Our home has a bad smell only in the winter months when the air conditioning is not running. We can smell it coming from several vents and in the basement where the heater and hot water tank are located. The smell is similar to rotten eggs, sewage and we had someone mention sulfur. We cannot seem to pinpoint what triggers the odor, except that we do NOT smell it when the A/C is on. It seems to come and go both when the heater is running and when it is turned off.

We moved in a couple of years ago and have smelled the odor both winters. We cleaned the de-humidifier (which was disgusting – we thought was the source) and have turned it off completely, but the odor still returned. We have an aerobic septic system which has been checked out; we've asked a plumber about the smell, who didn't have an answer. He did suggest we fill the traps with water, but that has not made a difference.

From researching online, we have some theories – we'd like to have the ductwork cleaned, but are discouraged after reading that other home owners have spent thousands cleaning their ductwork and without eliminating the odor. My husband has a few theories, including replacing the hot water tank. He's also tempted to tear down a wall to see if there is a possible leak; however this would not explain why we do not smell the odor in the summer months.

We appreciate any feedback, or suggestions on what professional might be able to help find the source. Thank you!

The first thing I would do is get a plumber/hvac tech in to do a carbon monoxide test on the furnace and water heater. Most likely this is what you are smelling (even though carbon monoxide is odorless, you are most likely smelling the byproduct of burnt fuel, or combustion, which can contain carbon monoxide, which can be deadly). How old is the furnace, and does it have a metal vent, or plastic vent? I would get this checked out asap. Sure sounds like it is coming from the furnace, which could signify a bad heat exchanger, or venting issue. Please let us know what they find.