Why My AC Smell Musty & How To Stop Musty Smell From It?

An air conditioner brings us peace of mind on summer days, but it is something that needs upkeep. It is not uncommon to see an air conditioner that smells musty, and many people don’t know how to get rid of it.

Mold and mildew in the air filter, evaporator coil, and drain pan are common causes of musty smell from air conditioners. A poorly sized ac unit could also be to blame. To eliminate the musty odor, you must clean each component.

I will analyze all the common causes of this problem and teach you how to easily remove the musty smell from your ac. A lot of helpful information has been included in this post. Let’s get started!

Why does my air conditioner smell musty?

In general, the musty smell is a damp, stale, and moldy scent that is often produced by mold and mildew.

If your air conditioner begins to blow air with a musty odor, it is an indication that mold and mildew are thriving somewhere in your unit. 

Filthy air filter

dirty ac air filter

The air filter is located in front of the evaporator coil, which is responsible for filtering out the dust and dirt lingering in the air.

These contaminants can become the food source for mold and bacteria. Meanwhile, they can also absorb some moisture, which will promote the proliferation of these microbes and release musty smell. 

As a side note, some types of bacteria and mold can give off a urine smell, that’s why some ac will smell like pee. But the root cause is the same. 

Dirty and frozen evaporator coil

While the filter can remove a certain amount of dust and dirt, some impurities will pass the filter and settle on the evaporator coil. 

Since the evaporator coil is where the condensation will happen, meaning that the coil is humid while running. In addition, when the evaporator coil is dirty, there is not enough warm air to warm up the coil, causing the ac to freeze

That is to say, the coil will have even more moisture left on it because the ice will thaw, which creates an even better environment for mold and mildew. With time, the moldy smell will disperse from your air conditioner. 

Clogged drain hose

Another place you need to check is the condensate pan. There is a high likelihood that the stagnant water in the pan causes a musty smell. However, the clogged drain hose is to blame, not the pan itself.

When the water can’t drain properly, it will accumulate in the pan and even overflow inside your ac. When the water is stale, mold and bacteria will grow happily in it. 

Air ducts are leaking

The cool air from the air conditioner is delivered to each room via the air ducts. The air duct can crack due to extreme weather or wear and tear, allowing cool air to escape.

Mold spores, bacteria, dust, and dirt, on the other hand, can get into the ducts along the way. As you are aware, the ducts will pass through the crawl space or attic, which are the areas that are contaminated.

As a result, the air that comes out of the vents will contain these impurities and smell musty.

High humidity

If you live in a humid area, the excess moisture may be the root cause of the musty smell. 

That is to say, condensation can form in the vents and ducts, causing the mold to grow, even if there are no cracks in the duct. 

Wrong-sized air conditioner

An air conditioner that is too small or too large can lead to the house being humid with the ac on.

When the ac is too small, it does not have the capacity to dehumidify the amount of air, whereas when the ac is too large, it stops quickly and does not have enough time to dehumidify the air.

When the air is humid, the mold can grow faster, making your entire house or room smell musty. 

How to remove a musty smell from an air conditioner?

The previous section has summarized the common reasons for the musty smell of air conditioners. Because mold and mildew can result in an array of respiratory issues, you can’t leave it unattended. Let’s look at how to stop musty odors from an ac unit.

Replace or clean the dirty filter 

This is a task you can do it yourself easily. After turning off the machine, open the cover of the indoor unit, take out the filter, and rinse it under the running water. 

You can use a brush to get rid of the sticky grime on the filter. If you notice the filter is too hard to clean, you can consider purchasing a new replacement. 

Once the filter is dry, you can replace it to see if the musty smell is gone.

Clean the evaporator coil

Cleaning the evaporator coil may require more effort than cleaning the filter. You can access the evaporator coil after removing the filter.

If it is not too filthy, you can remove the grime with a can of compressed air. If that doesn’t work, apply some coil cleaner, use the brush to clean out the buildups, and let the coil dry before turning on your air conditioner again.

Clean the drain hose

If the musty smell comes from the drain hose in ac, you can simply use a vacuum cleaner to suck away the buildup. Or cut the hose down and use a  brush on a wire to remove the debris. 

Clean the air vents

Air vents are easy to access and clean. You can often find brown stuff on it. To clean it, you can make use of a vacuum cleaner to suck away the light lint and grime. Next, use a soft damp cloth to remove the stains and debris.

Clean the air duct 

I recommend that you hire an HVAC technician to do the job for you. They can use tools to locate the leak, then repair it and remove any debris that has accumulated inside the ducts.

If you attempt to do it, it may void the warranty.

Use a dehumidifier 

use a dehumidifier

If the musty smell in your ac does come from the surrounding environment, you can try to run a dehumidifier. 

A dehumidifier is helpful in mitigating musty smell. However, you should still remove all the mold and mildew if you want to stop the musty smell entirely because dehumidifiers can’t kill the mold.

Final thoughts

Musty smell from air conditioners is a common problem. It is more likely to occur when cleaning is overdue, or the environment becomes too moist.

Fortunately, cleaning your ac, air vents, and air ducts can eliminate the musty odor. Indoor humidity control is also beneficial.

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Liz Yang is the founder of Airsmartly. She has been working at home for a few years and realizes that the performance of the HVAC system plays such an important role in our life. She has tested a lot of products in person, like humidifiers, air purifiers, dehumidifiers, and ACs, and wants to share tips about using or troubleshooting these products with you. Her uncle is an HVAC expert with over 30 years of experience in the field, and often offers assistance when she is unsure how to handle a situation. He is also in charge of reviewing the articles on this site.

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