man inspecting home vent for cause of musty smell from his air conditioner

A musty smell from air conditioning is a common complaint, but it’s not normal for air-conditioned air to smell musty; air-conditioned air shouldn’t smell at all. Strange smells from your air conditioner indicate underlying issues.

Musty smells are typically caused by excessive moisture within the HVAC system, resulting in bacteria, mold, and mildew growth. It’s essential to determine the source of this extra moisture since bacteria and mold spores can spread in the air throughout the house and cause health issues and trigger allergies. Fortunately, musty and other unpleasant odors can often be easily resolved.

Why Your Air Conditioner Smells Musty

If your air conditioner smells musty, the cause might be a dirty air filter, clogged drain line, full drain pan, frozen evaporator coils, or excessive moisture in the ducts and vents. You can easily determine if any of these issues is the source of the odor.

1. Dirty Air Filters

HVAC systems have air filters that collect dust, dirt, hair, and debris from your home’s air. Air filters keep the HVAC system clean and improve the quality of the air you breathe. Air filters need to be changed regularly to keep the system functioning correctly and the air fresh. How often to change the air filter depends on the filter’s quality. Filters should generally be changed every month to three months.

When filters are not regularly changed, dust, dirt, and debris accumulate and restrict airflow through the HVAC system. Restricted airflow reduces moisture evaporation. The air filter and other components remain damp, and mold and mildew begin to grow on them. Air passing over the mold and mildew spreads a musty odor throughout the house.


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2. Clogged Drain Lines

The drain line, also known as the condensate line, is typically a PVC pipe that drains water from the indoor unit’s drain pan to the landscaping or a storm drain outside the house. The exterior section of the drain line is a small, sometimes capped, pipe located next to the condenser.

Drain lines can become clogged with sludge and debris and block water outflow. Mold and mildew grow in moist conditions and cause musty odors.

3. Full Drain Pan

The drain pan, also known as the condensate pan, collects condensation droplets that form on the evaporator coils located in the indoor unit. The drain pan’s accumulated moisture empties into the drain line and is carried away from the unit.

Drain pans can rust and become clogged with hair and other debris that restrict drainage. Moisture accumulates in the pan, and mold grows and emits an odor.

4. Frozen Evaporator Coils

The evaporator coils are located in or near the indoor air handler unit, positioned by the blower fan. Refrigerant runs through the evaporator coils to remove heat and humidity from the home’s air, and condensation forms on the coils. Normally, the condensation droplets evaporate or drip off the coils into the drain pan.

If airflow through the HVAC system is restricted by a dirty air filter or the evaporator coils are especially dirty, the condensation droplets freeze on the coils. If your AC is freezing up, the droplets can thaw between cooling cycles and remain damp. Mold can begin to grow on the constantly moist coils.

5. Excessive Moisture in Ducts and Vents

Drainage leaks and holes in the air ducts and vents allow moisture from humidity to enter. The dark, damp environment inside the ductwork is ideal for mold growth.

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How to Stop Musty Smell from Air Conditioner

Homeowners can eliminate musty air conditioner smells by changing the air filter and cleaning the evaporator coils, ducts, vents, and condensate lines. Regular HVAC maintenance is another critical component of preventing and eliminating air conditioner odors.

man changing HVAC air filter to prevent musty smell air conditioning

Change Air Filter

Turn off the air conditioner to eliminate suction, make removal more manageable, and prevent electrocution. Remove the old filter. Air filters are either single-use or washable. Single-use filters should be replaced with filters of the same size and type. Reusable filters can be sprayed with a garden hose and allowed to dry in the sun or near a fan before replacing.

When replacing the filter, note the arrows on the filter indicating the proper way to orient it. Slide the filter into the slot. The filter should fit snugly without folding or falling out. Some filters need to be secured with screws or clips. Turn the air conditioner back on.

toothbrush being used to clean  evaporator coil because of musty smell air conditioning

Clean the Coils

Switch off the power to the air conditioning unit and open the access panel of the indoor air handler unit. The coils can be blown with compressed air or cleaned with a no-rinse evaporator coil cleaner by following the cleaner’s directions. Another option is to hire a professional to perform the maintenance.

person in yellow rubber glove cleaning the air vent of home

Clean the Ducts and Air Vents

Turn off the air conditioner to prevent dust from blowing at you if the AC turns on while you're cleaning. Use a brush attachment to vacuum the dust outside the vent. Remove the faceplate using a screwdriver. Clean the faceplate with a sponge, soap, and warm water. Be sure to let the faceplate dry thoroughly before replacing it. Replace bent or rusty faceplates and screws with new ones.

The air ducts should be professionally cleaned to ensure a thorough job. Mold will return if it’s not eradicated.

man using plastic tool to clean out condensate line

Clean the Condensate Line

Turn off the air conditioner at the thermostat and the circuit breaker panel. Locate the drain line outside near the condenser. The condensate line is typically a PVC pipe with a plastic cap. Remove the lid, and use a flashlight to look for a clog. Remove any clogs, but avoid pushing them further into the line. Slowly pour 1 cup of distilled white vinegar into the pipe. Replace the cap, and wait 30 minutes before turning the air conditioner on.

If you cannot get to the clog or the drain line has never been cleaned, use a wet/dry vacuum to suck up the clog and the trapped water. Let the vacuum run for 3-4 minutes. Replace the cap, and wait 30 minutes before turning the air conditioner on.

man performing HVAC maintenance to  prevent musty smell air conditioning

Regular HVAC Maintenance

Regular HVAC maintenance by trained technicians will help prevent air conditioner odors and costly repairs. Yearly maintenance calls generally include inspections of the air conditioner’s components and ductwork, and other tasks, such as clearing the condensate line if necessary. Schedule your service calls in early spring before you begin using your AC.

Other Common Air Conditioner Smells

Air conditioners emit various smells that correlate to HVAC issues. If your AC smells like vinegar, it could be another sign of mold and bacteria growth due to a dirty air filter, clogged drain line, full drain pan, frozen evaporator coils, or excessive moisture in the ducts and vents. It might also be the odor of ozone from a failing electric motor.

A burning smell from the AC could indicate an overheating motor due to a dirty air filter or worn-out bearings. A burnt odor can also point to a damaged capacitor and other electrical issues. Turn off your air conditioner, and call a professional to inspect your system.

A fishy smell from the air conditioner is a serious indication of an electrical issue that could spark a fire. When heated or burnt, the plastic coating on electrical wires emits a fishy odor. The source of the electrical problem might be within the HVAC system, or the scent of a nearby electrical issue might be sucked into the system’s intake and spread throughout the house. Shutting off the power and calling a professional electrician or HVAC technician is recommended.

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Musty air conditioner odors are typically caused by moisture issues that lead to mold and mildew growth. You might be able to resolve your air conditioner issue without calling an HVAC technician. Start by changing the air filter. If the problem persists, investigate the drain line, drain pan, and evaporator coils. Cleaning these components is not overly complicated and can be done by DIYers. However, if mold grows inside the ductwork, it should be cleaned by professionals to prevent a recurrence.

Future air conditioner issues can be prevented by scheduling yearly maintenance calls with a local HVAC company.

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