Why Does My AC Smell Musty When the Fan or Cooling System Is On?

If you’ve noticed a musty odor coming from your HVAC Packaged Units or AC Units, the situation is more than just unpleasant. It can be a sign that something isn’t right within your system. Musty smells typically point to excess moisture, mold, or mildew, and ignoring them can lead to more serious problems with your indoor air quality and system performance. Understanding the root causes behind these air conditioning smells is the first step toward fresh, clean air and reliable cooling comfort.

Why Does My AC Smell When in Fan Mode?

A musty smell from your AC, especially when it’s running in fan or cooling mode, is often a sign of humid air and even microbial growth inside your unit. Whereas heating mode dries out internal components, fan mode tends to circulate humidity. This can lead to mold and mildew buildup on the evaporator coil or in other damp areas. That’s why an AC smells musty while a heating unit doesn't.

When air goes over dusty coils or moves through damp ductwork, those particles get pushed into your home, creating a stale, damp odor. This is especially common in systems that haven’t run for a while. If you’re powering up your system after a cold winter, then it’s not uncommon that your air conditioner smells musty when first turned on.

Even smaller systems can have problems. Maybe you have noticed that your portable AC smells musty or have been wondering, Why does my window air conditioner smell musty? In these cases, it’s likely that you’re dealing with trapped moisture, and cleaning is required. Whether it's your home system or a musty smell from AC car vents, regular maintenance is key to keeping air fresh and healthy.

Is It Bad if the AC Smells Like Mildew?

The short answer is yes. If you notice a musty smell from your car AC or central AC unit, or if your portable AC smells musty, there is likely reason for concern. A musty smell from an AC usually indicates the presence of mold or mildew inside the system. Breathing in these odors over time can aggravate allergies, trigger asthma, or lead to other respiratory concerns, especially in vulnerable individuals.

Moisture and organic buildup inside HVAC components can significantly reduce indoor air quality over time. This is especially common when an AC smells musty after winter, since having your system turned off lets mold and bacteria grow undisturbed. Your AC smells musty, but the heat doesn't because heating mode typically dries out internal parts that could otherwise collect moisture.

Ignoring the odor can also lead to system damage. Mold or mildew on coils can reduce airflow and strain the system. This can ultimately shorten its lifespan. If you notice lingering odors (especially in older units) or are wondering why your window air conditioner smells musty, it’s best to have a professional inspect and clean the unit before it turns into a more serious and more expensive problem.

How Do I Get My AC To Stop Smelling Musty?

If you want to know how to stop a musty smell from air conditioner systems, the first thing to do is a little maintenance. This is true for whole home systems, car systems, and portable units, too.

Start by turning off the unit and inspecting the air filter. A clogged or dirty filter is one of the easiest things to address, so changing or cleaning air filters regularly should be your first move. Next, check the drip pan and condensate lines. If these components are clogged or filled with standing water, that’s a good place for mold and mildew to thrive. Carefully drain and clean the drip pan and flush the lines to get the old moisture cleared out.

To target odors directly, you can use antibacterial sprays or AC cleaners designed specifically for HVAC systems. These products help eliminate bacteria. They also go after fungal growth inside coils and ducts. After running the AC with these cleaners, consider turning the system to fan mode for a few minutes to help dry out moisture after operation. This little precaution can help reduce mustiness the next time it’s used.

The process is similar if you’ve got a musty smell from a car AC. You’ll want to replace the cabin air filter and clean around the vents. Then you can use a specialized AC disinfectant spray inside the intake system. These simple steps can make all the difference. Many people want to know how to remove a bad smell from an AC in a car. It usually comes down to reducing moisture and clearing out microbial buildup.

Lastly, if your air conditioner smells musty when first turned on, it may just be due to dust and dampness that settled during downtime. Running through regular cleaning and turning on your fan periodically during the off-season can go a long way to keep odors at bay.

How Do You Get the Musty Smell Out of the Air?

When your AC smells musty after winter or a long break, the stale air can linger, even after the system has been cleaned. To help freshen up your space, start by ventilating your living areas. This is relatively easy. Open windows and doors when the weather permits to circulate fresh air and push out trapped odors.

Next, consider investing in air purifiers and dehumidifiers. Air purifiers with HEPA filters can capture mold spores, dust, and allergens. As another option, dehumidifiers reduce moisture in the air, making it harder for mildew to return. These tools are especially helpful in high-humidity environments where the musty smell from an AC tends to keep coming back.

And of course, preventive maintenance is essential. Change filters on schedule, keep coils and pans clean, and make sure your system runs long enough to dry out after cooling. All these habits work together to help you learn how to stop a musty smell from air conditioner units. If you're wondering how to remove bad smells from AC car systems, running the fan works well in cars, too. Don’t let your system sit too long without being able to dry out, and remember to keep on top of filter replacements.

Restore Efficiency for Less With Surplus City

If mildew buildup has your AC working harder than it should, it may be time for help from Surplus City. Don’t let worn-out parts hold back your system’s performance!

We carry brand-name replacement parts for HVAC packaged units/AC units, so you can make affordable, lasting repairs. Whether you’re fighting persistent odors or dealing with components that need to be replaced because of moisture damage, you can get both clean air and an efficient system at unbeatable prices—all at Surplus City!