Why Does My Air Conditioner Smell?
Why Does My Air Conditioner Smell?

It can be a real bother to be relaxing in the cool air blowing from the air conditioner on a hot day when all you can think is, “Why does my air conditioner smell?” If you notice a smell permeating the air when the air conditioner is running, the smell is likely coming from the AC system. 

Air conditioners can emit various odors indicating something is wrong with the system. Dirty socks, rotten eggs, and mildew are just a few of the smells. You may be able to isolate the problem by identifying the particular type of smell coming from the system. Once you have identified the smell, you can determine how to fix the issue and eliminate the odor.

Why Does My AC Smell?

“Why does my AC smell?” is a good question, but a better question is, “What does my AC smell like?” Articulating what your AC smells like is the first step to determining the problem and finding a solution. Here are some common smells:

  • Vinegar
  • Urine
  • Dirty socks
  • Rotten eggs
  • Musty


A sour smell from your air conditioner could result from an electric motor emitting ozone. Electric motors typically produce small amounts of ozone gas, but splits or breaks in the motor’s wiring can cause it to emit more ozone than usual. Call an HVAC technician or inspect the wires, but hire a professional to do any electrical repairs.

Excess condensation on the evaporator coils resulting from a dirty air filter can cause your AC to smell like vinegar. A dirty air filter impedes airflow, which can cause the coils to freeze and have excess condensation. Replace the air filter and continue to replace it regularly. It would help if you also cleaned the coils or called a technician to do it for you. To 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 cleaned with compressed air or a coil cleaner. 

A malfunctioning filter can also spread the smell of vinegar around your home. Change the air filter and make sure it clicks into place. Check the connection points to make sure the filter is held securely.

A clogged condensate drain pan can emit a sour smell. The drain pan collects condensation that drips off the evaporator coils. AC systems typically have an overflow drain pan as well. Drain pans and drain lines can become clogged by dirt and debris accumulated over time. Turn off the unit at the thermostat and the main switch. Check for pools of water around the unit and in the indoor air handler. Open the access panel to inspect the drain pans. The primary drain pan will be located under the evaporator coils, and the overflow pan will be below the primary pan inside or under the unit. Using a flashlight, check the pans for debris and apparent clogs. A clog in the primary pan or the drain line is likely if the overflow pan contains water, also known as condensate. Slowly pour a gallon of water into the overflow pan. An unclogged pan will drain freely into the drain line. If it does not drain, continue slowly pouring water into the pan until the clog opens up. 

Mold in the ductwork caused by excess moisture may smell like vinegar. The mold must be located and entirely removed. The source of the excess water must also be identified to prevent mold from forming again. Leaks in the ducts, an AC system that is too large for the space, and keeping the temperature too low can cause excess moisture to form in the ducts. Hire a professional duct cleaning service to ensure the job is done thoroughly and the mold doesn’t return. 


Live rodents like warm places to nest and may settle in your air conditioning unit, which is the most likely cause of a urine smell. If mice live in your AC unit, they are also urinating and defecating. Check the indoor air handler unit for dropping and other signs of mice. You can try eliminating them yourself with traps or poison, but if they die in places you can’t reach, you will have another smelly problem. Your best bet is to first to call a pest control company and then have your ductwork professionally cleaned. 

A urine smell could also be produced by mold in your AC system. Air conditioners recycle the air within your house; they don’t blow in the fresh air. Therefore, things floating in the air, like the dead skin cells you shed, get sucked into the system. Microbes that naturally exist inside your AC system consume and digest these skin flakes, producing the byproduct ammonium. If you are familiar with ammonia, you know it smells like urine. A dirty air filter plus a poorly maintained and seldom cleaned air conditioner worsen a mold problem. Mold may begin to form on the unit’s evaporator coils, too. The first step is to change the air filter. If you’re comfortable doing so, switch off the power to the air conditioning unit, open the access panel of the indoor air handler unit, and inspect the evaporator coils. The coils can be cleaned with compressed air or a coil cleaner. Otherwise, call a technician to check and remedy the problem.

Dirty Socks

The smell of feet or dirty socks is so common that HVAC professionals jokingly refer to it as “dirty sock syndrome.” The odor is caused by bacteria and mold growing on the moist areas of the AC system like the evaporator coils and the drain pans, especially if the pans are not adequately draining due to a clog. 

Changing the air filter regularly reduces moisture in the system by maintaining good airflow. Turn off the unit at the thermostat and the main switch to check the drain pans for excess moisture and mold; if there is water in the overflow pan, there is likely a clog. If mold or rust is present, replace the pans with new ones. While the power to the air conditioner is turned off and the access panel open, you can clean the evaporator coils. Remove the panel and spray the coils with compressed air or a coil cleaner. Alternatively, call a technician to clean and service your unit. 

Rotten Eggs

Dead animals in your AC system can produce a rotten egg smell. Birds, rodents, and lizards sometimes nest in warm places like the ductwork or air handler unit. Locate the spot from which the odor is emanating. If it’s in a duct, you will need to unscrew the vent cover first. Remove the carcass and wipe the area clean. Seal up any entry points. Call a duct-cleaning service to eliminate any animal debris and lingering smells. Duct cleaning professionals can also assist you if you can’t get to the remains or don’t want to deal with the issue yourself. 

Natural gas contains an additive that smells like rotten eggs or skunk spray. You may have a gas leak near your air ducts if you smell one of these smells. Turn off the gas supply, leave the premises, and immediately call the gas company.


The most familiar smell coming from your air conditioning is a musty smell caused by mold, mildew, and fungus growing in or around the air conditioner. One function of an air conditioner is to remove humidity or moisture from the air. Parts of the air conditioner which remain damp from excess water are the perfect place for mold, mildew, and fungus to grow. 

Turn off the unit at the thermostat and the main switch. Open the access panel of the indoor air handler unit to inspect the drain pans for excess water and mold growth. The primary drain pan will be located under the evaporator coils, and the overflow pan will be below the primary pan inside or under the unit. If mold is present, the drain pans must be replaced. Mold can grow on the system’s internal components, such as the evaporator coils. Check the coils while you have the unit open. The coils can be cleaned with compressed air or a coil cleaner. Alternatively, call a technician to clean and service your unit. 

Mold can grow inside your vents and ducts if the ducts are not adequately sealed and warm air leaks into them, your AC unit is too large for your space, or the thermostat is kept at a low temperature. Check for visible signs of condensation and mold around the vents and ducts. If mold is present, it must be removed by a professional duct cleaning service. 

A dirty air filter can give off a musty smell, especially in humid climates where moisture can accumulate in the filter. Replace the air filter with a new one. How How often to change an air filter depends on the filter’s quality. It is recommended that air filters be replaced every 45-90 days. Check the filter’s label for instructions.

Why Does My Air Conditioner Smell - Your Next Steps

While you may be able to replace the air filter, clean the evaporator coils, or clear a clog from the drain pan, repairing an air conditioner and removing mold are two jobs that you should leave to professionals. Changing your air filter regularly and scheduling routine inspections and maintenance of your AC unit can help you avoid problems in the future.


Air conditioning

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