What's That Smell? AC Vent Smells and What They Mean

When you turn on your air conditioner for the first time, you expect to be greeted by nice, cool air. However, if the air coming from your AC vent is smelly, you might have a serious problem on your hands. Some smells are merely irritating while others are hazardous. Learn to tell the difference and take the appropriate step to eliminate the odor.

Innocent AC Vent Smells


Since your air conditioner dehumidifies the air as it cools, it’s no surprise that mildew is the most common odor to come from an AC vent. If excess water collected from the air isn’t drained away properly, the lingering moisture can allow mould and mildew to grow. Even a dirty filter can harbor mould growth, so try replacing the filter first. If the odor remains, call a heating and cooling professional to clean your air conditioner.

Stinky Feet

Stagnant water collected in an AC may cause the smell of stinky feet to come from air vents. A quick cleaning and repair to help future water drain properly should remedy the problem.

Rotting Garbage

If an injured bird or sick rodent found its way into your air ducts during the winter, it may have died there and begun decomposing, producing a rotten egg smell. It’s a gross thought, but not particularly dangerous. However, you’ll still surely want to have a heating and cooling technician remove the animal without delay.

Old Cigarette Smoke

If you or someone in your home smokes, the fumes can be pulled into the condenser coil. The odor permeates the coil and blasts the smell of old cigarettes into your home every time you run the air conditioner. Cleaning the coil can fix the problem.

Hazardous AC Vent Smells


As opposed to old cigarettes, an active burning odor could indicate a mechanical problem with the AC fan or compressor. Turn off the air conditioner and check if you notice the burning odor near the equipment itself. If so, call an HVAC technician right away to avoid a fire.


The smell of raw sewage coming from an AC vent is never a good sign. It usually indicates a backed up sewer line or ruptured sewer vent pipe somewhere near the ductwork. All it takes is a bit of methane from the backup to fill your house with the nasty smell. Methane is a dangerous gas, so call a plumber or HVAC professional without delay.

Rotten Eggs

Natural gas by itself is odorless, but an added rotten egg smell helps homeowners detect gas leaks. If this odor is coming from an AC vent, you could have a leak somewhere near your air ducts. Get out of the house immediately and call your gas company first, and then an HVAC contractor.

Gun Powder

The smell after firing an old gun is very distinctive. A similar smell may come from an AC vent if a circuit board or fan motor in your air conditioner shorts out. Call an HVAC technician to take a look before you run the AC again.

Exhaust Fumes

While air conditioners are powered by electricity, not gas, it’s still possible to smell exhaust fumes if engine fluids leak out. To prevent a dangerous situation from developing, have a professional sort out the problem.

If you notice any of these smells coming from an AC vent, or if you have other heating and cooling questions, please contact Aire Serv® today.

For Further Reading:

How Heavy Rains Can Affect Your Air Conditioner

Air Quality Testing

Catch the Culprits: Allergens

What Is Duct Work and What Can Go Wrong?

Don’t Be Fooled by these AC Maintenance Myths