A musty smell when the air conditioner starts often comes from condensate that is not draining properly, a dirty filter, or damp surfaces in the air path. The priority is finding why moisture or debris remains in the system rather than masking the supply air with fragrance.
Check the filter and accessible return area
A loaded filter holds dust and can restrict airflow; replace or clean it according to the system design. Vacuum accessible dust around the return grille without reaching into equipment or wetting electrical components.
Look for condensate that is not draining
Standing water, a full drain pan, staining near the unit, or dripping suggests condensate may be trapped. Turn the system off if water is reaching electrical components and correct the drainage problem before continuing operation.
Startup-only odor can point to damp internal surfaces
The indoor coil and drain pan become wet during cooling; if they remain dirty or damp for long periods they can smell. Internal cleaning that requires opening service panels or working near wiring belongs to qualified heating-and-cooling maintenance.
Recurring or extensive mold requires moisture control
If there is extensive visible growth in ducts or porous materials, cleaning a grille does not solve the source. Identify condensation, leaks, or high indoor humidity and consider system inspection when the problem keeps returning.
Sources
- U.S. Environmental Protection Agency — Mold and Moisture — Moisture control, mold prevention, cleanup, and recurrence.