Symptoms
- Musty or mildew smell when the unit first turns on
- Smell fades after running a while
- Visible dark spots on the filter or coil
- Smell worse on humid days
Likely causes
- CommonMold or mildew on the filterA damp, dusty filter is an easy place for mold to grow.
- CommonMold on the evaporator coil or in the drain panCondensation that doesn't fully drain leaves standing moisture that grows mold.
- PossibleStagnant water in a clogged drain panWater that can't drain sits and develops odor-causing bacteria.
- PossibleUnit sitting unused with moisture inside for a long periodOff-season storage without drying out first invites mold growth.
What to do
- Unplug the unit and remove the filter; wash it with warm soapy water, rinse well, and let it dry completely before reinstalling.
- Remove the front panel and look at the coil and drain pan for visible mold, dark spots, or standing water.
- Spray the coil with a coil cleaner made for AC units, or a mix of water and a little white vinegar, and let it sit briefly before wiping.
- Clear the drain pan and drain hole of any debris so water flows out instead of pooling.
- Run the unit on fan-only mode for 30 minutes with the room dry to help air out any remaining moisture.
- At the end of the season, run fan-only mode for an hour before storing to dry the coil out fully.
Tools and parts
- Soft brush
- Mild dish soap or AC coil cleaner
- White vinegar (optional)
Replacement window AC filterSearch: “window air conditioner filter”
When to call a pro
Call an appliance technician if mold keeps returning after cleaning or if it's grown deep inside a coil you can't reach, since deeper cleaning may require disassembly.
Safety
Generally low risk
Generally low risk for a careful person. Stop if anything looks different from what this guide describes.