5 Reasons Your AC Smells Musty When You Turn It On—and How to Fix It

On a sweltering summer day, you turn on the AC, excited for some reprieve from the heat. But once the system is kicking, you catch a whiff of a musty smell. It could be from many things, experts say. Regardless of where it’s coming from, it’s worth getting to the bottom of it fairly quickly. 

HVAC techs Bob Cardwell and Robert Dykes walk us through the potential reasons your AC unit might smell stale. They also share valuable maintenance tips (and when DIY'ing a solution is appropriate versus when it’s time to call the pros). 

Evaporator Coil Buildup

One of the most common reasons your AC smells bad? Mildew-like buildup on the evaporator coil is often to blame. “The coil operates at temperatures below the dew point, meaning condensation forms on its surface every time the system runs,” Dykes explains. “When the coil is coated in dust and debris, that layer of organic material stays wet and becomes a growth medium for mold and mildew.” 

Since the coil is continuously damp during operation, even a small amount of buildup can cause a noticeably musty smell when the AC initially kicks on. But that doesn’t mean it’s time to dive in with aggression to fix the issue. “What homeowners should not attempt is cleaning the evaporator coil with anything beyond a no-rinse coil spray available at hardware stores,” Dykes says. More significant contamination should be handled with care (by a professional) to avoid damaging sensitive components.

Standing Water in the Drain Pan

Beneath the evaporator coil is the drain pan, which collects condensation as it forms and funnels it safely out of the system. When everything is going as planned, the water doesn’t sit in the pan. 

However, problems can begin when the drainage process is interrupted. “If the drain line is even partially clogged, water can sit stagnant in the pan for days or weeks,” Dykes says. “In our experience, a drain pan with standing water will develop visible biological growth within 48 to 72 hours in a Houston summer.”

This is one of those issues you can visually check but not always fully resolve on your own. If you see standing water or signs of slime, the first step is to confirm whether the drain line is flowing properly. A simple clog may be cleared with basic maintenance, but recurring pooling (or any visible growth inside the system, like Dykes mentioned) is typically a sign it’s time to call a professional for a deeper cleaning and inspection.

Clogged Condensate Drain Line

A clogged condensate drain line is another common reason your AC smells musty, Cardwell explains. When the line becomes restricted, water can’t properly leave the system (which backs it up). This creates the perfect storm for odors and pesky buildup.    

In many cases, the blockage comes from sludge in the line, which can sometimes be cleared with a simple flush of water or with a wet/dry vac at the exterior drain outlet.  

Once the flow is restored, Dykes recommends pouring a cup of distilled white vinegar down the line. “It’s important to use bleach or vinegar after you clear and flush the drain line because they work on a chemical level and kill algae, mildew, and bacteria,” Dykes explains. This keeps it cleaner longer and removes that musty smell. 

Dirty Air Filter

A neglected air filter is another common reason an AC unit might smell musty. When the filter gets clogged with dust (and other particles), it restricts airflow and throws off how the system is meant to run.  

“This can cause the coil to run colder and wetter than designed, and introduce additional organic material into the system, which compounds the factors that generate odor,” Dykes explains. 

To combat this, Dykes recommends changing your air filter more frequently than the box says (which is generally 90 days). He explains that changing it every 30 days can nip this issue in the bud, and costs around $30. 

Presence of Mold, Mildew, or Bacteria Deeper in the System

If the musty smell persists after the basics above have been checked, the issue may indicate biological growth within the system itself.

“When an AC system sits dormant over winter, or even just a few days in a humid climate, moisture accumulates on cold surfaces inside the air handler and ductwork,” Dykes says. “That standing moisture, combined with organic debris like dust and skin particles, creates an ideal environment for microbial growth. When the system kicks on, it blows that odor directly into the living space.” 

If the musty odor persists even after the system has been running for a while, or if it’s accompanied by allergy-like symptoms such as irritation or congestion, it may indicate established growth inside the ductwork rather than surface-level buildup. In those cases, Dykes notes that remediation is typically more complex and may require professional treatment rather than routine cleaning.