Waterfront Office Smells Musty: Drain Tray Film, Not Gas

A musty smell filling a room in the first minute after the unit starts, then easing noticeably as it keeps running. It is most obvious after the unit has been off for a day or more. Cooling is normal throughout.

The shape of the smell over time settles this, and observing it costs nothing. A source spread across a surface the air passes over releases steadily for as long as the unit runs. A source sitting in a pool releases what has accumulated above it in one burst when the fan starts, then very little afterwards. That pattern points at the tray rather than the coil, and a long idle period beforehand confirms it.

Condensate collects in a tray and leaves through a drain. A tray that does not fall away cleanly keeps a shallow pool behind. Film establishes on that wet surface and produces its by-products continuously, and those build in the still air above the tray while the unit is off. Starting the fan pushes that accumulated air into the room at once, then delivers only what is being produced live, which is far less.

The tray is cleaned out and the drain checked to fall properly, because a tray that empties fully has nothing to grow in. It is the standing water rather than the film that is the real target. Where the fall cannot be corrected, the tray needs to be part of the service routine rather than something looked at when it overflows.