AC Smells Musty

No one likes a musty smell. It can make you wrinkle your nose and degrades your indoor air quality (IAQ) and home enjoyment. If you’ve eliminated a wet basement, crawlspace, or a mold issue in your home as the source of the smell and pinpointed it to when your air conditioner comes on, we can help.

If your musty smell is coming from a window AC, wall unit, or portable AC, consult your owner’s manual.

If you have a musty smell coming from the AC vents when your central air conditioning system turns on, you have a moisture issue. The trick is to find the cause and eliminate it.

In a standard split system, your air conditioning system consists of an outdoor condenser unit with a condenser coil and an indoor coil called the evaporator coil. Refrigerant lines connect the two components and together, they complete the refrigeration cycle that cools your home (and removes humidity).

Read more about how an air conditioner works, and then come back. We’ll wait.

If the smell is noticeable when you turn your thermostat to COOL for the first time, the smell may dissipate in a day or two. If it persists and permeates your home, turn the AC off and do some investigating and troubleshooting, which we’ll explain here.

You will likely end up needing to call an HVAC professional, but here are some things to you (and they) should do if musty odors permeate your home when the AC comes on.

The first thing you should do if your AC smells bad is check the air filter. If it’s dirty, change it. If it’s wet, change it, and find the root of the problem. Your air filter should never be wet.

A clogged or dirty filter can become a breeding ground for bacteria, mold, and mildew in the humid months. Check out our HVAC filter maintenance guide for some tips.

Your air conditioning system needs good air circulation to function properly. Reduced airflow can cause a frozen evaporator coil and other issues. Proper airflow also helps increase your comfort. Ensure that drapes, furniture, or other items are not blocking or covering registers or return air vents.

While you’re at the indoor unit, check the area underneath the furnace and paired coil. The cooling process creates condensation that drains away from the unit.

Do you see signs of condensation or puddles of water? You may have an issue with a cracked or overflowing drain pan. You may also have a clogged AC drain line.

With either of these issues, standing water can allow mold and mildew to thrive. This can cause a musty smell when your AC is running. Call an HVAC professional to check the system out, replace the drain pan, and clear the drain line.

If you see signs of water leaking around your AC unit or signs of water damage, you may have a longstanding issue. Excess moisture in ducts can cause very big problems. Check for signs of visible mold at the registers and return air vents. You’ll need to call an HVAC professional with special equipment to check your ductwork for mold.

Learn how to protect your HVAC system from mold.

If an HVAC technician discovers signs of mold in your ductwork or around your evaporator coil, you should schedule thorough air duct cleaning. Be sure to hire a company certified by the National Air Duct Cleaners Association (NADCA). Some HVAC companies offer air duct cleaning, but if yours doesn’t, they can recommend a reputable contractor.

Before making recommendations, they should conduct an assessment of your system. Any cleaning should clean your entire HVAC system, not just the ductwork.

Leaky ductwork lets moist and humid air into what should be a sealed air duct system. This in turn can allow excessive condensation to form inside the ductwork and air vent system, which provides a perfect environment for mold and mildew growth.  

Your HVAC contractor can assess your ductwork and seal it from the inside. You can also seal exposed ductwork from the outside.

You can help prevent a musty smell and other air conditioning issues by taking care of your HVAC system. Air conditioner maintenance includes changing the air filter every 30-90 days depending on the filter quality.  

Make it a habit, because something as simple as a clogged or dirty air filter can cause some big problems. See our HVAC filter maintenance guide for tips.

Keep the area around the indoor unit clean and sweep away dirt, dust, and cobwebs. Keep landscaping at least two feet away from the outdoor unit.

In addition to DIY maintenance, schedule an AC tune-up every spring for optimal performance. With a yearly visit, an HVAC technician can catch problems while they are still small and before they lead to increased AC repair costs.

Ask your local HVAC company if they offer a service plan. Most do. They will reach out to you each fall and spring to schedule service.

Finally, consider opting into Trane Diagnostics. This allows your Trane dealer to monitor your HVAC system remotely via data sent by your smart thermometer. They’ll receive alerts of any issues with your system and can act quickly to help you, even when you’re out of town.

Anne Fonda, Content Writer at Trane Technologies

A Content Writer with Trane Technologies, Anne Fonda researches topics and writes for Trane® and associated residential HVAC brands. She works in collaboration with Trane Technologies subject matter experts, offering easy-to-understand, informative content on complex topics. Her goal is to help consumers make informed decisions on the products and services they need.

She has written for HVAC and other service provider websites for over 16 years. Before transitioning to web content writing, Anne had a 14-year stint as an award-winning journalist. She graduated cum laude from the University of Missouri-Columbia School of Journalism. When she’s not working, Anne enjoys playing word games, reading, gardening, spending time with family, and visiting gardens and museums.

Expert review by Kimberly Sexton, Senior Systems Engineer

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