Symptoms
- Musty smell noticeable for the first minute or two after the AC kicks on
- Smell fades as the system keeps running
- May be more noticeable after the system has been off for a while, like overnight
Likely causes
- CommonCondensation sitting on the coil between cyclesMoisture that lingers on the coil when the system is off can support light mold growth that smells musty at startup.
- CommonHumid air sitting in ductwork overnightDucts, especially in humid climates, can hold damp air that smells musty until it's flushed out by running air.
- PossibleDirty filter holding moisture and dust togetherA damp, dirty filter can develop a musty smell that's most noticeable when air first starts moving through it.
- PossibleStanding water in the condensate panWater sitting in the pan between cycles can develop a musty odor that gets pushed out when the fan starts.
What to do
- Replace the air filter and check it for dampness or visible mold.
- Set the thermostat fan to run for 15–20 minutes after each cooling cycle ends to help dry out the coil and ducts.
- Check the condensate drain pan for standing water and clean it if present.
- Run the system with the fan on Auto for a few days and note whether the startup smell lessens.
- Consider a UV coil light or antimicrobial coil treatment if the smell persists and mold is visible on the coil.
- Have a professional inspect the coil and ductwork if the smell doesn't improve after these steps.
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 coil and ductwork if the startup mildew 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.