Sewer smell in Winter

Hello,

I bought my house around 6 years ago. Every year, starting in the fall, I get a sewer gas smell in one of my bedrooms. This seems to happen anytime that a load of laundry is ran or someone takes a bath or shower.

I have had a few plumbers come out and they have checked all of the traps and vents in the house. None of them have been able to find a cause for this smell.

Last year I replaced all of the old drain lines with new PVC lines and added a trap to the main line. This seemed to fix the problem for a few days but then it returned.

The room where I am getting the smell most has no plumbing in the walls and is two rooms away from any bathrooms.

Any advice would be greatly appreciated.

Can you notice that the smell is stronger in one room or location than another? This might help locate it. Do you have central air conditioning? During the winter, that generally doesn't run. If the condensate line has a trap, (it should) it can dry out in the winter, and that can leave a direct path to the sewer to allow gases to be sucked into the air ducts. If this is the closest room to the furnace, it could be from that. If the laundry drain line does not have a proper vent to it, the high velocity of the pumped water can suck a drain trap dry. Once a trap is sucked dry, there's nothing to stop gasses from coming through. A cold air return could suck that into the air ducts and distribute it around the house.