Solved! Why Your AC Smells Musty—And What To Do About It

Q: I recently noticed my AC smells musty when it turns on, and I’ve never noticed this before. Why does my air conditioning smell, and what should I do to get rid of the scent?

A: AC units can sometimes give off a mildew smell for a host of reasons. If you’re wondering, “Why does my AC smell?” it’s helpful to know that there are various factors that could be causing the issue.

But what does musty mean? “Musty” describes a stale, moldy, or damp smell—and that type of odor can come from one of several different sources, including an AC unit. If you think your air conditioner smells musty, you’ll want to understand potential reasons to determine how to address the scent and remediate the problem. Some of these fixes are easy to make on your own, but more serious issues may require the help of an HVAC technician.

The air conditioner may smell musty when you first turn it on after the winter, or you may need to change the filter.

After a long winter, it’s normal for an air conditioner to smell musty when you first use it, as dirt, dust, and debris can build up on its coils and filter. However, this buildup can also mean mold and mildew growth. Cleaning the coils before you start using your AC regularly can avoid this unpleasant smell and keep mold spores from circulating the house. Only a professional HVAC technician should undertake coil cleaning.

Dirty filters may be another reason why an AC smells musty. It’s a good idea to change the AC filters at least once every 3 months as part of regular maintenance. Some other signs it’s time to change the AC filter include a spike in energy bills, dust near the registers, and an uptick in allergy symptoms.

There may be mold or mildew in the vents or ducts.

When there’s a musty scent wafting throughout the house, you might panic and wonder, “Does mold smell? What does mold smell like?” Mold and mildew can give off that musty scent, so you’ll want to check—or hire someone to check—the vents or ducts for signs of growth. Mold and mildew in an AC unit or the ducts can be a health hazard for residents. Inhaling mold spores can cause symptoms such as headaches, sore throats, and respiratory distress.

A full drain pan can cause a musty smell.

If a house smells musty, but there’s no mold, you may want to check if you have a full drain pan. A pan filled with water can indicate a clogged drainpipe, which can lead to more severe issues. Leaving a full drain pan can cause flooding or mold and bacteria growth, so you will want to check and empty AC drain pans regularly. Most AC units have two drain pans: one under the indoor evaporator coils and another auxiliary pan.

Musty smells can come from frozen evaporator coils or a clogged drain line.

A musty-smelling AC unit can point to frozen evaporator coils or a clogged drain line. If evaporator coils freeze and go untreated, it could lead to expensive repairs. Some signs of frozen evaporator coils include ice outside the refrigerant line, condensation and moisture around the air handler, a full condensate drain pan, and noticeable ice around coils. If you notice any of these signs, it’s best to turn off the thermostat and power to the unit and contact an HVAC repair technician.

Signs of a clogged drain line include standing water near the indoor unit, water damage, trouble cooling your home, complete dysfunction of the AC, and a musty smell near the indoor unit or in the air from registers or vents. You may be able to unclog and clean a drain line yourself, but it’s always a good idea to contact an HVAC repair technician if you’re unsure how to proceed.

The AC unit may be the wrong size for the home.

Many homeowners don’t consider that their AC unit might be too big or too small for their house. However, an AC unit that’s the wrong size could lead to unpleasant smells. The unit might not adequately cool the home and leave the air humid, creating a prime environment for mold and mildew to thrive. Some other signs an AC unit is the wrong size for a home is if there are hot and cold spots around the house or if the AC is running constantly or continuously cycling on and off.

An experienced HVAC repair technician can find and remedy the source of a musty-smelling AC.

When in doubt, it’s wise to hire an expert to identify the source of a musty smell and make any repairs. Exploring an AC unit without understanding the system can lead to more expensive issues or damages. Experienced HVAC technicians have the proper training and equipment to find and remedy areas of concern. Hiring a professional will also ensure that you are not missing mold or mildew growth—which can be detrimental to your health and wellbeing. If an expert finds these common allergens in HVAC ducts, registers, or air handlers, they can guide you on how to eliminate them.