A while back we had a plumber install a new tub surround in our upstairs bathroom. After he was finished he said there was a vent in the wall that wasn't connected so he connected it. We never had the smell before but it seems ever since he did that there's been sewer gas leaking into the downstairs bathroom. The funny part is that it's only so often. The drain pipe and toilet seal are all fine. Also it might just be coincidence but when we use the washer downstairs it sometimes seems to cause the smell in the bathroom. Any ideas? How could connecting a vent cause that? I thought if anything it would prevent sewer gas.