Why Does My AC Smell Musty? 15 Top Reasons

You count on our AC to keep you cool, but when it starts to smell musty, that’s a sign that something may be wrong. So you ask yourself, why does my AC smell musty?

15 Top Reasons Your AC Smells Musty:

Although a musty smell is most commonly described, there may be other ways of describing an “off” smell associated with an AC and HVAC system that may also be a concern. We will try to cover all the reasons your AC may smell bad.

Air conditioner (AC) with musty smell

1. Mildew and Mold in Vents and Air Ducts

The off-putting musty smell may only be noticed when using your AC or heating system. Mildew and mold in vents and air ducts happen when a home is exposed to a high level of moisture in the air, also known as humidity. High humidity and limited ventilation in a home are strong contributing factors that encourage mold growth.

You will likely smell the onset of mildew and mold in vents and air ducts before you begin to see it. The mold spores will already be present in the air, yet not visible to the human eye.

Note that when you do see mold growing in your vents and air ducts, it took a while to get that way. You’ll know the infestation has been there for quite some time and needs to be addressed.

2. Drain Pan Needs to Be Emptied

The most frequent reason for a musty odor coming from your air conditioner is a full drain pan. Mold and bacterial growth might start to form in the drain pan when old water is left there for a long period of time. If you begin to notice odd coloring near the AC unit or dripping water down your wall, it’s time to check that drain pan.

Water accumulates into these drip pans by way of condenser coils that pull moisture from the air. At times these can clog, making an ideal place for mold to grow.

With a wrench, detach the condensate drain from the connection at the drip pan and take out the drip pan. Clean the tray and drain well. Using either bleach, vinegar, or hydrogen peroxide (3%) to clean the tray is recommended, as these solutions will kill any existing mold.

3. Filters Are Clogged

At the first sign of an AC odor, a good rule of thumb is to start by checking the air conditioning filter. It’s possible that the things that get trapped within the filter begin to smell. Odors or no odors, filters need to be changed regularly nevertheless

Clogged, dirty AC filters are full of things like dust, pollen, and mold spores. Because AC units are prone to having a rather moist environment, those trapped mold spores can begin to blossom, contributing to that musty, moldy smell.

4. Incorrect Unit Size

It’s possible that you have an AC that is not the appropriate size for the needs of your house. AC units that are the wrong size might not cool the home sufficiently, leaving the air in the home humid. The excess moisture in the air is what mold thrives on to grow and spread within your AC unit and home.

Air conditioners that are constantly running or working hard and turning on and off often are sure signs you need to re-evaluate the size of your AC unit.

5. Evaporator Coils Are Frozen

That unusual smell from your AC could be due to frozen evaporator coils. Evaporator coils enable your air conditioner to convert hot air into cold air and then distribute the resulting cool air through the vents in your home.

Signs the Evaporator Coils Have Frozen or Partially Frozen

  • Your evaporator is half covered in ice or condensation.
  • Your AC won’t turn on.
  • Your refrigerant line is surrounded by ice.
  • Odd odors coming from your air conditioner.
  • Near your air conditioner, there is a refrigerant leak.
  • Your AC is making banging or hissing sounds.
  • You have warm air flowing from your vents.
  • The AC frequently starts and stops, but the house never becomes cold.

Causes of Frozen or Partially Frozen Evaporator Coils

  • Bad Airflow
  • Dirty Evaporator Coils
  • Old Air Conditioner
  • Leaking Refrigerant
  • Drain Clog

To repair frozen or partially frozen evaporator coils, stop running your AC to allow your coils to thaw. The thawing process may take up to a full day or so. Before turning your AC back on, replace your filter. Doing so can extend the life of your AC. Once your evaporator coils are thawed, clean them before turning your AC back on.

6. Filthy Evaporator Coils

Dirty evaporator coils could be the reason for the musty smell when you turn on your AC. When the evaporator coils come into contact with humid air, this creates condensation. Condensation drains outside without creating any issues.

Condensation, on the other hand, can combine with dirt on your evaporator coil to produce mold, which would account for the musty odor.

Because the evaporator coils are not necessarily easy to access, you may want to call a professional to help you with this.

7. AC Drain Line Is Clogged

As time passes, debris such as dust, dirt, and other airborne molecules can be transferred to the condensate drain line where it can become trapped by the condensation. This buildup can be transferred through the condensate drains from the coil, causing an obstruction in the drain line.

Yearly preventative maintenance is recommended to clear this debris and keep your AC and HVAC equipment runny efficiently, prolonging the life of your unit.

You may notice the following signs of a clogged AC drain line:

  • The AC unit is not cooling your house.
  • Places close to the interior unit have water damage.
  • You notice a moldy, musty odor near your interior unit or registers and vents.
  • AC does not come on.
  • Accumulated water close to the interior unit.

If you notice any of the above, consider making a service appointment.

8. Leaking or Damaged Air Ducts

Leaking or damaged air ducts can contribute to a musty smell as the small cracks allow humidity and mold spores to come in and mingle, making a perfect breeding ground for mold growth.

Damaged or leaking air ducts many times cannot be seen by the homeowners, leading to the ability for mold to flourish for quite some time before being detected.

9. The AC Is New

Brand-new air conditioners have a tendency to emit a burning smell that resembles a dusty, musty odor. A unit that has been sitting for quite some time prior to being used will likely be burning off an accumulation of dust. This smell should subside after the dust finishes burning off.

10. Deceased Rodent

As much as we don’t want to think about it, it can happen. When a small rodent gets trapped inside your AC unit and is unable to survive with no food, they eventually die. As their body decomposes, you’ll notice an offensive odor that is difficult to not notice.

Open up the AC cover to find and remove the culprit. A vinegar and water solution in a spray bottle can help eliminate residual odors before replacing the AC cover.

11. Living Rodents

Live rodents who do not become trapped in your AC unit but rather are using it as their home will create a pungent odor of urine and feces. If you notice a urine-like odor coming from your AC unit, you may not be imagining things. It’s not advised to set traps or set out poison, as you could be doing yourself a disservice.

You already have the smell of rodent excrement. The last thing you want to add to the mix is a decaying body.

12. Acetone Smell

An acetone smell could be a sign of a refrigerant leak in your AC unit. Acetone has been described as smelling exactly like fingernail polish remover and is quite an unmistakable odor. Freon, sometimes referred to as refrigerant, contains chemicals that can enter your lungs and influence the quality of the air you breathe.

13. Rotten Egg Smell

If you notice a rotten egg smell coming from your air vents, this could be an indication that you have a natural gas leak. Ventilate your home immediately and turn off your AC unit. Do not turn on anything either electrical or ignite any flames in your home, including using your cell phone. This may cause an explosion or a fire.

Go outside a safe distance away from the home and call the gas company and or the fire department to report the possible gas leak.

14. Sewage Smell

The smell of sewage coming from an air vent may indicate a ruptured sewer pipe or a backup sewer line. This methane odor is not only beyond repulsive, but it’s also considered a hazardous gas. You will need to immediately call a plumber to have them investigate the source of the smell.

15. Dust Accumulation

When an AC unit has been sitting for an extended period of time, dust can collect inside. If you notice a dusty, musty, burning smell when you first turn on your AC unit, there is no need to be alarmed unless that odor does not subside after 30 minutes.

How to Prevent Mold in an Air Conditioner

Mold growth in air conditioners may be avoided by managing moisture. Unfortunately, moisture may also come from air conditioners.

The following actions may be taken to manage moisture when operating an air conditioner:

  • Regularly change out your air filters. In addition to giving mold a food supply, clogged filters can also obstruct airflow.
  • Check the efficiency of your AC drainage system to prevent moisture buildup around the unit and entrance into the ductwork.
  • In order to keep rain and humid air from entering the inside, window units should fit the window securely.
  • Grates and filters in window units should be cleaned often to avoid dust accumulation, which can obstruct airflow and serve as a food source for mold.
  • To ensure that air can adequately circulate through the system and reach all components, keep all grates and air returns clear and clean.
  • Portable AC units need to be drained regularly. Before storing portable appliances for the winter, make sure they are dry and empty.
  • In order for condensation to adequately drain, window units should be angled slightly in the direction of the outside.
  • Never use water to clean ducts; instead, use dry vacuuming or dry wiping.

With minimal effort, you should be able to prevent mold from growing in your AC unit.

Can Mold in an AC Make You Sick?

According to the CDC, exposure to mold can have mild to severe health effects. Mold allergy symptoms can include, but are not limited to, the following for people with no other health problems:

  • Runny nose
  • Sore throat
  • Sneezing
  • Coughing
  • Wheezing
  • Burning eye
  • Skin rash

Those with serious lung diseases like asthma, COPD, and other autoimmune deficiencies, may have much more severe reactions.

You have a higher risk of developing a respiratory illness if you live or work next to an old, moldy air conditioner.

How to Get Rid of Mold in an AC Unit

Refer to the following steps in order to remove mold from an AC unit:

Step 1. Turn Off Your AC Unit

The EPA recommended that if you suspect you have mold in your AC unit turn it off to prevent spreading mold spores throughout the home.

Step 2. Disassemble AC Unit

If you have a window AC, you will need to detach the unit and remove it from the window. Once the AC has been removed from the window, take off the grill on the AC.

It’s recommended to read the disassembly instructions for your particular AC make and model to ensure that when you take it apart, you are doing so without damage to the unit.

Step 3. Inspect the Extent of the Mold Growth

Using a flashlight, look for signs of mold. If you see groups of black, brown, or greenish discoloration. These patches of mildew may or may not be fuzzy-looking, producing white or gray, powdery colors.

A few places here and there that indicate mold and mildew should be cleaned. Yet if a large amount of the unit appears infested with mold, it’s time to consider replacing your AC unit and not even bother with the cleaning process.

Step 4. Clean the Mold

The following steps will assist you in cleaning the inside of your AC unit:

  1. Using a vacuum cleaner attachment, vacuum out the inside of your AC.
  2. Remove the filter and let soak in the sink, with soapy water, and about 1/2 cup of bleach.
  3. Clean the coils with a commercial coil cleaner.
  4. In a large bowl or small bucket, mix together a solution of 1/4 cup of bleach to 1 gallon of water.
  5. Using a scrubby sponge, use the bleach solution to thoroughly clean the inside of the air conditioner.
  6. Remove the soaking filter and spray the hose to remove any excess debris.
  7. The unit will need to air dry for at least 24 hours before resembling.


Trina Greenfield, Author
SmackDown Media LLC

About the Author:
Trina Greenfield, the owner of SmackDown Media LLC, is passionate about providing information to those interested in the air quality in and around their homes. Trina writes content about things she’s passionate about, such as safe, in-home air, educational platforms for children and adults, as well as all things family-related.