Why Your AC Smells Musty–and What You Can Do About It

Key Takeaways

• A musty AC smell usually means mold, mildew, or trapped moisture in your system.
• Dirty filters, clogged drain lines, or standing water are common causes.
• Start by changing the filter and checking for leaks–if the smell lingers, call a pro.
• Prevent it with regular maintenance and by keeping your system dry and clean.

If you’ve ever turned on your AC and noticed a musty smell from it drifting through your home, you’re not alone. That weird, mildew-y scent is more common than you think, and it’s not something to ignore. A musty smell from the AC unit can be more than unpleasant; it’s often a sign that something deeper is going on inside your system.

Here’s what might be causing it, and how to clear the air.

Musty Smell from Your AC Unit? Here’s What It Means

When your AC smells musty, the most common culprit is mold or mildew growth inside the system. Your air conditioner pulls moisture from the air, and if that moisture doesn’t drain properly, it can linger in the drip pan or ductwork, creating the perfect environment for mold.

You might notice the AC smells like mildew especially when it first turns on after being idle for a while (Like at the start of summer). That initial burst of stale-smelling air is often your system clearing out built-up condensation or dust.

Why My AC Smells Musty

Here are some of the most common reasons your AC unit smells musty:

• Clogged drain lines - If the condensation can’t drain properly, it pools inside the unit.
• Dirty air filters - Filters trap moisture, dust, and debris. If they aren’t changed often, they can start to smell.
• Mold in the ductwork - This is especially common in humid areas or older systems with leaks.
• Standing water in the drip pan - A full or rusted pan can quickly lead to that mildew smell from the AC unit.

What to Do If Your AC Smells Moldy or Mildew-Like

If your AC smells moldy, start with a few simple steps:

• Change the air filter - Easy, quick, and often the fix.
• Run the fan - Sometimes, circulating air helps dry out the residual moisture.
• Check for visible water or leaks - If you see standing water, that’s a red flag.
• Schedule a professional inspection - If the smell doesn’t go away, it’s time to bring in an HVAC technician to check for mold inside the unit or ductwork.

How to Prevent Musty AC Smells in the Future

Once you’ve cleared out the smell, here’s how to keep it from coming back:

• Change your filters regularly, every 1-3 months, depending on your system and usage.
• Run your AC for short bursts during off seasons to keep moisture from building up.
• Keep vents open and unobstructed to ensure good airflow.
• Consider installing a dehumidifier if you live in a humid area.
• Get seasonal maintenance (our MintCare tune-ups help spot issues before they turn into smells).

When the AC Air Smells Off, Trust Your Nose

A musty smell from your AC unit isn’t something you have to live with. If your HVAC smells musty, it’s your system’s way of asking for help. Whether it’s a quick filter change or something more involved, dealing with it early can prevent bigger issues and keep your home from smelling fresh.

Need help figuring it out? Our licensed techs can take a look, clean your system, and make sure everything’s working (and smelling) the way it should with our MintCare maintenance plan.