Updated On

November 8, 2023

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    The truth is, most of the time if your AC smells foul, there is something wrong. Your HVAC system conditions the air in your home by filtering it and removing heat and humidity. 

    A clean air conditioner that is functioning correctly should not produce a foul odor. If anything, the air coming out of AC vents should smell similar to or better than the air in your home. 

    If you smell dirty socks, rotten eggs, or other off-putting scents when you turn your cooling system on, keep reading to find out what is causing it and how to make it disappear. However, if you suspect that it might be time for a new system, you’re probably better off filling out this form:

    Get HVAC Estimates In Just 30 Seconds
    Then connect with local experts to get the held you need.

    How Can You Identify Different Air Conditioner Smells? 

    Surprisingly, an experienced HVAC technician can diagnose air conditioning malfunctions just by how a home smells when they enter. Here are the most common odors homeowners report smelling, the causes, and how to fix them. 

    Musty Dirty Socks

    Would it shock you if we told you that “dirty sock syndrome” is a real AC problem? Well, it is. 

    HVAC systems provide an excellent environment for fungus, bacteria, and other germs to grow. If you do not change your air filters frequently, contaminants can more easily enter your air conditioner.

    Over time, these particles buildup on the evaporator coil, in the air ducts, on the blower motor, and other essential central air components. Once you start to smell the musty odor, merely changing your air filters may not be enough, and you may need to deep clean your AC or disinfect your AC.

    For more information on dirty sock syndrome and how to fix it, check out this video:

    Rotten Egg Smell

    Homeowners who own a gas furnace or other gas-powered appliances need to be careful if they smell rotten eggs. Natural gas smells like sulfur or rotten eggs due to an additive meant to alert people that there is a gas leak. 

    Without the additive natural gas is a colorless and odorless gas. So, you would have no way of knowing the harmful and highly flammable gas was leaking into your house. 

    When your house is filled with gas even a tiny spark from static electricity could cause a massive gas explosion. That is the reason, you must immediately leave your home and contact the gas company if you smell natural gas. 

    The utility company will send a technician who will shut off the gas supply to your home before inspecting your gas lines, valves, and appliances to be sure it is safe. If there are repairs you need to make before the gas company can restart your service, they will notify you. 

    Fishy Odor Or Burning Smell

    If you wake up or return home and smell a mild burnt fish smell, an electrical component in your air conditioner or electrical system may be burning or melting. Electrical insulation gives off a fishy scent when it heats. 

    The smell is distinct and not easily ignored. If you encounter this scent, you should turn your HVAC off and contact an electrician or HVAC professional. Running your air conditioning system with melting electrical wires could cause a house fire or damage your AC unit or your home’s circuit board. 

    A Rotting Odor

    Sadly, a small animal can dig through your air ducts and get stuck inside. The result is a horrible rotting stench.

    If you think a dead animal is in your ductwork, you need professional assistance. An HVAC contractor will locate the animal in your duct system, remove it, clean the ducts, and repair any damage to your ductwork.

    Mildew Or Moldy Smells

    Like the other unpleasant odors on our list, if you smell mildew, there is an underlying source. Mold growth can build on dirty air filters and spread to other components. 

    Smelling mold may also mean you have an overflowing condensate drain pipe or condensation on your ducts is causing water damage. If your condensate drain line has a clog, you can fix it yourself. 

    To remove the clog, follow these instructions:

    1. Turn your AC off.
    2. Locate the outdoor drain pipe.
    3. Place a bucket under the pipe.
    4. Use a shop vac to remove any water or debris from the condensate drip pan.
    5. Pour a solution of bleach and water mixed at one cup of bleach per gallon of water. 
    6. Let the solution sit for 20 minutes. 
    7. Check the bucket to see if anything came out of the line. 
    8. If the line is still clogged you can use a pipe cleaner brush to gently move the clog.
    9. Repeat steps 5 through 8 until the drainpipe is clear. 

    Dirty drain pans can also emit an odor. So, you should clean and sanitize it regularly.

    A Sweet Scent 

    Refrigerant can smell sweet. So, if you smell a sweet scent, and you aren’t baking, it could mean your refrigerant lines that run from your indoor unit to the outdoor unit are leaking coolant. 

    You will need to call an HVAC repair company if you think a refrigerant line needs repairs. The EPA has strict regulations on who can perform repairs involving refrigerants. 

    A Dusty Or Musty Smell

    Smelling dust could mean your AC ducts are due for a cleaning. Dirty or improperly installed air filters can also cause your home’s air to smell musty. 

    So, try changing your air filters to see if that makes a difference. If not, you should schedule an appointment with an HVAC contractor. They can go over your system to let you know what is causing the malodor. 

    Plastic Or Chemical Odors

    When you run a new air conditioning system, it is not uncommon for it to smell like plastic or chemicals. Give the unit some time to breathe and cycle through the air in our home. 

    If the smell remains after a few days, contact your HVAC technician and ask them to take a look to be sure that none of the components are rubbing or burning. 

    How Can You Prevent Your Air Conditioner From Being Smelly?

    HVAC systems contain many parts. If buildup forms on your evaporator coil, in your ductwork, or on other components can make your AC stinky. 

    To keep your AC system smelling fresh, you need to:

    • Change your air filters every 30 to 60 days. 
    • Make sure that you are using the correct filter sizes and that you install them correctly. 
    • Have a licensed HVAC technician perform biannual tune-ups on your HVAC. 
    • Have your air ducts professionally cleaned every 3 to 5 years. 
    • Clean your condenser and evaporator coils once a year.
    • Repair damaged air ducts. 
    • Keep air intakes clear and free of dust and debris. 
    • Open your windows to let fresh air into your home. 
    • Seal holes in your attic to keep critters out. 

    Using an air purifier in your home can also help to keep your AC from smelling stale. Your HVAC is not an air purifier and it will not eliminate all malodors. 

    A quality air purifier will reduce the number of pollutants in your air which will help to keep your AC fresh longer. Even if you do everything right, an older air conditioner can develop an unpleasant scent and it could negatively impact your indoor air quality.

    Is It Safe To Use Your Air Conditioning Unit If It Has A Strange Smell?

    When your air conditioner smells strange, it is almost always a sign that it needs attention. If you smell a burning, rotten egg, or sweet smell, you should turn your air conditioner off immediately and call a professional. 

    Breathing air blowing through ducts filled with mildew, mold, bacteria, and possibly a decomposing animal is not a great idea either. It can be especially dangerous if you or a family member is in a high-risk category. 

    So, if you still smell a strange odor after: 

    • Changing your air filters,
    • Cleaning your condensate drain, 
    • Cleaning your evaporator coil, 
    • Letting fresh air in your home, 
    • Removing any sources of odor in your home, 
    • And Letting your HVAC system rest, 

    You should schedule a system tune-up.  

    Final Advice When Your Air Conditioner Smells Bad

    Your home’s indoor air quality depends on the state of your air conditioner. If your system is filthy, it will develop an odor, which it pushes into your home. 

    However, an older system that is not running correctly will turn on more often and stay on longer. That means there is more time for condensation to develop and the more water vapor your system produces, the more mold and mildew can grow. 

    If you’ve been putting off HVAC maintenance or repairs because they haven’t been in your budget, it might be time for a brand-new air conditioner. Fill out the form below to get started on the road to finding a new AC system:

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    Editorial Contributors
    avatar for Jonathon Jachura

    Jonathon Jachura


    Jonathon Jachura is a two-time homeowner with hands-on experience with HVAC, gutters, plumbing, lawn care, pest control, and other aspects of owning a home. He is passionate about home maintenance and finding the best services. His main goal is to educate others with crisp, concise descriptions that any homeowner can use. Jon uses his strong technical background to create engaging, easy-to-read, and informative guides. He does most of his home and lawn projects himself but hires professional companies for the “big things.” He knows what goes into finding the best service providers and contractors. Jon studied mechanical engineering at Purdue University in Indiana and worked in the HVAC industry for 12 years. Between his various home improvement projects, he enjoys the outdoors, a good cup of coffee, and spending time with his family.

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    photo of Roxanne Downer

    Roxanne Downer


    Roxanne Downer is a commerce editor at Today’s Homeowner, where she tackles everything from foundation repair to solar panel installation. She brings more than 15 years of writing and editing experience to bear in her meticulous approach to ensuring accurate, up-to-date, and engaging content. She’s previously edited for outlets including MSN, Architectural Digest, and Better Homes & Gardens. An alumna of the University of Pennsylvania, Roxanne is now an Oklahoma homeowner, DIY enthusiast, and the proud parent of a playful pug.

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