Why Is Your AC Smelling Musty When You First Turn It On?


Sinclaire Enterprises, Inc.

140 South St., Walpole, MA 02081
P 508.668.8200 Toll Free 877.668.8822 info@sinclaireboston.com
Master Plumber License #9530,
Sheet Metal Master-Unrestricted #5442

When the hot days start up again, you’re naturally looking forward to the comfort and sanctuary of your air-conditioned home. However, after all those months it sat unused, you might notice a musty smell when you turn the AC on again for the first time. 

The problem might be benign and might take care of itself. For example, dust that settled in the unit might just be burning off, and the smell will be gone pretty soon. 

However, if the smell is persistent, then there probably is a bigger problem, and unless you know what’s causing it and can fix the issue yourself, it’s going to be necessary to call an AC technician. For AC repair in Ashland, Canton, and Norwood, you can trust Sinclaire, a trusted regional leader for years.

Woman cover her nose due to the musty smell from her AC

Why Bad Odors Are a Problem 

A burning smell might just be those dust particles burning off. Other unpleasant odors are a sign of something much worse. An odor like rotten eggs can indicate a gas leak or a dead animal somewhere in the vents. If the odor is musty or smells like dirty socks, then there may be mold or mildew growing in the system. 

If you view an AC unit as a habitat, you would describe it as dark and humid, and those are perfect conditions for bacteria and fungus to grow. 

When there is mold present, its spores can be distributed into every room of the house. Some of those spores can be harmful to people’s health, and anyone with allergies or breathing issues is especially at risk. 

So it’s never a good idea to ignore a strange or unpleasant odor coming from your AC system. These are issues that need to be addressed as soon as possible when you notice them.

Causes of Musty Odors 

There are some ways to check for the source of the odor so that you or your technician can plan what to do.

  • Your unit has a drip pan directly below the evaporator coils. Check to see if it’s full. The drip pan collects water droplets as the unit dehumidifies a home, and when the drip pan isn’t working correctly, mold can grow, and you might need a new drip pan.
  • Inside or near your unit’s blow fan are the evaporator coils. When dirt and debris obstruct airflow, water droplets can form on the evaporator coils and freeze. These damp conditions also can attract mold growth.
  • High humidity or a drainage leak can result in excessive moisture accumulation in air ducts and vents. As you’re now well aware, mold loves to grow in moist settings. Mold spores in your ducts and vents are more likely to reach people and cause respiratory issues.
  • Next to your condenser unit is a small pipe, probably made of  PVC, called the condensate line. Its job is to drain water from the unit into the drip pan. Because it’s so small, it’s easier for mold, debris, and other materials to cause a clog, causing musty odors to get inside the home.
  • Finally, the size of the AC unit might be the problem. When a unit is too large for the space it needs to cool, air cycles too quickly for proper dehumidification to occur, creating more moisture and conditions for mold growth. It has to work harder when it’s too small, turning on and off more frequently. That can lead the system to fail before it should, and as it starts to go bad, that sets the stage for some of the above conditions to occur.

What You Can Do About Must AC Odors

AC systems are complicated, with many components, and they’re not easy to service on your own. Two things you probably can do yourself are the following: 

  • Change air filters. Air filters trap dust and debris and keep them from circulating the air you breathe. Dirty or clogged filters can result in mold growth and unclean air. At the start of AC season, change air filters.
  • Check the evaporator coils to see if they’re cool, damp, or frozen. 

If those don’t take care of or identify the problem, the best thing to do is call a service technician. For Ashland, Canton, and Norwood AC repair, Sinclaire can handle any job, getting you back to coolness and comfort as soon as possible. We also perform recommended annual maintenance and inspection to identify and repair problems before they become more significant issues. Schedule with us today!