Cassette Airflow Weakening: Corrosion Slowing the Fan Motor

Airflow from the ceiling unit weakening over several weeks, with the filter already cleaned and no change. The air still comes out cold, just very little of it. The unit is more than a decade old and sits in a ceiling void.

Cold air with weak flow separates the cooling side from the moving side immediately: refrigerant is doing its job and something about air movement is not. With the filter cleared, the fan is the remaining candidate, and it splits again. A motor can fail, or a motor can be starved. Measuring power at the motor while it runs under load is what tells them apart, and measuring at rest does not.

A ceiling void holds warm, damp air around wiring for years at a time, and exposed metal connections corrode slowly in it. Corrosion adds opposition in the supply path, which does not matter at rest and matters a great deal under load. The motor then receives less than it needs, turns slower than it should, and moves proportionally less air. It is undamaged throughout, which is why a bench test of the motor alone would have cleared it and sent the search somewhere else.

Clean the contact points and protect them, which restores full supply to a motor that never failed. No part is replaced at all. The point worth carrying forward is environmental rather than mechanical. The void that corroded these connections is still a void, so the same points deserve a look at each service rather than at the next slow fade.