Bad Air Conditioner Smells: Common Causes for Nasty Odors

bad air conditioner smells

When Your Air Conditioner’s Breath Stinks!


Your air conditioning system is what you count on most to bring you comfort at home. So when you turn it on, you expect it to deliver nice cold air to keep you fresh. And you want to feel it, not smell it! But what happens when you turn it on and get a foul smell coming out of it? The last thing you want is bad air conditioner smells ruining your day!


Bad air conditioner smells are telltale signs that it’s time for service. But an odor problem doesn’t necessarily mean that an expensive repair is in order. It could be something as simple as a dirty air filter or a clogged drain pan. But some smells could mean potentially dangerous hazards for you and your family. So it’s important to know what these bad odors mean before you call us at 813-609-5015 to take care of it.


What Causes Bad Air Conditioner Smells?

Mold and mildew growth is the most common cause of bad smells in your AC system. Also known as bacterial growth, mold and mildew will circulate through your indoor air once it has settled in your air conditioner. Other contaminants in your HVAC system will do the same if left unattended. But bad air conditioner smells might not necessarily be originating from the system itself. Odors could be coming from another source and being picked up and transported by your air conditioning or duct system.


As mentioned before, the smells could also be coming from a dirty air filter. That’s because your AC system’s air filter pulls odor-causing contaminants out of the air as it circulates. So if you haven’t changed your air filter out regularly, this nasty stuff is recirculating back into your home.


Aside from your air filter, here are some common areas that can produce bad air conditioner smells:


Evaporator Coils

Your cooling system’s evaporator coils collect dirt, dust and other debris over time with regular use. Add to it the cool, damp environment your evaporator coil creates and you have a potential breeding ground for mold and mildew. Without regular maintenance and cleaning, your evaporator coils will start emitting bad air conditioner smells over time.


Drain Pan and Condensate Drain Line

Your AC system’s evaporator coils carry out moisture to the condensation line and drain pan. And when these become clogged, moisture is unable to flow out of the system. This leads to mold and mildew buildup. The drain pan has a float switch to prevent excess condensation. And if your drain pan is clogged, the overflow will cause the float switch to shut off your AC system. So if you’re picking up bad air conditioner smells and it’s shutting itself off, your drain pan and condensate drain line are gathering mold and mildew and it needs to be cleared immediately.



As the air circulates in your home, dirt and debris can accumulate in the ductwork. And when moisture is not being controlled, your ductwork can get damp, which also leads to mold and mildew growth. Any holes, gaps or damage to your ductwork create the opportunity for outside odors to enter the duct system and circulate through your indoor air. And mold and mildew from a roof leak or wet crawl space could also enter your duct system and produce the smell. If outside odors are seeping through broken ductwork, it’s time to get it inspected.


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Poor Ventilation

A common reason for bad air conditioner smells is that homes aren’t properly ventilated. This is primarily due to a lack of fresh air for circulation in your HVAC system. Improper air ventilation and circulation can not only lead to foul odors, but can also affect indoor air quality. If your home is not properly ventilated, try letting more air into it when possible. Don’t be afraid to open windows when you can. And call us at 813-609-5015 to check out your home for an inspection. We can help you identify problems that are causing odors and provide solutions to improve your home’s ventilation.


Types of Bad Air Conditioner Smells


Musty or Moldy Smell

A musty or moldy smell could be likened to smelly feet or dirty socks. Since your AC dehumidifies the air as it cools, mildew is one of the most common smells from the condensation it produces. As mentioned before, the odor created from bacterial growth will give off that musty or moldy smell. Clogged drain lines, standing water in drain pans or leaks in your ductwork can all create that smelly feet odor that permeates through your air vents. A simple AC maintenance and cleaning usually does the trick.


Sewage Smell

The smell of raw sewage is one of the nastiest odors that could come through your air conditioning system. And if that stench is coming from your vents, it’s not a good sign. It could mean there’s a backed up sewer line or ruptured sewer pipe somewhere near your ductwork. And that will emit a smell of methane, which can fill up your home with a foul smell. Methane is a hazardous gas. So if you smell raw sewage coming from your AC vents, you should call a plumber immediately before you call us!


Rotten Egg Smell

A natural gas leak has a foul stench that smells like rotten eggs. And if this odor is coming from an AC vent, you could have a leak somewhere near your air ducts. Natural gas is odorless. But gas companies add a harmless chemical that creates a smell akin to sulfur to alert people of a leak. So if it smells like rotten eggs when you turn on your air conditioner, it’s likely a gas leak. Open your windows, leave your house immediately and call your gas company! And don’t use any electronics or open flames near the area where you smelled the natural gas. Doing so could lead to an explosion or a fire. And speaking of fire …


Burning Smell

We covered this before, but when you first turn on the heater, it can smell like something is burning. The smell is usually created when dust settles on the heat exchanger after a long period of heater inactivity. So when the heater is turned on, the dust gets burned off. It’s normal for this to happen. But if the burning smell doesn’t go away after a while, you should call us immediately!


When using the the heater for the first time in a long time, you might also pick up the smell of burning electrical parts. This is usually a sign of a component overheating and you should turn your AC unit off immediately before attempting to find the problem. Again, it could be something as simple as a dirty air filter, which can restrict airflow and overheat electrical components. But if changing out the air filter doesn’t solve the problem, the odor is a sign of a motor overheating or bad wiring.


An active burning odor could indicate a mechanical problem with the AC fan or compressor. And unless you’re a professional electrician, do not attempt to fix the problem yourself! Dealing with electrical problems when not properly trained can lead to severe injuries. Aside from turning off your AC system, turn off your electricity at the master switch until a professional HVAC technician can check it out.


Dead Critter Smell

We hate the thought of animals dying in our HVAC system. But it’s not uncommon for small critters such as rodents or birds to enter your air ducts and die. So if there’s a dead animal in your ductwork, you will smell its carcass every time you turn on your air conditioner. When not taken care of, the decomposition of a dead animal will create a foul stench that will permeate throughout your home. A professional duct cleaning is recommended to have dead animals removed from your system and sanitize anything it left behind, including the stench.


The best way to find out if you have a dead animal in your AC system is by following your nose. Using your sense of smell will identify whether a dead creature is in your ductwork, your air handler or anywhere else. By the way, the air handler right behind your air filter. And if a mouse crawled in there and died, you will know about it soon!



Living Rodent Smell

We just mentioned small critters like mice dying in our HVAC systems. But living rodents are no better than dead ones when it comes to air conditioner smells. That’s because live rodents will pee and poop in your AC system. So a strong urine smell coming from your register vents is a sign that you have live creatures in your system. Mice in particular urinate and defecate frequently. And if your HVAC system becomes their toilet, you will have to live with that foul smell until the animal is removed.

To eliminate the problem, the best place to look is inside the air handler or another area where warm air is present. Although you can set up traps or poison to get rid of them, you could end up creating a bigger problem, as they could end up dying in places you can’t reach. If you suspect rodents are living in your HVAC system, it’s best to call a pest removal company first before calling us to clean your ductwork.


Cigarette Smoke Smell

This one is pretty self-explanatory but if you or someone in your home smokes indoors, the fumes can be pulled into the evaporator coil. The tobacco smell permeates the coil and blasts the cigarette smell into your home every time you run the air conditioner. Smoking indoors kills your indoor air quality. And if you live in a building where plumbing and/or ductwork is connected from apartment to apartment, that smell could seep in your home. If there’s a nasty tobacco smoke odor in your home, stop smoking indoors and call us today so we can clean and sanitize the air for you.


Acetone Smell

A refrigerant leak in your AC systems can create a smell similar to acetone. So if you smell something akin to nail polish remover in your home, a refrigerant leak is the most likely culprit. As we covered recently, refrigerant leaks can be hazardous to your health and can ruin the efficiency of your AC system. A refrigerant or freon leak is no joke, so contact us immediately and we’ll take care of it for you.


Did we leave enough calls to action in this post? Good! Because we want you to know you can count on us to take care of all your air conditioning needs! So call us anytime, 24 hours a day, 7 days a week at 813-609-5015 and we’ll take care of those bad air conditioner smells!

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