5 Reasons Why Your AC Vents Put Out A Musty Smell

July 06, 2017

“Eww! What’s that gross smell in here?”

Every time you turn on your AC, do you smell a musty odor coming from your air vents?

Well, that gross smell is most likely mold and/or mildew, which is usually created by 1 of these 5 problems:

  1. Dirty evaporator coil

  2. Clogged condensate drain line

  3. Leaky ductwork

  4. Filthy air filters

  5. An oversized AC

In this article, we’ll go into more detail about each of these issues and how to solve them. Let’s start with a dirty evaporator coil...

Problem #1: Dirty evaporator coil

Your air conditioner contains an important component called an evaporator coil, which is the part that actually cools your home’s air.

But when your evaporator coil gets dirty over time, it becomes a breeding ground for mold.

Mold needs 4 things to grow, and an evaporator coil provides all 4:

  1. Organic materials like dust and dirt: These come from your home’s air (what doesn’t get picked up by your filter) and the naturally dusty places like an attic or crawlspace.
  2. Darkness: There’s no sunlight (UV rays kill mold) near your evaporator coil.
  3. Moisture: Condensation (moisture) forms on the cold evaporator coils when warm air flows over it.
  4. Warm temperature (80°+): Warm air constantly flows over the evaporator coil.


Contact an AC professional to clean your evaporator coil. Since Florida is a naturally humid area, consider installing a germicidal UV lamp to kill mold growth.

You’ll want to clean a dirty evaporator coil because in addition to mold, you’ll get weak airflow coming out of your vents and your evaporator coil could freeze over.

Problem #2: Clogged condensate drain line

Air Condition Drain Line

An AC condensate drain line.

Do you see pools of water around your AC’s inside unit? And do you smell that musty odor?

If so, those symptoms point to a clogged condensate drain line. When condensation forms on your evaporator coil, it drops into a drain pan and exits outside via a drain line.

But if your drain line is clogged because dirt falls into the pan (another reason to clean your evaporator coil) then water stagnates in your drain pan, which facilitates mold growth.


You can try using a wet/dry vacuum to clear the line yourself, but if that doesn’t work, contact a professional to clear the condensate drain line for you.

Problem #3: Leaky ductwork

According to Energy Star, 20–30% of the air that moves through a typical home’s duct system is lost due to leaks and holes. Since most ductwork is located in dirty, non-living areas like attics or crawl spaces, it’s easy for dust and mold spores to enter your ductwork.

This dirt and mold goes 1 of 2 places:

  1. It stays in your ductwork, forms mold/mildew then causes your air to smell bad

  2. It goes to your evaporator coil, which makes it dirty (see Problem #1)


You need an HVAC professional to seal and clean your ducts to get rid of leaks and any mold/mildew growth.

Problem #4: Filthy air filters

Dirty Vs Clean Filters

If your filter looks like the one on the right, it’s filthy and you should change it.

Since a clogged air filter reduces airflow in your system, it can allow moisture and dust to settle in your ductwork, which facilitates mold growth.


Change your air filter if it’s dirty. Here in Florida, it’s important to change your air filter frequently (every 3 months at least) because we have high humidity.

Tip: Consider upgrading to a higher MERV filter to stop mold spores from entering your home’s ducts and air.

Problem #5: An oversized AC

“But I thought bigger was better?”

Well, not when it comes to air conditioners. If your AC is too big (meaning it’s too powerful) for your home, it will short-cycle. Short cycling occurs when an oversized air conditioner blasts your home with cold air, then shuts down quickly.

If your air conditioner is oversized, it means your AC won’t run long enough to remove moisture from your home’s air. When that moisture meets dust (in the evaporator coil or ducts) mold/mildew can form.


Unfortunately, this is an expensive fix. You’ll need a new air conditioner that matches your home’s specifications.

But before you despair, contact an AC expert to perform a Manual J Load Calculation to help you know what size air conditioner you need.

Need a professional’s help getting rid of that nasty smell?

Contact Red Cap Air to schedule an appointment with one of our trusted technicians. We’ll get your AC back in tip-top shape, and we’ll get rid of that musty smell in no time.

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