We have one bathroom in our house used by 4 people. Every once in a while, there is a strong sewage smell. It is intermittent, there is no certain time that it is present. I cannot pinpoint it to a certain drain or area in the bathroom. When the heat comes on, it is VERY strong when it is present (the furnace is next to the bathroom). I figured I would check here before I tried anything. Any help would be VERY appreciated.