Moving Tips: Getting Ready for the New Owners

What Can Cause Foul Odors To Come From Your Central Air Conditioning

When you run your central heat and air, the air should be mostly odorless. If you detect a strong or foul smell coming from your vents, this is usually a sign that something is in your system that shouldn't be. A bad smell doesn't necessarily mean something dangerous, but it's still something you should look into soon rather than later. The source of the smell and what's causing it may vary depending on what type of odor you're experiencing.

Mold And Mildew

If the air coming out of your vents smells like mold or mildew, it's a pretty safe bet that mold or mildew is the cause. It's most likely to form in or around your air conditioning unit, because this is what's responsible for removing moisture from the air. Debris on the coils can cause moisture to build up, and debris in the condensate drain can create a clog, which leaves moisture unable to drain.

If you have mold or mildew inside your air conditioning unit or in some other part of the system, it's best to have this cleaned by a professional. Trying to take apart your unit may void warranties or end up becoming a more complex job than you might think. However, you can locate and clean the condensate drain yourself. Clear the drain of any debris, then wash it with a mix of bleach and water. Flush the drain pipe or tube with water to clear any debris that might be in it, then pour some of the bleach and water mixture down it as well. Reattach it when you're finished.


The smell of exhaust coming from your vents can mean there are fluids from your unit or other component leaking into your air conditioning system. Your air conditioner itself runs on electricity, so your unit itself isn't leaking fuel, but you should still get your air conditioner investigated quickly by a professional service, like HomeSmart From Xcel Energy. If you can, turn off your air conditioner and fan until it can be investigated.

Rotting Eggs or Flesh

Since this odor tends to be more potent, you may start to smell it before there is even any air flow from your ducts. Most rotting smells are caused by an animal that has found its way inside your vents or ducts and died. This can sometimes happen when the grills over vents are not secured properly or have large slats. While the animal's body is easy enough for someone to remove, you should invest in a full-service duct cleaning once the body has been removed.

Recurring Odor Problems

If you think you've taken care of the problem but the odor keeps coming back, you should do a thorough investigation of your system or hire someone to do it for you.

  • Look for leaks in your ducts that could let creatures or odors from other places inside.
  • Make sure all your vents and other openings are covered securely.
  • Investigate your ducts for moisture that could be promoting mold and mildew growth.
  • Make sure your condensate pan drains cleanly and leaves no stagnant water.
  • Check your air filter once a month; if it's covered with dust and debris, replace it.