LASTING LEGACY PLUMBING HEATING & AIR
BLOG

What to Do If Your AC Smells Musty

June 13, 2025

If your air conditioner kicks on and a musty smell fills the room, don’t ignore it. That “dirty sock” or strange odor usually points to mold, mildew, or stagnant water somewhere in the system—and the longer it lingers, the more damage it can cause to your AC and your indoor air quality.

Here’s what that smell means, how to get rid of it, and how to prevent it from coming back.

What Causes Musty AC Smells?

Mold or Mildew on the Coils: Moisture and dust on the evaporator coils create the perfect breeding ground for mold.

Clogged Condensate Drain Line: If water can’t drain properly, it pools inside the unit and becomes stale.

Dirty Air Filters: Filters that haven’t been changed trap moisture and contaminants, pushing a musty odor back into your home.

Leaky or Dirty Ductwork: Moisture or mold in your duct system can spread unpleasant smells throughout the house.

How to Fix That Musty Smell

1. Replace Your Air Filter
Start simple. A dirty filter can trap moisture and create odors. Replace it monthly during high-use months.

2. Flush the Drain Line
You can clean your AC’s drain line using warm water or a mix of water and vinegar to clear out algae or buildup. This helps prevent standing water and the mold that follows.

3. Clean the Evaporator Coil
If accessible, carefully clean your indoor coil with a no-rinse coil cleaner. If it’s caked with debris, it’s best to have it cleaned professionally.

4. Check for Duct Leaks or Contamination
Leaks can pull in dust and moisture from unconditioned spaces, like attics or crawlspaces. Sealing and cleaning ducts helps remove odors at the source.

5. Improve Airflow
Ensure all vents are open and unobstructed. Poor airflow can trap humidity in the system, accelerating microbial growth.

When to Bring in a Pro

If the musty smell doesn’t go away after changing your filter and cleaning the drain line, it’s likely that mold or bacteria is present in deeper parts of the system. Our team can do a full inspection, clean affected components, and recommend air quality improvements like UV treatment or duct cleaning. Schedule your appointment today!

Don’t Ignore the Strange Smell!

A musty AC smell is more than a nuisance—it’s a warning sign. Acting early can prevent long-term system damage and protect your family’s indoor air quality. Our local HVAC experts are here to keep your system running clean, quiet, and fresh all summer long.