Why Does My AC Smell Bad?

What Causes Smelly Air ConditioningCentral heating and air has one very specific job:  Adjust the temperature of your home to a more comfortable level.  Unfortunately, HVAC systems do this by moving and regulating airflow throughout a home.  If any portion of that system develops a strong smell you’ll find that same smell filling every room in your house very shortly.  Just as some smells aren’t as bad as others, some problems that cause smells aren’t as terrible as others.  In fact, you can identify some HVAC problems by the smells they cause so you can identify who you need to contact for help.

Leaks (Gas, etc)

  • Skunk Smells: If you can smell that horrible scent of skunk whenever the AC is running, but can’t seem to find a source (especially if you can’t detect the smell outside), then you should inspect your gas system.  Methyl mercaptan, which smells very similar to skunk spray, is added to natural gas and propane to help detect dangerous leaks.  A skunk smell in your HVAC system is usually caused by a gas leak and should be inspected immediately.
  • Stinky Feet: The damp, musty smell that’s somewhere between dust and wet leaves may not be coming from your socks after all.  When running your central air gives a smelly feet odor to room, it’s probably due to undrained water that has collected and turned stagnant within the HVAC system.  Draining the water and cleaning it thoroughly will solve this problem.  It’s a good idea to have a technician examine it to make sure it will continue to drain properly so a repeat event doesn’t occur.
  • Sewage: Most sewer lines smell like they do because of methane.  If you smell methane in your home the most likely cause is a ruptured sewer main near the ductwork.  Methane gas can seep into the system and spread throughout the house.  Methane is harmful in large quantities, flammable, and has a very unpleasant smell.  If you smell sewage running through your HVAC system call a tech, preferably from a service that’s well versed in plumbing as well, and get the problem fixed without delay.


  • Rotten Eggs: The sickly, nasty smell of rotten eggs is a downer any day of the week, especially when it’s coming from the vent.  A rotten egg smell is usually caused by an animal in the ventilation system.  Sometimes a small rodent or bird will crawl into the vent while sick, or become trapped inside.  Once the animal dies, the decomposing carcass gives off the smell.  Once you turn on your HVAC system, the smell fills the entire house.  Ventilation and duct cleaning is the best method to solve this smell.
  • Mildew: Does a damp, heavy, almost fungus-like smell fill the room when the AC or your heater turns on?  This moldy-odor is probably caused by mold or mildew within the system or around the air conditioner itself.  HVAC units don’t just control the temperature of your home atmosphere, they also control the humidity of the environment by removing excess moisture.  When that moisture builds up, it can lead to mold and mildew growth anywhere water builds up, even in the filtration system!  Clean your unit and replace your air filter.  If it keeps happening (or you just want to be certain it doesn’t happen again), bring in an HVAC technician to inspect and professionally clean and maintenance the unit to prevent future issues.

Electrical Issues

  • Gun Powder or Carbon: When something is smoking, the scent and visual signs are unmistakable, and they can usually be traced to their source.  Unfortunately, when an onboard motor burns out, or a circuit board shorts and pops a resistor, the scent of smoke smells more like carbon or gun powder than typical fire-made smoke.  If your room smells like gunpowder during the initial cycling of your HVAC unit, there’s probably some damage there.  We highly recommend calling a service technician before more problems develop and an air conditioner repair is needed.
  • Exhaust Fumes:  Don’t worry, there’s not a small rodent on a motorbike racing through your vents.  Air conditioners are powered by electricity rather than combustion, but some fluids can still leak out.  Many of these can smell similar to exhaust fumes.  If you smell exhaust when your HVAC is running, call a technician to inspect the unit itself, rather than the ductwork.

If there’s a smell coming from your vents, it’s a clear sign of a more serious problem than just terrible odors.  If the smell matches anything we’ve discussed here, or even if it doesn’t, it’s never a bad idea to call a professional to get things cleaned up and repaired.  After all, no one wants to live in a house that smells bad, no matter how cool the environment is.

More pro tips on combatting common air conditioning problems:

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