I have a center townhouse unit in a 4 unit complex which I rent out. For years now I get an intermittent sewer gas smell. None of the other units have the same problem. It appears to be coming from the basement and happens when the furnace fan comes on apparently drawing the smell through the rest of the house. I have made sure there is water in all the traps. When it happens I can not locate the exact source by nose. It does not appear to be coming from the floor drain which I have pretty much stuck my nose down. A plumber has been called several times and has checked to make sure the vent is clear but says nothing is wrong it has to be dry traps but has never done any smoke, perppermint or electronic testing. The roof vent stack, which I suspect as the potential problem, comes down to the floor drain against the outer wall about 6 feet from the furnace. I have suspected the smell from this area but have not been able confirm it with my nose however, I sealed all the visible pipe connections in the vent stack in the basement with silicone sealer. I also added a fresh air intake for the fuirnace to try and minimize drawing air directly from the basement. Even after all that I still get the smell occassioanally when the funace fan cycles. Any advice is appreciated. What is my next step ? Thanks
