7 Reasons Why Your Air Conditioner Smells Bad

Is there a bad smell in your home that seems to get worse when your air conditioner is running?

While of course you want to get to the source of the bad smell and to eliminate it, you may not realize that a bad smell in your home can also be a sign of serious health hazards.

Here is useful information on the smells that may be coming from your air conditioning vents.

The Homeowner’s Guide To The 7 Causes of Bad AC Smells

Must, Mildew, and Mold

If there is excess humidity or moisture in the HVAC system, you could end up with mold and mildew flowing through the ductwork. To help prevent mildew and moisture problems, fix all water leaks, improve drainage, schedule bi-annual HVAC tune-ups, and frequently replace HVAC filters.

If you have exposed earth in your crawlspace or basement, you may want to investigate ways to cover the soil.

The best way to protect your belongings and prevent mold and mildew from forming in your home and HVAC system is with a whole-home dehumidifier. Connected directly to your existing HVAC system, the dehumidifier automatically removes the correct amount of moisture from the air and deposits the condensate outdoors.

When scheduling your air conditioning tune-up, make sure the company you hire includes a full cleaning. This annual inspection will clear your condensate drain line, clean evaporator coils, and detect any mold or mildew problems.

“Dirty Socks or Stinky Feet” Smells

Dirty evaporator coils, clogged condensate drain pans, and stagnant water can cause the smell of dirty socks or stinky feet. The solution is a professional cleaning.

Cigarette Smoke

If someone has smoked in the home, the odors could accumulate in the filter and evaporator coil. When you turn on the air conditioner, you may smell stale cigarette smoke.

Change the filter, schedule a professional tune-up and cleaning, and only smoke outdoors.

Burning Smell

If something smells like it’s burning, it probably is. This could be an overheating motor, a wiring issue, or some other mechanical problem. This is a serious thing – turn the HVAC system off immediately from the breaker box.

Sometimes the smell may be more similar to gun powder, which can indicate a short from the AC’s circuit board or fan motor.

If the burning odor persists, evacuate the home, and contact the fire department. Sometimes, melting plastic components can give off a fishy or rotten egg smell. Turn off power to the circuits from the breaker box, evacuate the home and contact the fire department.

After it has been deemed safe to re-enter the home, schedule service with an HVAC technician to make sure there are not electrical or mechanical problems with your air conditioner.


Sewer gases can back up into the home if you have dry P-traps. Try running the water in all of your sinks (about every month) to fill the P-trap and create a seal against sewage smells.

If dry P-traps aren’t your problem, you may have a backed-up sewer line or ruptured sewer vent pipe. Sewer gases can be dangerous. Contact a professional plumber or HVAC technician right away.

“Rotting Carcass or Dirty Trash” Smells

If a critter has died in your HVAC or duct system, the smell will quickly become impossible to ignore. The quicker you detect the decomposition the better.

First, turn off the AC system.

Next, open some doors and windows for ventilation.

Contact a professional HVAC or pest management company immediately.

Sulfur (Rotten Egg Smell)

Never ignore the smell of rotten eggs in your home. It could be the smell of leaking gas!

While natural gas is naturally odorless, manufacturers are required to add a chemical called mercaptan to give it that distinctive rotten-egg gas smell. Another sign if a gas leak is a path of grass or shrubs that have changed to a sickly brown/orangish color.

IMPORTANT… if you smell rotten eggs coming through your vents:

  • Don’t operate anything electrical.
  • Don’t operate matches or lighters.
  • If there are any open flames, such as candles, put them out.
  • Open as many doors and windows as you can.
  • Turn off your gas supply at the meter. If you can’t turn the gas off at your home, the gas company to do it for you.
  • Evacuate the building.
  • Contact the gas company.
  • Do not turn the gas back on. Have the gas company turn the gas back on for you.

Still not sure what the smell is or if it is related to your HVAC system? Don’t hesitate to Ask a Champion.

Professional Indoor Air Quality Solutions

The U.S. EPA has labeled poor indoor air quality as one of the main health threats in the U.S. Dust, dirt, hair, dander, pollen, fungus, and microbes that gather in your HVAC equipment and air ducts are responsible for many health issues. Additionally, the pollutants can get inside the HVAC system and damage internal parts, leading to more frequent repairs and breakdowns.

Covering Smells Up Is A Bad Idea

If you are trying to cover up the smell with aerosol sprays, scented candles, and the like, you are only worsening the air quality. Save yourself from irritated eyes and skin, nausea, headaches, breathing difficulties, trouble sleeping, and many other health problems by scheduling a professional indoor air quality inspection. Learn more about our Indoor Air Quality Solutions.

Service Champions is known for trustworthy, on-time heating and air conditioning service throughout the East Bay, South Bay, and Sacramento areas. We can help diagnose bad smells related to your HVAC – Contact Us.

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