I have a 20 year old house with an intermittent sewer gas smell that has been going on for about 2 years; although it has been increasing. At this time, it appears almost daily, but sometimes it is undetectable. The gas company has checked for leaks. We've topped off the floor drain with water, flush toilets and showers regularly, and have had a plumber come out. By the time, he arrived, the smell was gone. He could find no answer. Oddly, the smell is in a bedroom in the basement. No water line or bathrooms are nearby or above it. Next to the bedroom in the furnace utility room. No smell in that room ever. But, on the other side of the furnace room, there is a room that picks up some of the smell, we're assuming from a shared air vent with the bedroom. The rest of the house is fine. Just those two rooms which are not side by side, but do have the funace room in between them. Any ideas?? We're getting ready to sell our house, and need this smell taken care of. Also, the plumber told us we do not have a sump pump, if that has any consequence. Thanks!