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Explaining the Weird Smells Coming Out of Your AC

Odd smells coming from your air conditioner are never normal. Air conditioning unit odors like burning, rotten eggs, a musty smell, or other bothersome aroma impact the indoor air quality of your home, can indicate damage or problems with your heating and air conditioning system, and even lead to health issues amongst members of your household. Always address air conditioner smells right away to avoid these potentially harmful issues at home.

Thomas & Galbraith helps Cincinnati area homeowners better understand what could cause that smell from your air conditioning unit and HVAC equipment. Learn how to address smells coming from air conditioners and when to contact a professional air conditioning company so that a local HVAC technician can perform the repair services your heating air conditioning unit needs.

Why the Air Conditioner Smells Musty

When your air conditioning is on and the air smells musty, mold and mildew are probably causing the AC smells you notice. Mold as well as mildew often grows inside the AC unit or in the air ducts of the home. Airborne particles deposit organic matter inside the system and moisture forms through the cooling process – these elements plus the dark, cool interiors of the system create a perfect spot for mold to feed and reproduce.

If your air conditioner smells musty, take action to solve the odor immediately. Mold inside the HVAC system impacts indoor air quality, and it can spread and grow in other areas of the home as well as cause health issues or increased allergy symptoms due to breathing indoor air heavy with spores. Inspect these common areas of the air conditioner and HVAC system where mold or mildew is likely to form.

1. Evaporator Coils

As the refrigerant within the evaporator coils removes heat from the air, water vapor in the air condenses and settles on the coils. When combined with organic debris tracked in by moving air, mold or mildew has the conditions and food it needs to thrive. Cleaning the evaporator coils will remove mold and mildew growth.

  1. Shut Off Power to the AC Unit.
  2. Take Off the Metal Panel to Reach the Evaporator Coils.
  3. Create a Solution of Warm Water and Mild Soap. Put It in a Spray Bottle and Apply to the Coils.
  4. Allow the Solution to Soak on the Evaporator Coils for a Few Minutes, Then Clean Off the Coils Using a Cloth or Soft-Bristled Brush.
  5. Reattach the Panel and Restore Power to Your Air Conditioner.

Prevent mold as well as mildew on the evaporator coils by changing your air filter regularly and making sure the condensate drain is clear.

Condensate Drain Line

The condensate drain system of an air conditioner includes the drip pan below the evaporator coils and the condensate drain line attached to the pan. Moisture from the cooling process falls from the coils and is collected in the drip pan, then drained outside the home through the condensate line. The pan and drain line can grow mold and become clogged, causing moisture to back up in the air conditioning unit which further contributes to mold or mildew formation inside the HVAC system. Clean the drip pan if you see mold growing inside of it and clear clogs from the condensate drain tube to prevent moisture collection inside your heating air conditioning system.


Vents in the home can form mold or mildew as moisture collects on their metal surfaces from high humidity levels and leaks. Bathrooms without exhaust fans become saturated when hot showers run, leaving moisture to collect on the vents – repair malfunctioning exhaust fans or have fans installed if your bathrooms do not have this equipment. Mold can also form from moisture that collects on vents naturally, as cool air from the supply duct mixes with warmer room air, causing condensation. Clean vents to remove mold or mildew growth.

  1. Remove the Cover From the Vent.
  2. Clean the Cover With Bleach or Vinegar and a Cleaning Brush.
  3. Make Sure the Cover Is Totally Dry Before You Reattach It to the Vent.

Air Ducts

Air ducts form mold or mildew due to issues affecting the ducts or the growth is spread into the ducts from other areas of the air conditioner. Circulating air picks up spores from one part of the unit and carries it through the HVAC system and home, and some of the spores settle in the air ducts during an air conditioning cycle.

You won’t be able to look deep into your ducts for mold, but you may be able to spot external issues causing mold or mildew within.

  • A leaky roof lets rain in, which can soak the ducts and lead to mold or mildew. Examine your attic and roof for damage and watermarks. Tarp the damaged roof section until a repair can be completed to protect your ducts from further water infiltration.
  • Plumbing leaks can also cause water inside of air ducts. Leaking water supply lines near ductwork in the basement, crawlspace, or attic can create problems, as well as leaking plumbing fixtures above the ducts. Search for signs of water damage or standing water and shut off the water supply until the plumbing repair is made.
  • If your home has an air conditioner that is too big for the space, it can add moisture to the air ducts. Oversized heating air conditioning equipment doesn’t cycle long which causes more humidity to stay in the air – this moisture can be deposited inside the ducts as vapor condenses along metal duct walls. Unfortunately, the only fix for this issue is to call a professional heating and air conditioning company to have a local HVAC technician install a new air conditioner.

Anytime you have mold or mildew growing in your home’s air ducts or AC unit, it’s wise to have your ducts cleaned. Duct cleaning services eliminates any mold growth in the ducts to improve home indoor air quality and stop future growth.

Vinegar-Like Air Conditioner Smells

An odor of vinegar, when your air conditioning is on, can be due to sitting water and organic buildup inside the AC unit. These issues also lead to mold growth and mildew in the AC unit, so you may smell something musty at the same time you notice a vinegar smell. Be sure to check the aforementioned areas of HVAC systems for mold.

Ozone produced by electronic air cleaners or an electric fan motor inside the air conditioner can create an odor similar to vinegar. Address this smell right away as exposure to high ozone levels could cause health issues as well as poor indoor air quality. If you have an electronic air cleaner, lower the setting to reduce the amount of ozone generated. If the odor persists or you notice a vinegar smell but don’t have an electronic air cleaner as part of your heating air conditioning system, contact a professional for repair services.

The Air Conditioner Smells Like Dirty Socks

Dirty sock syndrome refers to air conditioner smells that are similar to a pile of dirty gym socks or dirty feet. This air conditioner smell results from bacterial growth within the AC unit, often on the evaporator coils as organic particles collect in this area. Mold, as well as mildew growth, are common under these conditions, so these air conditioner smells may be noticed together in the home.

  • Clean the evaporator coils of the AC system to remove the dirty sock smell.
  • Change air filters regularly to keep particulate matter out of the interior of the HVAC equipment.
  • Use a filter with a MERV rating of 8 to 12 to capture smaller matter and more particles, preventing them from entering the unit.

When There Is a Burning Smell Coming from an AC Unit

A burning odor, electrical smell, or gunpowder scent from an air conditioner can indicate overheating and damaged system components such as a fan motor, circuit board, compressor, or electrical wiring. If a component is overheated or fries it may emit a burning smell without causing a fire. The burning odor could also indicate active burning, from damaged electrical wiring for example.

It is hard to tell the source of a burning odor, but don’t ignore this smell. If a fire is burning somewhere in the system, call 911 and leave the home right away. Contact a professional local HVAC company to inspect the AC unit and make repairs before resuming use of your air conditioning at home.

The Air Conditioner Smells Like Rotten Eggs

The smell of rotten eggs from an HVAC unit usually concerns natural gas heating air units like furnaces. The rotten egg odor is added to natural gas so that homeowners can be more easily alerted to a gas leak. If you smell rotten eggs when using a natural gas furnace for heating air, turn off your gas right away and call the fire department to inspect your home. It may be necessary to have the heating air unit repaired by an HVAC technician before further use.

When a rotten egg smell comes from an air conditioner, it’s usually due to an unpleasant issue – a dead animal in the ducts. Small animals and insects can make their way into ducts for shelter and may spend their last days there, leaving a carcass to decay. When this happens, you will experience the rotten eggs smell when the air conditioning is running and maybe even when the air conditioner is off.

  1. Trace the odor to the vent where the smell of rotten eggs is the strongest – this is probably where the dead animal is located.
  2. Take off the vent cover and check inside the duct to see if the carcass is within reach by hand or using something to assist with its removal.
  3. If you are able to reach the body, put on gloves and grab the carcass. Pull it out of the duct and dispose of it in a garbage bag. Immediately take it out of the home.
  4. If you are unable to find the carcass or reach it inside the ducts, call a professional to assist you.
  5. Improve indoor air quality by having ducts cleaned after an animal or insect infestation to remove droppings and biological hazards left behind.
  6. Have your local HVAC company inspect ducts for damage caused by pests and make repairs.
  7. Find the animal’s entry point into your home and repair it so future pests cannot enter your home.

The Air Conditioner Smells Like Chemicals

Air conditioner smells that seem chemical-like can be traced to chemicals in the home, nearby vehicles, and even air conditioning refrigerant leaks. If you notice chemical air conditioner smells, these sources could be where the odor comes from:

  • Fumes from chemical cleaning products, air fresheners, paints, adhesives, and other substances can linger within the indoor air. If these containers have loose lids and are stored near intake vents or the air conditioning unit, the fumes may be carried throughout the house while the air conditioner cycles. Check your containers to make sure they are sealed and store them away from the air conditioner.
  • If your air conditioner blower fan motor is situated within a garage attached to the home, an idling car’s exhaust fumes can easily make their way into the indoor air supply through this HVAC equipment. Always open the garage door before running a vehicle and pull the car out of the garage if you intend to leave it running in idle for more than a few brief moments.
  • Older air conditioner units with Freon refrigerant may produce a sweet, chemical smell if a refrigerant leak is present. When leaked Freon mixes with indoor air and spreads through the home, air quality declines and occupants may suffer health issues due to exposure. Call a professional HVAC company for repair services to seal the leak and recharge the air conditioner with refrigerant.

Correct Air Conditioner Smells Now

Thomas & Galbraith helps Cincinnati area homeowners improve indoor air quality and preserve the home’s AC unit by solving AC smells quickly! If you have AC smells at home that cannot be solved through the advice above, give us a call to request an appointment for air conditioner repair.

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