I have an musty odor that comes out my forced air system under two scenarios:
1. The furnace blower is running and the smell will come out of the hot air vents
2. The furnace is not running and conditions are just right and the smell will come out of a couple of the cold air returns.
I believe that the smell is originating from the cold air returns because I performed the following experiment:
- Run the furnace blower...smell the hot air vents...confirmed smell is there.
- Put tin foil over the furnace filter and open up the door to the blower on the furnace...run the furnace blower (only draws air from the basement now)...smell the hot air vents...confirmed there is no smell.
The smell seems to be worst when it is rainy and humid, although I am not sure which one of these factors is causing the problem.
The smell also seems to be worse in two ducts that run on the same wall from the basement. The ducts are basically just run between studs in my wall with no sheet metal liner. I stuck a flash light and a video camera down the ducts and didn't notice any mold or water...but I'm not exactly an expert.