Air Conditioner Smells Musty: 7 Causes & Solutions

Are you noticing a musty smell coming from your air conditioner? Not only is it unpleasant to be around, but it could be a sign of a more serious problem. 

In this blog post, we will discuss 7 common reasons why your air conditioner smells musty. Keep reading to learn about the causes of musty odors in air conditioners, and how to stop and prevent them.

7 Reasons of Musty Smell from Air Conditioners

Here are the 7 causes of musty smell from your air conditioner.

#1 Build-up of mold and mildew

Mold and mildew are both fungi that can grow inside your air conditioning unit. It is often caused by excessive moisture and warm temperatures inside the unit. A musty smell is a common symptom of mold and mildew growing in the air conditioner.

As an air conditioner specialist, I have repaired hundreds of air conditioners that smell musty, and this was the most common problem. In some of the cases, the growth may appear as dark spots or patches on the internal components of the unit.

Common issues of mold and mildew growth inside an air conditioner can be:

  • High humidity levels
  • Poor ventilation
  • Leaks or condensation
  • Ductwork issues
  • Lack of use

Symptoms of mold and mildew growth inside your air conditioner can be:

  • Musty or moldy odor, the most obvious signs of mild and mildew growth.
  • Visible mold or mildew
  • Discoloration on the unit

But how can you remove mold and mildew from your air conditioner? First, let me tell you that mold and mildew can be difficult to remove from the unit, so it will be best to have a professional handle the job.

However, if you want to do it yourself, here are the super actionable steps to remove mold and mildew and get rid of the musty smell coming from your AC unit. Just follow it.

  1. Turn off your unit and disconnect it from the power source.
  2. Use a mold and mildew cleaner, to clean the surfaces of the air conditioner, including the coils, and other visible mold or mildew.
  3. After applying the mold cleaner, use a stiff brush or scrub brush to clean the surfaces of the air conditioner, paying close attention to areas where mold or mildew is visible.
  4. Now, use a clean cloth to dry the surfaces of the air conditioner. You can also use a rag for the procedure. But make sure the unit is completely dry.
  5. After the unit has dried, use a disinfectant to kill any remaining mold or mildew spores. And you are done!

#2 Dirty or clogged air filters

Air filters are important to keeping your air conditioner running efficiently and smoothly. When the filter becomes dirty or clogged, it can cause a musty smell in the air conditioner. 

This is because air can no longer flow freely through the system once the filter gets clogged. A few signs of clogged air filter are:

  • Poor airflow
  • Unusual noises coming from the unit when it is running.
  • Bad air quality
  • Reduced cooling 

To avoid it, keep in mind to check your air filters regularly and replace them if they are excessively dirty, ideally after 3-4 months. 

And filter cleaning it by far the easiest process, you can do it yourself by watching this video: Cleaning air conditioner filter

#3 Dampness or moisture in the ducts or unit

Dampness or moisture in the ducts or in the unit can be one of the most common causes of a musty smell in an air conditioner. The presence of moisture in the system indicates that your unit is not properly ventilated and can lead to mold and mildew growth. 

Moisture can also cause dirt, dust and other debris to accumulate in the unit and cause an unpleasant odor. To prevent this, ventilate your air conditioning unit and have all ducts and vents inspected regularly for any signs of moisture buildup.

#4 Presence of pests or other debris in the unit

Here we will provide FAQs that cover the most common questions regarding air conditioners smelling musty. This is going to be extremely helpful for you, so keep reading.f

Not only that, they can also introduce allergens and pathogens into the air. To prevent this from happening, make sure your unit is securely sealed and check the interior for any signs of pests or debris. Some signs are:

  • Visible pets
  • Droppings or nests
  • Chewed wiring or ducts
  • Unusual odors
  • Strange noises

#5 Dirty coils and fins

The coils and fins inside an air conditioner are extremely essential for the unit to function properly. As the time goes, these components can become clogged with dust and debris, causing a musty odor.

Signs of dirty coils could be:

  • Unusual noises
  • Poor airflow
  • Higher electricity bills
  • Ice on the coils
  • Higher temperature inside

To clean the coils and fans, you’ll need to access the interior of your unit and clean off any dirt or debris. We have recently published a post about this topic which you can check here: A step-by-step for cleaning air conditioner coils 

#6 Refrigerant leaks

Refrigerant leaks can be a major contributor to musty odors in air conditioners. As you may already know, refrigerant is responsible for cooling the air, and if it’s leaking from the system, the air that’s blowing through will be hotter than expected.

And the fact that the refrigerant itself will create an unpleasant smell. Signs of refrigerant leaks could be: 

  • Decreased cooling performance
  • Hissing or bubbling sound
  • Icing on the refrigerant line
  • Skyrocketing energy bills
  • Water droplets near the air conditioner
  • Room temperature spike

More: Signs of refrigerant leaks

To diagnose and repair a refrigerant leak, I would suggest contacting an HVAC technician, who will have the tools and expertise to locate and repair the source of the leak.

You can do it yourself, but refrigerant leaks can be difficult to repair, so it would be best to have an HVAC. If you want to do it yourself, learn from here: How to Fix a Refrigerant Leak in an AC Unit?

#7 Ductwork issues

Ductwork is a common source of musty odors from air conditioners. If ducts are poorly insulated, condensation can build up and cause an accumulation of moisture that can lead to mold and mildew growth.

And if there’s a crack or leak in the ducts, it will allow outside air to enter the system, causing a musty smell. You may need to inspect the ducts to make sure they are properly sealed and insulated and to make sure there are no gaps or breaks that could allow air to enter the system.

How To Stop Musty Smell From the Air Conditioner?

There are some easy steps you can take to prevent that musty smell from your air conditioner. Here we will provide you with 4 tips to help you remove the musty smell from air conditioners.

#1 Regular cleaning and maintenance of the unit and air filters

Maintaining air conditioners

Believe it or not, a musty smell from your air conditioner can be a sign of insufficient cleaning and maintenance. Regular cleaning of your AC unit is by far the best solution to any problems with your air conditioner. 

The first thing to do is to make sure the unit itself is clean and free of dust and dirt. Use a damp cloth or vacuum cleaner with an upholstery attachment to remove dust and dirt. 

Now, the next thing to do is to make sure that the air filter is cleaned or changed regularly. It is recommended to clean it every month and replace it every 3 to 4 months. This is also mentioned in the manufacturer’s instructions. You can determine how often it needs to be changed.

#2 Use of air purifiers or dehumidifiers to remove moisture

Use a dehumidifier

When it comes to getting rid of that musty smell in your air conditioner, using an air purifier or dehumidifier can be an effective solution. Air purifiers help remove dust, pollen and other airborne particles from the air, making it much easier for your AC unit to keep your room cool.

And in the same way, dehumidifiers help reduce the humidity level in your room, which can help prevent mold growth and bad odors. 

The most important thing when using an air purifier is to replace the air filter timely, just like the air conditioner filter, but the point to be noted is that the quality of the air purifier’s filter is generally higher than that of the air conditioner. So instead of replacing it every 3 months it is advised to replace it every 3-6 months.


#3 Address and prevent leaks or moisture in ducts or unit

Any leaks or moisture in the ducts of your air conditioning unit can be one of the primary causes of the musty smell. So to prevent musty smells from an air conditioner, it’s important to inspect your ducts regularly for any signs of moisture or leaks.  A few sings could be: 

  • Dampness or water spots on walls in ducts or units.
  • Rust or corrosion on ducts or unit
  • Condensation on ducts or unit
  • Peeling paint near the ducts or unit

The first and most important step in preventing leaks or moisture in ducts is to make sure that your duct is properly sealed. If there are any gaps in the seams or the sealant has loosened, you should reseal them as soon as possible. You can use a special foam sealant or rubberized mastic sealant to re-seal.

Another way to prevent moisture from getting into your ducts is to insulate them. This will help keep the air inside the ducts warm and reduce the amount of moisture that gets inside. You can use foil-faced insulation board, aluminum-faced insulation board or spray foam insulation for this job.

You should also make sure to clean your ducts regularly. This will remove any dirt and debris that could potentially block air flow and lead to moisture build-up. 

Learn: How to clean the air conditioner duct?

#4 Professional cleaning or disinfection of the unit

If your air conditioner still smells musty after following the above tips, then it may be time to call a professional. It is always best to hire a professional to perform any type of inspection on your air conditioning unit.

A professional can assess the problem and determine the best solution for removing the musty odor. When calling a professional, make sure they are experienced in cleaning and disinfecting air conditioners.

Having your air conditioner professionally cleaned and disinfected can eliminate any mold or mildew build-up, which can help reduce the musty smell from your air conditioner.

Learn: How to hire a professional for AC inspections?

FAQ: Air conditioner smells musty

Here we will provide FAQs that cover the most common questions regarding air conditioners smelling musty. This is going to be extremely helpful for you, so keep reading.

Q1. How do I prevent the musty smell from my air conditioner?

Ans: You can prevent the musty smell from your air conditioner by regularly cleaning and replacing the air filter, inspecting the ducts, and cleaning the entire unit in a timely manner.

Q2. Is it harmful to smell musty air from my AC unit?

Ans: Yes, smelling stale air from my AC unit can be harmful. This can indicate the presence of mold and mildew, which can release spores and other irritants into the air. This can lead to various health problems like allergic reactions, respiratory problems and other health issues.

Q3. Is it bad if my AC smells musty?

Ans: Yes, it is bad if your AC smells musty and it may indicate that mold or mildew is growing inside the unit. Mold and mildew growth can cause poor indoor air quality and can be harmful to your health if inhaled over a long period of time.

Q4. What are some signs of pests or debris in my air conditioner that could cause a musty smell?

Ans: Visible signs of pests, such as droppings or nest material, hearing strange noises and finding chewed wires are some signs of pests or debris in my air conditioner that could be causing a musty smell.

My final thoughts on how to stop musty smells from air conditioners. 

The musty smell in your air conditioner can be an annoying and even unhealthy problem. However, with regular maintenance and cleaning of your unit, you can help prevent and eliminate musty odors. 

Pay special attention to the filter and vents, you can also use an air purifier or dehumidifier to remove moisture, and plug any leaks or moisture in ducts or the unit itself.

Also, professional cleaning or disinfection of the unit is necessary. You can’t ignore it. By following these tips you can stop musty smells from your air conditioner.

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David Jack

David Jack

David Jack is a veteran when it comes to air conditioners, he love helping people find solutions to their problems with AC units. His specialty is helping people find the right Air-Conditioner for their needs, whether that means replacing a broken unit or adding on to an existing system. He is also great at diagnosing problems and fixing them quickly. He learned this from my father Jack Smith who was an air conditioner repairer, he has 2 years of experience in this field.

1 Comment
  1. I was hunting for this type of post for a long time. The way you explain every causes was just amazing.

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