5 Reasons Why Your Air Conditioner Smells Musty

If you’ve ever turned on your air conditioner only to be met with a musty smell, you know how concerning and unpleasant it can be.

This smell musty issue can make using your air conditioning unit a less than enjoyable experience.

In this guide, we will delve into the five common reasons why your air conditioner smells musty and explain how you can fix them.

5 Reasons Why Your Air Conditioner Smells Musty

Dirty Filters

Dirty filters are a common reason why your air conditioner might emit a musty odor. Over time, these filters can collect dust, dirt, and other airborne particles, which when combined with moisture can foster mold spores and mildew growth.

How to Fix It: Regularly replace or clean your air filter, ideally every one to three months, depending on usage and manufacturer recommendations.

Mold in the Air Ducts

Mold spores find the environment for mold in your air ducts very conducive, especially when there’s excess moisture. This is a common source of the musty smell.

How to Fix It: Regular air duct cleaning can prevent mold growth and eliminate the musty smell. This task is best left to an HVAC professional to ensure thorough cleaning and prevent mold spores from spreading throughout your home.

Issues with the Evaporator Coil

When your air conditioner is off, especially when the compressor is off, moisture can accumulate on the evaporator coil, creating a ground for mold and mildew growth, resulting in a musty smell.

How to Fix It: Have an HVAC professional clean the evaporator coil. This is a delicate task that requires professional expertise.

Drainage Leaks

Excess moisture from drainage leaks in your air conditioning unit can lead to mold and mildew growth, causing a musty smell when the compressor is off.

How to Fix It: Have an HVAC professional check for and repair any drainage leaks. Regular preventative maintenance can also help identify potential issues before they become serious problems.

Dirty Socks Syndrome

Dirty Socks Syndrome, named for the unpleasant smell it produces, is a condition where bacteria and mold build upon the heat pump, producing an odor similar to dirty socks or feet. This issue is more prominent when your air conditioner smells like feet when turned on.

How to Fix It: This condition requires a thorough cleaning of the indoor unit, which should be done by an HVAC professional to ensure that the issue is completely resolved.

When Should You Call an HVAC Professional VS Do It Yourself?

While tasks like changing air filters can be done by homeowners, other tasks, such as cleaning the evaporator coil, repairing drainage leaks, and performing duct cleaning should be left to professionals. HVAC professionals have the expertise and tools to handle these tasks effectively and safely.


A musty-smelling air conditioner can detract from the comfort of your home and even trigger allergic reactions in some people. Identifying the reasons behind the musty smell and taking appropriate measures can help maintain your air conditioner’s performance and improve your home’s air quality.

Remember, an air conditioner is a significant investment in your home and comfort, and regular maintenance is key to keeping it running optimally. If you’re not sure what to do, don’t hesitate to call an HVAC professional – they’re there to help!

About Roy Cohen

Roy Cohen has a burning passion for heating, cooling, and ventilation. He aims to help you save money on expensive repairs and bring you the best HVAC products. He has years of experience behind him in HVAC repair and garage maintenance.

You can find Roy at his LinkedIn or his email: [email protected]

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