Why My Air Conditioner Smells

Your air conditioning just kicked in and now you can smell something funny coming from your vents. Is this a sign of a serious problem or just something unpleasant? Here’s what you need to know about air conditioner smells.

Types of Air Conditioner Smells

Dirty Sock Syndrome is a real A/C problem

Dirty Sock Syndrome

When your cooling system kicks out a smell that reminds you of dirty socks (or feet) then you may have Dirty Sock Syndrome. To steal a line from humorist Dave Berry…we are not making this up. It is actually the term for this smell used in the HVAC industry. Dirty Sock Syndrome is a bigger problem with heat pumps than with Air Conditioner/Furnace combinations, but A/C units can put out this smell as well. Research shows that Dirty Sock Syndrome is the result of mold, mildew and any other organic microbes in your cooling system.

The problem is worse in the spring and fall when the system (this is also why heat pumps have a bigger problem with this) may heat in the morning and cool in the afternoon. The result of the temperature flux is that moisture ends up in the system from condensation and feeds organic growth. Furnaces heat to much hotter temperatures and can actually kill organic material in your system (heat pumps don’t), which is another reason why this problem is more common in heat pumps.

Sulphur Smell

This type of smell is less common, but a possibility. It can be pretty mysterious because it doesn’t always last for long periods of times. Some homeowners have reported the sulphur smell only last for a couple of minutes when the A/C unit first turns on. There are a few possibilities that can be causing the sulphur smell.

  1. A dead animal in your vents or attic
  2. A gas leak. Natural gas has a scent put in it by the gas company.
  3. A problem with your plumbing vent stack which is allowing sewer gases to be sucked into the air handler.

Other Strange Smells

Occasionally, we’ll hear about other odd smells coming from the vents. These are not nearly as common as the musty dirty sock syndrome or even the sulphur smell, but here they are just in case you have this problem.

  • Exhaust Fumes (probably a fluid leak)
  • Gun Powder (Fan or Circuit Board failure)
  • Skunk (can be a gas leak)

How to Fix the Musty Dirty Sock Syndrome Smell

That nasty smell is caused by mold or mildew growth. Solving the problem depends on where the growth is happening. Usually, it is one of three locations.

  1. Drain pan or condensate line
  2. Evaporator Coils
  3. Ductwork

Checking Your A/C units drain system

The indoor air handler part of your split A/C system (the most common type in homes) will have a drain pan under it. In the Charlotte area, it is most commonly located in the attic. Check the drain pan to make sure that there isn’t any water in it. If there is, your drain has backed up.

Another way to check for backups is if there is water dripping from the secondary drain that comes out of the drain pan. Often, these drain are located over a window to improve the chances that somebody will notice there is a problem. If there is water in either place, then your primary condensate drain is clogged up. In newer systems, you will usually find a sensor in the drain pan that shuts down the air handler if the pan starts to collect water. This is a precaution to prevent flooding (especially when the pan is in the attic) and alert you to a problem.

If you have water in your drain pan, there is a good chance the musty smell is coming from mildew and mold in the drain system. You’ll need to clean out the main drain first to remove the moisture problem. We recommend calling a licensed and trusted HVAC provider to fix this problem for you.

If you are in the Charlotte, NC metro area, you can request service by email or call us at

Evaporator Coil

Cleaning Your Evaporator Coil

If your pan is free from water and everything else is working correctly, there is a good chance you have mold or mildew in your evaporator coil. Dust on your evaporator coils provides a place for mold to grow. When combined with moisture from the condensation caused by the A/C unit running, you can run into a problem. There are two steps to solving this problem, mold removal and prevention of future growth.

Removing the mold and mildew growth on your evaporator coil is really a professional job. You can find instructions online about how to clean the coil yourself, but also consider the risk. If not done correctly, some of the dirt in the coil could become trapped instead of being cleaned out. Coils are fairly delicate and expensive. Making mistakes when cleaning them can have a long-term negative impact on your system.

In some cases, cleaning the evaporator coil doesn’t stop the problem from coming back. We can also install a UV purification system which is very good at killing off bacteria, mold and mildew that cause the odor.

Duct Cleaning or Replacement

If both the drain and the coil look good, the last place to look for mold and mildew growth is in your ducts. A visual inspection is a good start, but you won’t be able to see very much of the inside of your ducts. The EPA recommends duct cleaning if there is actual mold growth in your ducts. Verifying this may require mold testing. If there is mold in the ducts they should be cleaned or in some cases replaced. Cleaning won’t be enough to keep the problem from happening again. You’ll need professional help to determine and correct the problem that led to the mold growth in the first place.

How to Fix the Sulphur Smell

Rotten Egg Smell

Getting rid of that rotten egg sulphur smell.

You can check a few things in your attic (assuming that is where your air handling system is located) or leave the inspection up to a professional. You may be able to smell a gas leak just by entering the attic and sniffing around. If this is your problem, get the gas company out to your home immediately to identify and fix the leak.

If you are pretty sure it isn’t a gas leak, then the next thing to look for is a vent pipe for your plumbing. Occasionally, the the vent stack can become loose or disconnected from improper installation, movement from work being done in the attic or maybe some other reason. If you see a large white PVC pipe that is disconnected, your next step is to have the plumber come out and fix the problem. This should stop sewer gasses from escaping into your attic and being pulled into your HVAC system.

Finally, you can check around for signs of animal problems. This can be listening for scurrying sounds, spotting droppings or finding damaged ductwork. You’ll probably want some help from a professional pest removal service to track down, remove and prevent further pest issues. You may also want your ducts cleaned or need some replaced if they have been damaged by the intruding animals.

How to Fix Other Smells

If your A/C unit is sending off other smells, then your best bet is to contact a qualified service technician to do some troubleshooting on your system. When things are weird, there is not substitute for experience and resources that an HVAC technician can provide.

If you are in the Charlotte, NC metro area, you can request service by email or call us at 803-327-2700

Don’t Forget About Prevention

Good maintenance practices throughout your home will reduce your chances of having smelly air conditioning problems. A simple way to help prevent dirty sock syndrome is to regularly change your air filters so you don’t end up with a lot of dust on your condenser coil. Beyond that, we offer routine maintenance services for your HVAC system, which include visually inspecting your coil and other parts of the system, so we can catch and correct problems before they lead to bad odors or worse.

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