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Sources of Weird Air Conditioning Unit Smells and How to Stop Them

Smells coming from your air conditioning unit are never a good sign. If your air conditioner smells musty, smells like rotten eggs, has a burning odor or other troubling scent, there is a problem somewhere within your HVAC system. Air conditioning unit odors aren’t something you should ignore or allow to linger very long, as they diminish indoor air quality, indicate trouble within your AC unit, and could lead to health issues depending on the source of your AC smells.

When your heating air conditioning unit smells bad, Jarboe’s Heating, Cooling & Plumbing shares what to look for within your air conditioning system and how to repair the issues causing the air conditioner smell. When air quality of indoor air is poor, and the odor persists in your home, call a professional for air conditioner repair services. Jarboe’s is your trusted local HVAC air conditioning company, with a professional HVAC technician team that is NATE-certified and ready to help!

If Your Air Conditioning Smells Musty, Mold Is Likely

Mold and mildew growth has a distinctive musty odor that many people are familiar with. If you notice the indoor air smells musty, more so when your air conditioner cycles, mold or mildew probably exists within the AC unit or your air ducts. Organic matter brought into the HVAC equipment by circulating air combined with moisture from the cooling process creates an ideal breeding ground for mold or mildew to grow inside the air conditioning unit.

Mold, as well as mildew, are typically found in the following areas of HVAC systems. When you detect a musty air conditioning odor, check these areas to correct the issue right away, as exposure to mold in the air can cause health issues such as increased allergy and asthma symptoms and other respiratory issues in home occupants.

Evaporator Coils

The evaporator coils within the indoor air conditioner unit are prone to mold growth and mildew. The cooling process causes water vapor in the air to condense, forming moisture on the coils. When moisture combines with organic matter inside the unit from air particle pollution, mold, as well as mildew, have all they need to grow. You’ll need to clean the evaporator coils to eliminate this growth.

  1. Cut the power to the AC unit at the switch and breaker.
  2. Take off the panel to reach the evaporator coils.
  3. In a spray bottle, mix a solution of warm water and mild detergent. Spray it on the evaporator coils and allow it to soak for several minutes.
  4. Use a cloth or soft brush to clean away mold or mildew on the coils.
  5. Reattach the panel and turn the power back on to your air conditioner.

Condensate Drain Line

The moisture that forms on the evaporator coils during the cooling process will drop into the drip pan below and flow out of the system and home through the condensate drain line. Because of moisture and organic matter, the condensate drain system is another prime location for mold growth within an AC unit. Clogs in the condensate drain line can also cause moisture to back up in the air conditioner, contributing to mold growth on the coils. Clean visible mold along with mildew from the drip pan. Unclog the condensate drain so moisture can freely flow out of your air conditioner.


Vents that deliver air conditioning inside rooms collect moisture due to leaks as well as when condensation forms as cool air combines with warm room air. You may be able to see mold or mildew growth on the vent covers, which needs to be cleaned.

  • Remove the cover from the vent.
  • Clean the vent cover with a brush and a solution of water and vinegar or bleach.
  • Let the cover dry before reinstalling on the vent.

Bathroom exhaust issues can also cause moisture to gather on vents, leading to mold. If your exhaust fans are not properly vented outdoors or you do not have them, have this issue corrected right away to prevent mold or mildew growth in your home and HVAC system.

Air Ducts

When mold is found in air ducts, it may have started in the ducts or was tracked into the duct system from elsewhere in the air conditioner unit. Because mold spores easily travel with circulating air, it’s best to have ducts cleaned anytime mold is found in air ducts or the air conditioner system. When mold forms in air ducts, it’s often due to these issues:

  • Leaky roofs let rainwater in, which can soak your ductwork and deposit water inside to fuel mold and mildew. Check your roof for damaged areas and look in your attic for signs of water damage. Tarp the impacted area until a roof repair can be made to protect your ductwork.

  • If nearby plumbing lines or fixtures leak, water can make its way into ducts to feed the growth of mold or mildew. Check water supply lines running near ducts in the attic, basement, or crawlspace, as well as plumbing fixtures situated above duct runs. If leaks are found, shut off the water supply until repair services have been performed.

  • When an air conditioner is too big for a home, indoor air retains more humidity because the air conditioning cycle doesn’t run long enough to remove it. When moist air hits metal ductwork, water vapor condenses on the duct walls, leaving moisture behind in the ducts to form mold or mildew. There is no easy repair for an oversized air conditioner – you’ll need to contact a professional local HVAC and have an HVAC technician install a new air conditioner unit for your home.

Vinegar Air Conditioner Smells

When you’re hit with a sour odor like vinegar as your air conditioning runs, it’s a sign there is pooled water and organic debris buildup inside your air conditioning unit. These conditions not only produce vinegar odors but also air conditioner smells that are musty – a clear sign of mold or mildew inside the unit. If you smell vinegar from your air conditioner, check the trouble spots mentioned above for mold.

If an electronic air cleaner or electric fan motor is producing excessive ozone, this may also cause a smell like vinegar from your AC unit. You can lower the electronic air cleaner setting to try and reduce the smell, but if that doesn’t work or you do not have an air cleaner to attribute the odor to, call a professional HVAC technician for repairs.

The Air Conditioner Smells like Dirty Feet and Socks

Air conditioner smells similar to feet, and used gym socks are sometimes referred to as “dirty sock syndrome.” These air conditioner smells are produced by bacterial growth inside the AC unit, caused by collecting organic particles carried in by the air. Again, these conditions also contribute to mold growth, so if the air smells musty, you may also notice that dirty gym sock smell along with it.

  • Clean the evaporator coils to remove bacterial growth inside your AC unit.
  • Protect your air conditioner from debris buildup by replacing the air filter on a regular basis.
  • Upgrade to an air filter rated MERV 8 to 12, which provides superior filtration for homes and removes smaller particles and more of them.

There’s a Burning Smell Coming from Your Air Conditioner

A burning odor may result when components in the system overheat, start to fry, or are actively burning. This includes electrical wiring, a fan motor, circuit board, or the outdoor compressor. It is hard to tell if a part inside your AC unit is actively burning or simply overheating, so don’t take any chances with your safety.

When you notice a burning smell from your air conditioner:

  1. Get everyone out of the house and call 911.
  2. Have your home cleared by the fire department before reentering.
  3. Call a professional HVAC technician to inspect and repair your air conditioner before using it once more.

The Air Conditioner Smells like Rotten Eggs

An odor that smells like rotten eggs is most often discussed in the HVAC world related to natural gas heating air equipment like furnaces. The natural gas company adds an odorant to natural gas to help consumers identify a gas leak, as natural gas is naturally odorless. When your home smells like rotten eggs and gas appliances are in use, turn off your gas and have heating equipment inspected and repaired before further use.

An air conditioner is a completely electric system that doesn’t run using gas. So when a rotten egg smell comes from air conditioning, it’s usually caused by a dead animal in the ducts. Small animals often seek shelter in the ductwork and sometimes don’t make it out, leaving behind droppings and even carcasses. Eventually, the dead animal decays, which creates a powerful odor that spills out into the home, especially during an air conditioning cycle.

  1. Follow your nose to identify the vent where the rotten egg smell is the worst – this is a sign the carcass is located in that duct run.
  2. Remove the cover on the vent and peer into the duct to assess if you will be able to reach and remove the carcass yourself.
  3. Glove up and pull the body out of the duct opening using your hands or some sort of tool. Place the carcass into a garbage bag, seal it, and take it outside of the home.
  4. If you cannot pinpoint the location of the carcass or it is too far into the air ducts for you to reach, call a professional to remove the animal.
  5. After an infestation, you need to have your ducts cleaned, or you will experience ongoing indoor air quality issues that can be harmful to occupant health.
  6. Animals cause damage to ducts to get inside them and while occupying them. Have ducts inspected and repaired by a local HVAC professional.
  7. It’s important to find and fix the area of your home where animals were able to come inside. Repair these points to prevent future pest issues.

A Chemical Smell from Your Air Conditioner

A chemical odor from the air conditioner AC unit is usually the result of chemicals or exhaust fumes making their way into the indoor air. If you notice the smell of chemicals as your AC unit cycles, check these areas:

  • Chemical fumes escape containers when seals are loose or broken. If these goods are stored near air conditioning equipment or intake vents, circulating air can pick up the fumes and spread them throughout the home, hurting your indoor air quality. Inspect cleaners, paint, and other household chemicals to make sure lids are secure. Store these items away from the HVAC system and vents.

  • In a home with an attached garage, exhaust fumes from idling vehicles can enter the indoor air through an air conditioner fan motor installed in the garage. Avoid this indoor air quality issue by leaving the overhead garage door open whenever starting a vehicle and pulling out of the garage when leaving the vehicle to idle for more than just a few moments.

  • A sweet, chemical-like odor is a sign of a refrigerant leak. An older air conditioner made prior to 2010 probably uses Freon refrigerant, which can be dangerous to the environment and cause health issues in those exposed through a leak. The spilt Freon can be carried into the home and mix with indoor air. Call a professional air conditioner repair company and have an HVAC technician find and seal refrigerant leaks before refilling your air conditioner with refrigerant.

Eliminate Air Conditioner Odors Today!

Air conditioner smells harm indoor air quality, cause health hazards, and indicate problems with HVAC systems. If you are unable to solve your air conditioner smells through the professional advice above, it’s time to call for help. Call Jarboe’s Heating, Cooling & Plumbing for air conditioner repair in the Louisville area.

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