african american woman holding her nose because of vinegar odors caused by air conditioner

The cool air of your house is a welcome retreat on a sweltering summer day unless your AC smells like vinegar. Air conditioner smells such as vinegar, rotten eggs, and dirty socks are not uncommon. These unpleasant odors likely indicate issues with your air conditioner, so it’s important to determine the causes. Most issues can be resolved by the homeowner, though some problems require professional help.

Common Reasons Why Your AC Smells Like Vinegar

There are several possibilities for why your AC smells like vinegar. You have to do a little investigating to determine the cause. Luckily, you may be able to resolve the issues yourself in many cases. If that makes you uncomfortable, you can call an HVAC service instead.

A vinegar odor may be caused by:

  • Mold and bacteria
  • The electric motor
  • Too much condensation
  • A dirty air filter
  • A blocked condensation pan

1. Mold and Bacteria

One function of your air conditioner is dehumidification of your air. The humidity removed from the air is collected and drained away from the home. A malfunctioning air conditioner can cause some AC parts to remain damp, which are the perfect spots for mold to grow. Mold, mildew, and bacteria growth inside your home’s ductwork emits a sour smell.

Excess condensation produced by your air conditioner or the improper drainage of condensation can lead to moisture and mold forming in the ducts. Moisture and mold growth can also be caused by improperly sealed ducts, an air conditioner that is too large for your square footage and keeping the thermostat at a low temperature.

mold and bacteria growing on air ducts in home caused by excess condensation from air conditioner


Your ducts should be inspected for excess moisture and mold. If mold is present, it must be thoroughly removed, or it will return. Hiring a professional duct cleaning service ensures proper removal.

Furthermore, the cause of the excess moisture must be determined. If no leaks in the ducts are found and the thermostat hasn’t been set extraordinarily low, your air conditioning unit should be inspected. You could hire an HVAC professional or do it yourself.

To inspect the unit yourself, turn off the unit at the thermostat and the main switch. Open the access panel of the indoor air handler to check the drain pans for excess water and mold growth. Drain pans can be cleaned by hand, but replacing a moldy pan is better. Mold can grow on the system’s internal components, such as the evaporator coils. Following the manufacturer's directions, the coils can be blown with compressed air or cleaned with a no-rinse evaporator coil cleaner. If these tasks seem too daunting, call an HVAC service.

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man removing the top of central air conditioner unit to fix damaged motor

2. Electric Motor

All electric motors naturally emit ozone, an inorganic molecule, during normal functioning. Ozone has a distinctly pungent odor and results from using electricity to create energy. A damaged motor might produce more ozone than usual, making the smell more noticeable. Besides having an unpleasant odor, ozone also releases harmful toxins into the air.


Hire an HVAC technician to inspect the motor and repair it if necessary. If you choose to inspect the unit yourself, be sure to shut off power to the unit at the switches and the breaker panel. Visually inspect the wiring near the motor for splices and breaks. Air conditioners contain high-voltage parts, so don’t poke around in the unit. If you see a wiring issue, hire a professional to repair it.

3. Too Much Condensation

Condensation is produced by your air conditioner during the cooling process. Warm air travels over the cold, refrigerant-filled evaporator coils, the air’s temperature is reduced, and the cooled air is blown through the house’s vents. Condensation forms on the coils, collects in the drain pan and exits through the drain line.

A dirty air filter causes excess condensation by restricting airflow and reducing evaporation. Excess condensation can also develop in high humidity environments.


Change the air filter and continue to change it regularly, every 30 to 90 days.

Cleaning the evaporator coils also helps to reduce condensation. Hire a professional to perform maintenance or clean the coils yourself. To clean the coils, switch off the power to the air conditioning unit and open the access panel of the indoor air handler unit. Following the manufacturer's directions, the coils can be blown with compressed air or cleaned with a no-rinse evaporator coil cleaner.

condensation caused by restricted air conditioner airflow
dirty air conditioner filter causing vinegar smell

4. Dirty Air Filter

The air filter cleans air entering and exiting the air conditioner. Contaminates from the air accumulate in the filter, and airflow inside the unit becomes restricted over time. If the air filter is not regularly changed, humidity within the unit continues to increase, mold forms on the filter and elsewhere within the unit, and an increasingly sour smell is produced.


Make sure the air filter is properly positioned and functioning. Change the filter if it appears dirty, wet, or has been more than 90 days since it was last changed. How often to change your air filter depends on various factors, but as a general rule, it should be changed every 30 to 90 days, depending on the filter’s quality.

5. Blocked Condensation Pan

The condensation pan collects moisture that drips off the evaporator coils. The pan is supposed to drain into the drain line, but sometimes it becomes clogged. Hair or other debris blocks the drain hole, causing water to collect in the pan. Over time, mold grows in the stagnant water. Clogs occur from running the air conditioner without a filter, neglecting to service the unit, and extremely dusty or dirty conditions.


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 should be replaced. Otherwise, the pan can be emptied and cleaned with hot water. Use a paper clip or wire to clear the clog from the drain hole.

clogged condensation drain pipe on air conditioner
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Other Air Conditioner Smells

The smell of vinegar is undoubtedly unpleasant, but air conditioners sometimes release other foul odors into your home’s air and leave you wondering, “Why does my air conditioner smell?” These various stinky smells can resemble:

  • Musty towels
  • Rotten eggs
  • Something burning
  • Dirty socks
mold growing on air conditioner condenser coil causing musty smell


The musty odor of wet towels left in the washing machine for too long is a common air conditioner odor. Certain parts of the air conditioner remain damp because one function of the AC is to remove humidity from the air. Mold, mildew, and fungus thrive in moist environments.

The musty smell from your air conditioning could be fungus or mold growing around or inside the outdoor condensing unit or on components inside the indoor unit. A clogged air filter, drain pan, or drain line is a common cause. Mold can also grow inside improperly sealed ductwork.

Air conditioner parts and ducts that contain mold must be thoroughly cleaned to prevent the spores from returning.

dead mouse found in air conditioner unit causing rotten egg smell

Rotten Eggs

The odor of rotten eggs should not be ignored. It could mean a gas leak. Natural gas has no smell but contains a stinky additive to alert people to gas leaks. First, rule out a gas leak by calling your gas company. Leave the premises until you know it’s safe to return.

A more likely, but a disgusting, cause of a rotten egg odor is a dead animal. In air conditioning units and ductwork, rodents and other small animals sometimes find shelter during the winter. The animal carcass must be located and removed, and the unit and ducts must be thoroughly cleaned and sealed to prevent a recurrence.

air conditioner with electric issue causing burning smell


A burning smell emanating from the vents likely indicates an electrical issue. Electrical issues occur more often in older air conditioning units than in newer ones. A bad circuit board or a burned-out motor is the cause. An HVAC professional can identify the issue and perform any necessary repairs.

mold growing in air conditioner drain pan causing dirty sock smell

Dirty Socks

The smell of dirty socks is so familiar 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 indoor AC unit, namely the evaporator coils and the drain pans. A dirty air filter or a clog in the drain pan are two other issues that can lead to excess moisture and mold growth.

The Bottom Line

The nasty smells of vinegar, musty towels, rotten eggs, something burning, and dirty socks indicate that your AC needs maintenance or repairs. Many smells indicate problems with moisture, mold, and mildew. Other smells indicate more complex issues such as electrical problems, dead animals, or gas leaks.

Luckily, there are DIY solutions you can try before you call for professional help:

  • Change the air filter.
  • Check for excess moisture, mold, and drain pan clogs.
  • Clean the evaporator coils.

Electrical and other complicated issues should be handled by an HVAC professional.

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