On 03/24/2017 3:02 PM, Bob wrote:
....

I think that when the waste water enters the tank, the gas gets pushed
back. After the fix it has dropped noticeably, but it appears it has now
been directed to the tank vent.
....

I'd assume since house is old the vent is in the waste line leaving the
house, not a vented tank (I guess some are done that way, never seen it
though and spent 50 yr in houses on septic systems).

I'd suspect besides the other note on dry traps possibly there's a
basement floor drain or unused washer connection or something down there
maybe?

Unless the exit vent is blocked or there's a leak in a vent itself,
other than coming back through a dry trap there really just isn't a path
for gas back into house...it would take quite a lot of pressure to force
it back through a full trap.

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