A musty or stale odor when an air conditioner first starts running is a common complaint that traces back to biological growth on the evaporator coil rather than any refrigerant or electrical issue. The evaporator coil operates in a consistently cool, moist environment during operation, conditions that favor mold and bacterial growth on the coil surface and surrounding drain pan area if organic matter, such as dust that has settled and become damp, is allowed to accumulate over time.
This odor is often most noticeable at startup because air passing over the coil picks up compounds released by this growth before the airflow has had time to circulate and dilute the smell throughout the room, becoming less noticeable as the unit continues running. A thorough evaporator coil cleaning, sometimes combined with an antimicrobial treatment designed for HVAC coil surfaces, typically resolves this odor issue, though addressing the drain pan and line simultaneously is important since biological growth there can contribute to the same odor even after the coil itself is clean.
Preventing recurrence generally involves more frequent filter cleaning, since a cleaner filter reduces the amount of organic dust reaching the coil surface in the first place, alongside periodic coil cleaning as part of routine maintenance rather than waiting for odor complaints to prompt a cleaning visit.