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Musty Smell Coming From Central A/C Vents? Here’s Why

March 8, 2023

Every time your AC comes on, the air leaving your vents stinks with a musty odor.

What in the world could it be?

Most likely it’s the smell of mold and/or mildew. That’s bad news because your AC is most likely distributing mold and/or mildew spores throughout your home. Both harmful to your health, especially if you have allergy issues.

In this article, we will cover 3 common causes for a musty smell coming from your air conditioning system along with solutions for each cause.

Let’s go over the sources of the mold/mildew and what you can do to get rid of it.

Source 1: Dirty evaporator coil

Signs this is the problem:

  • Iced up evaporator coil
  • Weak airflow coming out the supply vents

If the evaporator coil, the part that cools your air, gets dirty (and it usually does), it becomes a breeding ground for mold.


Well, mold grows when it has 4 things:

  1. Food (organic materials like dust and dirt)
  2. Darkness (UV rays kill mold)
  3. Moisture
  4. Warm temperature (80 degrees or so)

Your evaporator coil naturally has 3 of these elements already:

  • The coil is located in a dark area (the indoor unit).
  • The blower constantly flows warm air over the coil.
  • Condensation (moisture) forms on the cold coil when the warm air flows over it.

All that’s needed is food (dirt/dust).

Unfortunately, dust/dirt usually sticks to the coil because of the condensation, giving mold the food it needs (this is less likely to happen if you regularly change your air filter).

Solution: Have the evaporator coil cleaned by a professional. Many professionals will clean the coil as part of a maintenance visit, something your AC needs at least once a year.

Also, you can prevent mold from growing on the coil again by installing a UV germicidal lamp.

Source 2: Clogged condensate drain line

Signs this is the problem:

When enough condensation forms on the evaporator coil, it drops into a drain pan and then exits out a drain line.

But if the drain line gets clogged (dirt from the coil often falls into the pan) then water backs up into the drain pan. Now mold has everything it needs to grow (food, moisture, darkness, warm temperature).

Solution: Clear the condensate drain line. You can try a DIY drain clean. Or hire a professional to clean it for you.

Source 3: Leaky air ducts

Signs this is the problem:

If your cooling system was a body, the air ducts would be the respiratory system. They carry air to your living spaces.

But if air ducts leak/disconnect in a dirty, non-living area (like your attic or crawl space), they draw in the surrounding air, which is usually filled with dirt, dust and mold spores.

This can not only cause the evaporator coil to get dirty but can also cause mold to grow in the ductwork itself.

Solution: You need a one-two punch of duct sealing to eliminate the leaks and duct cleaning to get rid of any dust, mold and mildew buildup.

Live in Florida and need help? Call Cool Today!

Do you have leaky air ducts? Do you have a musty smell coming from your vents? Not sure what the problem is? No problem! Cool Today can sniff out what’s wrong and fix it in a jiffy. Contact us for service.

Cool Today provides award-winning air conditioning service to Sarasota, Tampa Bay, Orlando, Naples and the surrounding areas. View our full service area.

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