Check the wheel for any dirt or foreign material, especially in hard-to-see places: backside of the wheel, underside of the blades. Airfoil blades are usually hollow, and when exposed to excessive moisture can retain water. Drill a 3/16" drain hole near the trailing edge to fix the problem. Rebalancing is not usually necessary.
Check for corrosion or erosion, which usually occurs on the nose of the blade. On a paddle wheel fan the blade tips may be worn. An airfoil wheel exposed to abrasive material can develop pin holes in the nose of the blades. Eliminate these conditions and rebalance the wheel. If the wheel is seriously damaged, it will have to be replaced.
- Be sure fan speed is not too high. Fan may be operating without ductwork at low resistance so that too much air is flowing.
- The fan may be handling ambient air when it was originally intended for hot, less dense air.
- Fan may be running backwards.
- Air pre-spin into the fan inlet.
- Fan drive sheaves set for too low a fan speed.
- Resistance to airflow, such as caused by a closed damper, much higher than calculated.
the Fan Doctor below.