), and above 4.5 mm/s vibration is severe enough to cause damage (zone D).

Probably transmission, not severity: failed or shorted spring isolators, rigid conduit/duct connections bridging the isolation, or a duct acoustic resonance. Check isolator deflection (they should compress visibly), flexible connector condition, and whether the hum follows fan speed before rebalancing anything.

On the fan bearing pedestals, horizontal; on AHUs measure on the bearing, not the casing panel — as close to the bearing as possible, on stiff metal (never on covers or guards). Take horizontal, vertical and axial readings if you can; use the highest for the ISO grade and always re-measure at the same spot, load and speed.

The usual suspects are blade fouling and imbalance, belt wear and sheave misalignment, bearing wear, and isolator failure transmitting vibration into the structure. A frequency spectrum tells them apart: imbalance shows at 1× running speed, misalignment at 2×, bearing defects at non-synchronous frequencies, and looseness as a raised noise floor with harmonics.

Velocity in mm/s RMS, 10–1,000 Hz. ISO 10816 zone tables are defined on broadband RMS velocity because it weights low- and high-frequency faults evenly for general machines. Acceleration (g) suits high-frequency bearing analysis and displacement (µm) suits low-speed machines, but neither maps onto these zone boundaries.