Why Does Your Air Conditioner Smell Musty? Common Reasons + How to Fix

Kenneth Wilson

Does your air conditioner smell musty? This means your central air conditioner smells like a pair of stinky feet, dirty socks, or blue cheese– enough to make you wrinkle your nose and run from the room. There are plenty of reasons why your AC unit emits a musty odor. In most cases, it is because of bacteria, mildew, or mold build-up on your evaporator coil. Any excess water may also make your central air conditioner smell musty.

An air conditioner's musty smell is also a common issue if you are in an area with frequent rainstorms (moisture). Let's discuss the different culprits behind a window air conditioner that smells musty and some expert-backed tips to help you resolve the issue.

Why Does My Air Conditioner Smell Musty?

Picture this: You're finally back home after soaking in the scorching heat of the summer. You catch a strange, musty whiff when you turn on your AC unit for that much-needed comfort. That sounds like a deal-breaker, right?

The moment you notice this, it would be best to act at once and search how to stop the musty smell from your air conditioner as soon as possible. This may potentially indicate a potential bacterial growth build-up, allowing mold and mildew to thrive.

Is a Musty-smelling AC a Matter of Concern?

Once you notice a musty smell from your AC unit (similar to dirty socks or rotten eggs), this may suggest an underlying issue with the system. The most common culprit is the presence of contaminants inside your dark and humid AC system, which serves as an ideal breeding ground for bacterial growth.

So yes, the musty smell from your air conditioner can be dangerous– spreading mold spores into various living spaces inside the home. This may trigger health issues, such as severe allergies.

Basic Troubleshooting: How to Remove Musty Smell from Air Conditioner

As much as we recommend finding a professional to take care of this problem, there are steps you can follow to find the source of the musty AC smell and how to fix it. We've listed them below for your reference:

  • Check if you have a full AC drain pan (overflowing)
  • See if the evaporator coils are frozen
  • Observe if there is excess moisture in the ducts or vents
  • Determine if there is a condensate line blockage
  • Ensure that the AC unit is the right size for your home needs

Naturally, the air from the AC vents must not have a foul odor. It would be best to consult a local HVAC professional to assess the issue before the air conditioner's musty smell gets out of control.

Common Reasons Why the AC Unit Smells Musty

Here are some of the most common culprits behind the musty smell from your AC unit, so you know where to look and diagnose the problem.

  • A full drain pan ─ Since your air conditioner is designed to get rid of humid air inside the home, it's normal for water droplets to fall into the drain pan. This component is subjected to wear and tear over time, so you can expect its efficiency to deteriorate in the long run. If you are stuck with a poor-quality drip pan, this may only accumulate mold and cause the foul odor that is distributed throughout the home. Not all hope is lost, however. A local HVAC technician can quickly help you replace your old drip pan with a new one.
  • Mold/mildew in the ducts and vents ─ The most common reason behind an AC unit's musty smell is the presence of mold or mildew in your air ducts or vents. We recommend hiring a professional to care for this problem, as they have the proper protective gear and expertise to remove mold and mildew safely. To help avoid this issue from happening again in the future, opt to keep your AC on a low setting and use a UV filter.
  • Frozen evaporator coils ─ If dirt blocks the airflow, this causes water droplets to pool on the evaporator coils– which freeze once the refrigerant cools the air. An HVAC professional will allow the condenser coil to thaw before replacing the old filters. This helps ensure maximum airflow.
  • Excess moisture ─ High humidity levels or drain leaks lead to excess moisture inside your air ducts and vents, which may only encourage mold and mildew. Note that this is a matter of concern, as harmful exposure to mold can result in upper respiratory health issues. Have a local HVAC technician clean your ductwork and do all necessary repairs so your home smells fresh and avoids risking your safety.
  • Clogged condensate line ─ The condensate line is one of the most popular breeding grounds for mold, mildew, and sludge build-up. When overlooked, this may result in clogging and nasty, musty odors inside your home.
  • Leaking air ducts ─ If the air ducts are punctured, your AC unit may be at an increased risk of leaks. This makes it more susceptible to mold growth. Unfortunately, it's common for homeowners to overlook this issue, so the moment you notice the AC smells musty, have a team of HVAC technicians fix the problem.
  • Improper AC size ─ To ensure your AC system works efficiently (the way it was intended), you need to ensure it is the right size for your home needs. In homes with an air conditioner unit that is too large, the air may get cycled too rapidly– causing poor dehumidification and excessive moisture. But if it is too small, it may have fluctuations in its operations and may turn on and off constantly. Trust us; you won't want that to happen amid scorching summer days.

With that, have your HVAC technician look to ensure your air conditioning unit has the correct size for your home needs. This will depend beyond your home's square footage, as other factors will come into play: the number of people in your home, the insulation, and the local climate/environment.

Frequently Asked Questions (FAQs)

Can I spray Lysol on my home AC vents?

Although Lysol may be your preferred all-purpose cleaner, it is not recommended to use it inside an air conditioner. Lysol is very explosive due to the ethanol alcohol in it, which means that spraying it close to your air conditioner's motor might harm it or perhaps set it on fire or explode. Fortunately, many alternatives effectively clean and sanitize your home's AC ducts and vents without using ethanol alcohol or other combustible substances.

How does mold get in my air conditioner?

Regardless of whether you are indoors or outdoors, mold spores are always in the air around you. When they find a moist area to settle, these invisible spores develop into a mold, which is usually not a problem. Your air conditioner is a dark, confined chamber that regularly becomes damp from condensation and rain, making it difficult to completely dry, mainly if used daily. (Related: 5 Things To Do If You Find Black Mold In Your Air Conditioner)

How do you get the musty smell out of an AC?

You can clean your AC unit yourself. However, to avoid hurting yourself or causing harm to the AC, you should take basic safety precautions. The following procedures can assist you in getting rid of the musty odor coming from your air conditioner:

  • Remove the outer casing with caution. This will expose the fans, coil, and compressor.
  • Place the unit in a secure location where it may stay throughout the cleaning procedure.
  • To prevent internal components from being damaged, blow compressed air far enough.
  • The finer dust particles in the front and back of the unit can be removed using a brush or a smaller piece of cloth.
  • With some cleaning solution, gently clean the vent and the duct, and then rinse the cleaning solution out with water.
  • Apply slight pressure when flushing the drip tray to ensure that all dust and particles are removed.
  • After giving the air conditioner a few hours to dry, wipe it down with a fresh towel.
  • You may now reassemble the unit after it is scorched.
Kenneth Wilson
July 7, 2022
Contractor Tips, HVAC

Kenneth Wilson

Retired contractor. Currently residing in Southwest Florida. Now in semi-retirement, I write and manage this blog focused on helping home owners make savvy decisions when it comes to finding contractors and getting their projects done. I also operate remodeling design service for homeowners.

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