Each Wash Bought Less Time: The Coil Had Reached Its End

Output falling across two seasons, with cleaning booked twice a year. Each round helps, and helps less than the one before, and the gain fades within weeks rather than months. Nothing has failed outright and the unit still runs on demand.

A single inspection cannot answer this, because a freshly washed coil looks like a freshly washed coil. What answers it is the series: supply air measured immediately after each wash, kept on file, and compared. A recoverable coil returns to the same ceiling every time. A coil that is wearing out returns to a lower one, and the size of that drop is the diagnosis.

Abrasive dust does two things across a long service life, and cleaning reverses only one of them. It blocks the gaps between fins, which a wash lifts out. It also wears and bends the fin itself, which nothing recovers. Early on the first effect dominates and washing looks like a complete answer. Late on the second does, so each wash starts lower because more surface has gone permanently.

This is arithmetic rather than a service decision. Annual cleaning spend, set against a replacement sized for the floor, with the output lost while capacity sits under spec counted on the same sheet. On a shrinking-return curve that comparison usually favours changeover inside a few seasons. Keep washing until the new unit is in, because a smaller lift is still a lift.