Sewer gas comes up in the basement when upstairs shower used -- but only in the winter? Main floor bath and kitchen water do not cause any problems.

Check the trap seal in the basement floor drain. If the floor drain isn't used then pour a quart of cooking oil down the drain. That will prevent the seal from evaporating.

only in the winter? In warm weather sewer gas will raise so you don't smell it so bad. In colder weather it stays close to the ground where it can be smelled. Good luck, Tom

If the drain trap was the problem why would not all the water from the house cause the sewer smell? The very mysterious part of this problem is that it is relatively recent -- in the last five or six years -- and we have been in the house for thirty five. Also is the mystery of why only the second story water causes a problem and not main floor i.e.. Dishwasher and bathtub yet they use the same drain. I am wondering if the stack might be the problem as that is the only thing that is different -- upstairs vent/stack runs horizontally in the attic before it joins the downstairs one up to the roof. We live in the country and have a septic system as opposed to city sewage. We had the septic tank opened and drained this past summer thinking that might solve it but it did not.

Your nose is the best tool we have. Where, in the basement, does it smell the worst? Back to you. Tom