Office Unit Hums: Suspension Rods Sitting Uneven

A hum transmitted through the ceiling of an office whenever the concealed unit runs. Every suspension rod is tight when checked by hand. Nothing about cooling or airflow has changed.

Rods that are all tight can still be carrying very different shares of the weight, and that is the distinction worth making here. A unit hanging unevenly rocks slightly about the loaded rods rather than sitting still, and that movement is transmitted straight into the ceiling structure. Tightness is not the measurement; whether each rod is actually bearing load is. Checking that means testing them individually rather than as a set.

A unit hung from four rods is stable only when all four take a share of the weight. Where two are set slightly shorter, they carry the unit and the other two hang without load. The unit then pivots minutely about the loaded pair every time the fan starts and stops, and the ceiling grid conducts that movement across the whole area. All four rods are tight throughout.

The rod lengths are adjusted so every one carries load, and the unit is checked for level in both directions afterwards. Even loading is what stops the movement; adding more tension to the rods that were already carrying it makes matters slightly worse. On any suspended unit this is worth confirming at handover, since it is invisible from below.