Symptoms
- Damp, mildewy odor when the AC fan runs
- Smell may be strongest right at startup
- May be paired with visible moisture near the indoor unit
Likely causes
- CommonDirty or damp air filterA filter that's stayed damp can develop mold, especially in humid climates.
- CommonStanding water in the condensate drain panA pan that doesn't drain fully can grow mildew over time.
- CommonMold or mildew growth on the evaporator coilThe coil's constant moisture makes it a common spot for mold growth if not maintained.
- PossibleMold in ductwork, especially in a damp basement or crawlspaceDucts running through damp areas can develop mold over years.
What to do
- Replace the air filter, checking it for visible mold or dampness.
- Check the condensate drain pan for standing water and clean any slime or buildup you find.
- Have the evaporator coil inspected and cleaned if it's accessible and looks visibly dirty or slimy.
- Run the fan alone for 15–20 minutes after cooling cycles end to help dry out the coil and reduce mold growth.
- Consider a UV light or coil treatment product designed for AC systems if musty smells recur often.
- If the smell persists after filter and drain pan checks, have a professional inspect the coil and ductwork.
Tools and parts
AC air filterSearch: “central air filter size replacement”
When to call a pro
Call an HVAC technician to inspect and clean the evaporator coil and ductwork if the musty smell persists after filter and drain pan maintenance.
Safety
Generally low risk
Generally low risk for a careful person. Stop if anything looks different from what this guide describes.