What Are the Weird Odors Coming from My AC Unit?

When your air conditioner runs, you shouldn’t notice any odor or smells coming from your air conditioning unit. Air conditioning unit odors vary depending on the underlying problem – common AC smells you could encounter include rotten eggs, mold, and mildew that smells musty, burning odor, and more. If you detect a funky air conditioner smell this summer, don’t let these odors linger without professional attention as an air conditioning smell could result in damage to your HVAC unit and even health issues.

If you smell something odd as your AC unit cycles or suspect an odor in the home is related to air conditioner smells, you need to know what to do to protect your family, indoor air quality, and HVAC systems. Williams Comfort Air explains what issues are possible if your AC smells musty, smells like rotten eggs, has a burning odor, or another air conditioner smell. Learn what to do right away and when you should call a professional heating and air conditioning company to make an appointment for repair services with an HVAC technician.

If the Air Conditioner Smells Musty…

A musty odor is usually linked to mold and mildew growth. If you tend to be hit with the odor especially when your air conditioning unit runs, there is likely mold and/or mildew growing somewhere within your air ducts or AC unit. The dark, damp conditions inside an air conditioner create an ideal environment for mold to grow and spread.

When your air conditioner smells like mold, you need to address this problem right away. Indoor air that is contaminated with mold spores can spread mold throughout the home and expose your loved ones, which could lead to increased allergy and asthma symptoms or even serious health issues.

Mold and mildew can start to grow in various parts of an air conditioning system. When air from your air conditioner is musty, check these areas to find mold or mildew and take action to rid the HVAC system of this growth.

Evaporator Coils

Mold often forms on the evaporator coils inside the indoor air conditioning unit. The evaporator coils contain refrigerant that pulls heat from the air – the cooling process causes moisture to condense, which clings to the evaporator coils. Combined with dust and particles deposited by air onto the coils, mildew, and mold growth.

If you find mold or mildew on your air conditioner evaporator coils, the coils need to be cleaned.

  1. Turn off power to the heating air conditioning unit.
  2. Remove the panel to access the evaporator coils.
  3. Mix warm water and mild detergent in a spray bottle and apply the solution all over your evaporator coils.
  4. Let the solution sit on the evaporator coils for several minutes.
  5. Use a soft brush or cleaning cloth to remove mold and debris from the coils.
  6. Replace the panel and restore power to the HVAC system.

Condensate Drain Line

Condensate from the cooling process normally falls from the evaporator coils into the drip pan and is drained out of the AC unit through the condensate drain lines. Clogs in the pan or drain line can lead to the growth of mold or mildew, as well as contribute to mold growing on the coils. If you find mold or mildew in the drip pan or notice the condensate drain line is not draining, you’ll need to clean the pan and clear the clog.


Mold can form on supply vents throughout the home due to leaks and excess humidity. Moisture often gathers in this area and feeds reproduction of mold as well as mildew. If your bathrooms do not have fans to remove water vapor and exhaust fumes or the exhaust fans aren’t properly vented outdoors, moisture can gather on vents while the shower runs – contact a professional to repair this issue.

  • If you find mold on vents, remove the vent cover and clean it thoroughly with a bleach or vinegar solution.
  • Allow covers to dry completely before reinstalling them on vents.

Air Ducts

Mold or mildew can grow in a home’s air ducts when spores circulate and settle in the ducts from other areas of the HVAC system or due to issues impacting the air ducts. Anytime you experience mold or mildew growth in your ducts or AC system, duct cleaning services are advised to remove spores and prevent recurrence, as it’s easy for mold to spread into the ducts from other areas of the system through air circulation.

While it’s not possible to easily look inside the ducts, you can look for outside issues that are causing the mold or mildew problem.

  • Roof leaks can cause rainwater to seep into ductwork, forming mold or mildew – look in your attic as you may be able to see signs of water damage on roof decking or on the ducts themselves. Place a tarp on the affected area of the roof until the repair can be made to prevent further moisture from entering the ducts.

  • Plumbing leaks near ductwork can deposit water into the ducts to spark mold or mildew growth. Check the water supply lines and drain lines that run near or alongside the ducts in your crawlspace or basement for damage and look for leaks from plumbing fixtures that sit above ducts. Turn off water to the home or specific fixture to prevent further water damage and have a plumber repair the problem right away.

  • Oversized air conditioning equipment can deposit moisture in air ducts. Due to incorrect HVAC equipment sizing, air conditioning cycles don’t last long and fail to remove much humidity from the air. Inside metal ducts, water vapor in the air can condense into liquid which settles on duct walls. The only solution to correct this problem is to have a licensed, professional HVAC technician install a new air conditioner that is the appropriate size for your home.

If Your Air Conditioner Smells of Vinegar…

A vinegar smell coming through your vents while the air conditioning runs is typically caused by stagnant water in the unit and buildup of organic matter. This odor is often caused by the same issues that lead to mold in an air conditioner. If you smell vinegar or a sour odor from your air conditioning, check the system for mold.

A vinegar odor could also come from ozone generated by an electric motor in the air conditioning unit or an electronic air cleaner installed with the HVAC system. High levels of ozone negatively impact indoor air quality and can cause health issues such as respiratory symptoms. Lowering the setting on your air cleaner can reduce ozone production, if applicable. If you still notice the odor or smell vinegar and do not have an electronic air cleaner, call for professional repairs and turn your air conditioner off unit the system has been fixed.

If Your Air Conditioner Smells of Dirty Socks or Feet…

Air conditioner smells similar to dirty gym socks or feet are often caused by bacteria growing inside the air conditioning equipment. The evaporator coil can start to grow bacteria as organic matter collects in this area of the AC unit. Mold also grows under these conditions, so you may notice the dirty sock smell along with musty odors.

  • To rid your home of the smell and improve indoor air quality, clean the system’s evaporator coil.
  • Make sure to replace air filters on a regular basis to capture more airborne particles cycling through the system so they do not collect on the coil.
  • Consider switching to a filter with a higher MERV rating to catch smaller particles and more matter.

If Your Air Conditioner Smells like Something Is Burning…

If you notice a burning odor, gunpowder smell, or electrical smell coming from your air conditioner, the odor could be due to a few different issues.

  1. Overheated Fan Motor
  2. Fried Circuit Board
  3. Compressor Problems
  4. Burning Wiring

These issues may emit a burning smell but it is difficult to tell which one is present on your own. If your air conditioner smells as if something is burning, turn off the AC unit right away. If the AC unit or wiring is actively burning, evacuate the home and call the fire department immediately. Have your local HVAC company inspect your system to diagnose and fix the issue before running your air conditioning unit again.

If Your AC Unit Smells like Rotten Eggs…

When a rotten egg odor is associated with use of a natural gas heating system, the smell is likely due to a gas leak and you should turn off your gas at once. However, air conditioners do not use natural gas – they are powered by electricity. Because no natural gas is used, a rotten egg smell from an air conditioner is typically caused by a dead animal somewhere in the HVAC system.

Pests can enter your ducts through damage or they may create damage to get inside. If a pest passes away while inside your duct system, the dead animal will decay, creating a smell whenever you run the air conditioning.

  • To eliminate air conditioner smells of rotten eggs, you need to remove the carcass and repair damage. Follow the odor and check vents to pinpoint where the smell is strongest – the carcass is most likely in that portion of duct. Remove the vent cover and see if you are able to reach the body. If so, put on gloves and protective gear before handling it and dispose of the body in a sealed garbage bag.

  • If you cannot reach the carcass or pinpoint its location, call a professional to remove the body.

  • You’ll want to have your ducts cleaned to remove any droppings and filth that cause indoor air quality problems.

  • Have your ducts inspected by a local HVAC technician to identify damage and repair it.

  • Inspect the exterior of your home to locate the pest’s point of entry and repair it to prevent further access.

If You Have Chemical Air Conditioner Smells…

Chemical-like air conditioner smells can come from various sources either within your air conditioner or elsewhere in your house.

  • The smell may be caused by open containers in your home. If cleaning products, paint, adhesives, and other chemicals are stored near air intake vents or ducts, fumes can be carried into the system as air circulates and spread throughout the house by the air conditioner. Check chemical containers for leaks and open lids. Safely store these items away from your air conditioning equipment.

  • If your air conditioning blower fan motor is installed in an attached garage, exhaust fumes from a running vehicle may seep into the equipment and spread throughout the house when your air conditioner cycles. Make sure the garage door is open whenever vehicles are running and do not leave a vehicle to run for long periods without pulling out of the garage.

  • Refrigerant leaks can produce a chemical smell when using air conditioning. If you have an older AC unit that uses Freon (R-22), leaks can be damaging to the environment and cause harmful health issues when people are exposed through indoor air. Freon will have a smell that is slightly sweet – if you notice this odor, turn off your air conditioner and call your local HVAC technician to repair the refrigerant lines and recharge the AC unit.

Solve Air Conditioning Odors Today!

Air conditioner smells are unpleasant, lower your home’s indoor air quality, and can even cause problems with your HVAC equipment or your health! Don’t let these smells persist – if you are unable to solve the smelly problem with the tips above, call Williams Comfort Air for air conditioning repair in the Indianapolis area.

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