Summer is arriving, and everyone wants to stay in a cool, refreshing, and comfortable environment. Nevertheless, an unpleasant smell from your air conditioner can ruin your mood. There are significant reasons why an HVAC smells musty, and possible solutions exist to overcome this scenario.

Today, we will discuss what factors cause a musty smell in the house, including mold growth, accumulated dust and debris in the air filters, and stagnant water in the ductwork.

If you are still clueless about how to stop the smell from your air conditioner, The Bee Heat & AC has got you covered.

We cover a variety of air conditioning and repair services in Arizona & Phoenix. Contact us today for the annual maintenance and enjoy the new season hassle-free. 

Why Does My AC Smell Bad?

Different kinds of smells can result from a malfunctioned air conditioner. Some of them need simple fixes, while others are complicated and need detailed attention. If your AC smells bad, plenty of issues could be causing it, from moisture accumulation to malfunctioning components. 

Whatever is causing the smell can make your family members sick and provoke other health issues. So, one must be mindful of how to eliminate such smalls as soon as possible. 

Moisture is the source of mildew formation in the air conditioners, which potentially causes musty smells. 

Furthermore, what are the possible causes of excess moisture? It also needs attention to detail and potential solutions. 

Here, we will discuss some of the most common causes and possible solutions to come out of such circumstances effortlessly:

1: Frozen evaporator coil

Air conditioning systems rely on uniform airflow to absorb the warm air from the environment and return cycles of cool air through vents. The air filters in the cooling system trap the dust particles and get clogged easily.

In addition, these clogged filters cause moisture buildup and can freeze the evaporator coils of the indoor unit.

After some time, when the water thaws, it creates a pungent smell that is easily noticeable when the AC turns back on.


Turn off the AC and thaw the system. Then, wait until the coils melt and clear them yourself. Or hire a professional to inspect the system and replace the filters to resolve this issue.

2: Moisture buildup

Consistent high humidity increases the level of moisture in your ductwork. Moreover, a drainage leak also causes this issue.

You may notice a musty smell while turning on your HVAC or AC. Excess moisture breeds the mildew on the coils and air ducts, leading to the musty smells.

It also occurs when the air conditioner condensate line clogs due to dust and debris. In addition, it is prone to improper isolation around the unit.


  • To resolve this issue, inspect the system carefully. 
  • Clean the buildup.
  • Use warm water and vinegar to clear the blockage due to mold. 
  • Ensure proper insulation around the unit to prevent moisture buildup. 

3: Leaky air ducts

Leaky air ducts are a potential reason for musty smells from HVAC systems. What are the possible signs that reveal that leaked air ducts cause the scent?

When there is no significant cooling after turning on the AC for a long time, the home feels humid even though the air conditioner runs continuously. 

Leakage in the ductwork allows the debris and dirt to enter, which enhances the mildew growth inside the duck. The accumulated dust and fungus in the airways cause a terrible smell. 


Duct sealing and duct cleaning are necessary to overcome such a situation. It helps eliminate the dust, mildew buildup, and smell it causes. 

4: Mold and mildew growth

Mold thrives in dark and damp environments; undoubtedly, air conditioners offer these ideal conditions. If left untreated, mold growth spreads throughout the HVAC system, causing musty smells and potential health issues.

To detect mold in the air conditioner, look for green, yellow, or black stains developing on the system’s walls. Furthermore, check for a fuzzy and velvety texture on the air filters representing fungal growth. 


Clean the air conditioner components, including the blow motor, evaporator coils, and ducts. To remove stubborn mold, mix one part of bleach with four parts of water and rinse thoroughly.

Additionally, you can seek professional help to get yearly air conditioner cleaning services that will use the best practices for this task. 

5: Malfunctioning components

If your air conditioner is not running fine and is producing some unnecessary sounds and bad smells, you must look for an inspection.

Malfunctioning HVAC system components not only lead-lead or performance but can also call for expensive repairs.

Some common issues with an air conditioner include a leaking refrigerant line, a damaged condensate pan, or a failed fan motor.


Call an HVAC technician for an inspection. Explain your concerns and let them do their work. Let the experts diagnose the malfunctioning components. Replace the old components with new ones and ask for a regular cleaning.

6: Clogged filters

One of the common reasons why window AC smells musty is a clogged or dirty filter. Over time, dirt stucks in the filter and restricts the airflow. In addition, it also causes unpleasant smells, which ruin the atmosphere.

Regular check-up is needed according to the manufacturer’s recommendations to fix this issue. Generally, the air conditioner filters should be replaced every one to three months. 

Additionally, consider upgrading to HEPA (High-Efficiency Particulate Air) filters, which can easily capture the smaller particles and reduce the likelihood of unpleasant odors. 


Change your AC filters to avoid such smells. Call an HVAC technician and allow them to replace the dirty filters with new ones every three months. 

The Bottom Line:

To sum up, musty smells from heat vents and cooling systems are signs of wear and tears that need immediate attention.

Regularly inspecting the air conditioner with scheduled routine maintenance with an HVAC professional can ensure optimal performance and prevent such issues. 


How do I get rid of the musty smell in my HVAC system?

You can quickly eliminate the musty smell from your HVAC system by replacing the air filters, cleaning the coils, removing excess moisture, and removing mildew growth.

Why does my AC smell musty when I turn it on?

A misty evaporator, drain line, or clogged filter is an invite to the mildew growth and bacteria buildup. A musty smell indicates their presence and immediate action is needed to remove them to avoid further damage. 

Why does my HVAC smell?

When the HVAC is not used for longer, the bacteria and mold accumulate on the evaporator coil and other areas of your system. Bacteria and fungus buildup accumulation causes unpleasant smells. 

How do you deodorize HVAC?

If your HVAC has a new and clean air filter, you can deodorize it at home. For this purpose, put a bit of baking soda in your air ducts and let it sit there. It will help eliminate the unpleasant smells and mildew growth in the air ducts without professional help.