AC Smells Musty? Here’s What to Do

It won't be long before you start up your air conditioner for the first time this spring (if you haven't done so already). But what if your AC smells musty and unpleasant after sitting unused all winter? Air conditioner smells can happen for a number of reasons. Let's take a look at what can cause that musty smell coming from your AC vents (or the unit itself) and how to get rid of it.

Why Your AC Smells Musty

When an air conditioner smells musty, it's a sign of moisture where it doesn't belong. Moist conditions allow mildew to grow, leading to that musty odor. The question is, where are the moisture and the mildew coming from? It could be one (or more) of the following:

1. Your Ducts

Built-up dust and grime in your HVAC ducts, along with accumulated moisture, can cause a musty smell that spreads through your space along with heated or cooled air.

2. The Unit

If you have a through-the-wall type air conditioning system, moisture may have accumulated in the unit. Mildew spores can get in and grow as the air warms, bringing nasty smells when you turn on your AC.

3. Water Leaks Somewhere Else

It's possible that the musty smell may be coming from somewhere else in your space. It could be a poorly ventilated bathroom, locker room, or storage area that has a return duct delivering smelly air back into your AC system. If you have plumbing issues and water leaks somewhere, poor HVAC design can create airflow conditions that spread odors everywhere. You probably already know that trying to mask air conditioner smells with sprays or scented candles isn't going to get rid of the problem. Here's what to do instead.

3 Steps to Get Rid of Air Conditioner Smells

1. Have Your AC System Inspected and Thoroughly Cleaned

Start by bringing in a professional HVAC technician to look at your system components. They will change clogged filters and spot signs of water leaks and mold growth that could be causing odors. You might have condensate fluid leaking, a drain pan that's overflowing, or another issue that's gone undetected long enough to allow mildew to grow.

This can easily happen if you haven't had your system maintained for a while. Air conditioning maintenance prevents issues like this because the problems that cause them get fixed before any damage is done. (That's why it's called preventative maintenance.)

Residential systems should get maintenance at least once a year, ideally in the spring before you start it up for the first time. Commercial air conditioning systems may need maintenance more frequently, depending on the usage conditions. It pays to talk to the experts and get a recommendation. If you're in the NYC metro area, Express Tech Air can help. Here's an easy way to get an estimate for a commercial or luxury residential maintenance agreement.

2. Clean Your HVAC Ducts

If your equipment is working well and doesn't have any water leaks, the musty smell may be coming from the ducts that distribute cooled air throughout your space. (That's assuming you have a ducted system.)

Your HVAC technician can take a look at your ducts when they inspect your equipment, and they may be able to see moisture and even mold growth. If so, you'll need to bring in duct cleaning experts to clean and sanitize your ducts.

Get our helpful guide to learn more about the benefits of duct cleaning for improving indoor air quality.

3. Repair Water Leaks and Adjust Airflow

If your AC smells bad but your equipment and ducts are fine, it's time to look elsewhere in your space for the source of the odor. If water leaks are found, obviously you'll need to get that taken care of to get rid of the musty smell.

But why does the smell seem to be coming from your AC? The answer is airflow. Your HVAC system is designed to move conditioned air throughout your space. But sometimes it can spread odors along with the air moving through the ducts.

Airflow is something that can be managed with proper ventilation and HVAC design. Talk to your HVAC technician about this if you have ongoing odor problems. You might benefit from moving ductwork, adding fans, or adding ventilation to make your space smell fresh again.


A musty smell coming from your air conditioner can be unpleasant and indicate underlying issues. By following the steps outlined in this article, you can address the problem and ensure your AC provides clean, fresh air for a comfortable environment. If you've tried these steps and still have concerns about the smell or the overall performance of your AC, it's best to consult with a professional HVAC technician who can diagnose and resolve the issue.

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Q1. How often should I get my AC system maintained?

It is recommended to get your residential AC system maintained at least once a year, ideally in the spring before you start using it for the first time. Commercial AC systems may require more frequent maintenance depending on usage conditions.

Q2. Can clogged filters cause a musty smell in the AC?

Yes, clogged filters can restrict airflow and cause moisture buildup, which can lead to a musty smell in the air conditioner.

Q3. What are some signs of water leaks in an AC system?

Signs of water leaks in an AC system include condensate fluid pooling around the unit, an overflowing drain pan, or visible water damage near the system.

Q4. Can duct cleaning improve indoor air quality?

Yes, duct cleaning can help remove dust, allergens, and mold from the HVAC ducts, leading to improved indoor air quality.

Q5. How can I improve airflow and reduce odors in my space?

Consult with an HVAC technician to assess your ventilation and HVAC design. They can recommend solutions such as relocating ductwork, adding fans, or improving overall ventilation to enhance airflow and eliminate odors.

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