Woman pointing at the air conditioner and holding her nose as if she smells something bad.

If your AC unit smells musty, you should clean or replace your air filters, clear the condensate drain line, clean the evaporator coil, check and seal the ductwork, use a dehumidifier, keep regular maintenance, and install UV lights. 

Nothing is more off-putting than turning on your air conditioner and being greeted by a musty smell. Not only does it make your home feel less inviting, but it can also indicate underlying issues that, if left unchecked, could lead to more significant problems.

This guide will help you understand why your AC might emit that unpleasant odor and provide practical steps to eliminate it, ensuring your home remains fresh and comfortable all summer.

Why Does My AC Smell Musty? 5 Typical Causes

A musty smell when the AC turns on usually means mildew or mold in AC vents or other parts of the system. This issue can stem from several common causes that, when identified, can be effectively addressed to eliminate the odor. 

Check the detailed reasons why your window AC smells musty in the list below.

  1. Clogged Condensate Drain Line: The condensate drain line removes moisture that the air conditioner extracts from the air. If this line gets clogged with debris, it can cause water to back up and create a damp environment where mold and mildew thrive.

  2. Dirty Evaporator Coil: The evaporator coil is responsible for cooling the air. If it becomes covered with dust and dirt, it can trap moisture, providing an ideal breeding ground for mold.

  3. Wet Or Dirty Air Filters: Air filters are designed to trap dust, pollen, and other airborne particles. If they become damp or filthy, they can harbor mold spores circulated throughout your home.

  4. Excessive Humidity: High humidity levels in your home often lead to moisture buildup within your AC unit, promoting the growth of mold and mildew.

  5. Leaky Ductwork: If your ductwork leaks, warm, humid air can enter, causing condensation and creating an environment conducive to mold growth.

Understanding these causes is the first step in tackling the problem and preventing it from recurring. The next one is to try and solve the musty smell of AC vents.

How To Fix A Musty Smell From The AC Unit? 7 Tips

Once you have identified the potential causes of the musty smell, it is time to take action. Clean or replace your air filters, clear the condensate drain line, clean the evaporator coil, check and seal the ductwork, use a dehumidifier, keep regular maintenance, and install UV lights.  

Following the steps listed below, you can eliminate the odor and ensure your air conditioner operates efficiently and hygienically.

  1. Check your air filters and replace them if they are dirty or clogged. To do this step more quickly, read tips for AC filter location.

  2. Locate the condensate drain line and clear any blockages. You can use a wet-dry vacuum to suck out the debris or a long brush to dislodge the clog.

  3. Clean your evaporator coil with a soft brush and a coil cleaner solution. Before performing this task, turn off the power to your AC unit.

  4. Inspect your ductwork for leaks or damage. Seal gaps with mastic sealant or metal tape to prevent warm air from entering the system and causing condensation. Also, know the benefits of cleaning air ducts.

  5. If your home has high humidity, consider using a dehumidifier to reduce moisture in the air. Doing so can help prevent mold and mildew from growing in your AC unit.

  6. Schedule regular maintenance for your air conditioner. A professional HVAC technician can clean the components, check for issues, and ensure everything runs efficiently, preventing musty smells from developing.

  7. Install UV lights within the air handler to kill mold and bacteria. It is an effective long-term solution for keeping your AC unit smelling fresh.

By addressing these common causes of an AC that smells musty and following these steps, you can maintain a pleasant and healthy indoor environment. Regular maintenance with Wisercosts' HVAC contractors is vital to keeping your AC running smoothly and your home smelling fresh.