I have a terrible oder coming from the air return in my home. About 9 months ago we had the same thing and the serviceman said it was from an open 1 1/2 inch pipe where the previous air handler was located. We had a new system installed and the air handler was moved to the attic. The oder has returned and someone told me that I needed to open up the pipe and flush it with about a gallon of water. I did this and the oder seemed to be better, but it returned the next morning. I need suggetions as to how to fix this problem permanently. Thanks!

Where was the drain put from the airhandler in the attic? Does it have a trap?