I have noticed a smell coming directly out of my air conditioning vent inside the house. We have a 2 story home, the smell seems to be worse in an upstairs bedroom vent. I am sure it is coming directly out of the vent.

It smells like sewer gas, but I am not sure. I live in Michigan so we have not been running the air conditioner for months. We have central air.

My question: what could possibly be causing the air conditioning system to smell like sewer. Although I am a plumbing novice, the venting for sewer has nothing to do with the air conditioning or heating, correct? Any thoughts?

One way would be if the condensate from the AC unit is connected to a plumbing drain that has dried out due to the lack of moisture caused by not operating the AC.

Thanks HJ, that may be the problem. Is there any way to test this or to easily restore the condensate?

Find the condensate drain and follow it and see where it ties into the sewer. There should be a trap in the line and it has probably dried out. You might have to cut out the trap and fill with water then reinstall with couplings. I have had this same exact thing happen to a buisness. The 3/4 trap in the ceiling dried up and every time the heater kicked on the women who worked in the building would run from the horrible smell. They were slightly happy when I discovered the problem. Good LUck

You can pour water into the trap that handles the condensate for a temporary fix. A better solution would be to run the condensate to a location that has a trap that is used frequently, such as a washing machine or sink drain.

good call!