Help! We have built 30 houses total in the same subdivision and 2 houses have turned up with a bad sewer gas odor. They are not near each other and were built at different times of the year. The odor is only noticeable during warm weather when the windows are open. We have cleaned out chimneys,ductwork (looking for critters), and have done a smoke test. We don’t know the cause or what to do. Plumber has never had this problem before.
Sounds like you have a trap seal problem.You may have a fixture that is not properly vent,which will causes syphonage of the trap.Or your having a wicking problem.This is caused by hair or threading from clothing or even mop strings getting caught in the weir of the trap and hanging down the outlet side of trap causing a wicking action sucking the trap dry. or a vent not 10′ from an openable window or not three feet above a window these are U.P.C. codes.these are just a few things it could be. later insidecutter.