Why Does My AC Smell Like Mildew?

July 06, 2018

“Ew! Why does my AC smell like dirty socks?”

A musty, moldy smell is bad news because it means your AC could be distributing unhealthy mold/mildew spores throughout your home.

The source of a mildew or mold smell usually comes from 1 of these 3 AC problems:

Clogged condensate line
Dirty evaporator coil
Leaks in the air ducts

Let’s go into more detail about each of these problems and how to solve them.

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Problem #1: Clogged condensate line

As your AC runs, it removes moisture from your home’s warm, humid air. That moisture exits your home via a condensate drain line. Sometimes this drain line gets clogged with dirt or debris, so water backs up and collects in the drain pan.

Warm, dirty water in a dark place is a recipe for mold growth, which could be what’s causing your home’s air to smell bad. A clogged condensate line is a common issue here in Florida, especially during storm season when dirt and debris are constantly kicked up into the air.

Depending on the model of your AC unit, you’ll see 1 of these 2 signs that your condensate drain is clogged:

Pools of water around your indoor unit because the drain pan is overflowing.
Your AC shuts down after running only a short time. Some drain pans come equipped with an emergency switch that shuts off the AC when the water level gets too high.

*A clogged condensate line will create pools of water around your indoor air handler.*

Solution: Clear the condensate line

Use a wet/dry vacuum to clear the line of dirt/debris. Connect the vacuum to the main drain line (you’ll find it outside, coming out of a wall near the ground) and turn it on. If you don’t have the time or patience to do that, or if you tried it and it didn’t work, hire a professional to clean it for you.

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Problem #2: Dirty evaporator coil

Your AC evaporator coil is the part that cools your home’s air. It’s also where your system removes unwanted moisture from the air.

Evaporator coil sitting on cement ground — *An evaporator coil*

As moisture builds on the evaporator coil, it creates a possible breeding ground for mold—especially when the coil gets dirty.

You see, over time, as air passes through your AC, it brings dust and small particles with it. That dirt, mixed with water in a warm, dark place can foster mold growth. And that mold could be what’s causing that dirty socks smell.

If you have a dirty evaporator coil, you may also notice these 2 signs:

Water buildup around your air handler. A dirty evaporator coil can freeze over and when it melts, you’ll see water pooling around the air handler.
Weak airflow coming from the vents. When your evaporator coil is dirty it can’t cool your home’s air as efficiently.

Solution: Hire a professional to clean the evaporator coil

Evaporator coils are sensitive and easily damaged, so you’ll want a trained AC professional to clean them. A professional will have the right tools to properly remove dirt and mold from your coils without harming them.

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Problem #3: Leaks in the air ducts

Your home’s ductwork is located in unconditioned spaces like an attic or crawl space, which means dust constantly surrounds it.

Normally, that’s not a problem if your ducts are in good condition. But if your ducts have leaks, mold can easily grow when dust and humid air enter the ductwork.

If you have duct leaks, you’ll notice these problems:

Your home is very humid, even though the AC runs constantly (you’re losing conditioned air to leaks).
Your entire home gets dusty very fast (dust enters your ductwork through the leak and blows into your home).
Your AC runs for a long time, but doesn’t cool your home because the air is escaping from the leaks.

Related reading: How Can I Tell If My Air Ducts Are Leaking?