I have a two story house. Half way through last winter we started smelling a sewage smell coming for under the sink and vanity area in our master bathroom. The really weird thing about this is it only occurred when we would have a fire in our Great room fire place. After the fire had died out the smell would start to fade. The fire place is on the first floor and this bathroom is on the second floor. I started to investigate why this would occur. The fire place and the bathroom don't share any common vent pipes. Also the bathroom drain pipes and vent pipe share a common wall with our second bathroom upstairs. During these occurrences the second bathroom does not smell at all, just the master bathroom. This fall I took apart both drains in each bathroom and cleaned them out, even using a snake into the wall to get any build up cleaned out. Also I went onto the roof to check the vent pipe to see if it had an obstruction of any kind it did not. We have a prefab fire place that has an outside vent to let in outside air to help with effiency. We have had several fires this winter and the smell always comes back! I even went as far as putting insulation into the wall where the drain pipe goes and wrapping the pipes under the vanity with insulation and ducked tape to see if this would at least slow down the smell. This has not helped. The house is only 13 years old. I’m pretty handy but this has got me stumped!

Next time you build a fire, try duct taping over the drains and overflow. The fireplace could be drawing air in through the vent. Not sure why it would have suddenly started. Were you out with the caulking gun and can of foam just before? You may have the house so tight, the fireplace has to pull air in through the plumbing vents. Does your furnace have a good air supply?

Can you buy a kit to add an air intake to your fireplace? I couldn't, but after looking at the ones for other brands, I punched a 4'' hole from the back of my fireplace to the outside of the chimney in the garage, lining it with 4'' stove pipe. My fireplace draws better now.