Sewer Smell in Bathroom ONLY when Cold Outside

I have a sewer smell in two of my bathrooms that only occurs for a few hours on the day a cold front arrives. It does not happen every time, maybe 1 out of 4 times a northern blows in. The sewer smell has never happened during warm summer or spring weather. The smell does not happen that frequently on the days following the arrival of the cold front. There was one time when a northern blew in that it smelled strong in the stair well in the middle of the house. Any ideas?

I live in a 2 story split level built in 1979. The house is shaped like a rectangle, with the front door facing east. One of the bathrooms is a half bath near the front door on the east side, the other one is near the south west corner. Niether bathrooms have windows. The smell does not seem to be comming from the sink, the overflow drain, the tub, or the toilet. The toilets in both bathrooms are secure on the floor and everything is used frequently and works well. Both bathrooms have an original electric ceiling vent.

One theory is that the north wind blows the fumes from some vent into the bathrooms somehow. I don’t know why it would only go into the bathrooms. Another theory is that when it gets cold, we turn on the heater, and somehow the smell gets sucked in through the ceiling vents.

The bathrooms will have vent pipes connected to the plumbing that will vent the sewer gasses out through the roof. The bathroom ceiling fans will also have some kind of a vent that lets them exhaust the humid air out from the bathroom. Do these two happen to be in fairly close proximity on your house? And is your house a newer one, fairly airtight?

If so, it could be possible that when the furnace kicks in on a cold day, it needs air, and it will suck air in through they bathroom ceiling air exhaust vents, and that might pull in sewer gas that is coming out of the nearby plumbing vent pipes. It might happen more often with a northern wind, if the plumbing vents are on the north side of the ceiling exhaust vents. This probably wouldn’t happen much in summer, since the furnace won’t be kicking on then.

But I don’t see why it would only happen on the first day of a cold spell, instead of anytime the furnace kicked in.

A possible way to test this: just before the next northern cold snap, seal off the bathroom ceiling exhaust vents, and see if that prevents the smell. You could probably seal them pretty easily using the double-stick tape and heat-shrink window treatments that are all over in hardware stores this time of year.

If this is the problem, the best solution, by the way, is running a vent from the outside air directly into the furnace, to provide it with combustion air without sucking air from inside the house. That’s usually not very expensive to do.

I don’t know the answer, but when the wind is really blowing, the toilet water level in my toilet visibly rises and falls, sometimes quite rapidly. Vents above my stove will also begin to rumble and howl with backflow. I imagine that, in your case, air bubbles could be escaping through the traps in your sinks, etc., into your house. Perhaps the reason you only see this in winter is because your house is more tightly sealed, so you have a greater pressure differential between your house and the outside. The result is movement of fluid in your pipes when the pressure changes.