Hello:
Following hookup to the new sewer line (previously had septic system), my downstairs bathroom has a noxious sewer smell. I believe there is a faint smell upstairs too. The sewer line has an electric pump with an alarm. I called the installers and they said they did everything correctly and there is no way smell could be coming into the house beyond the pump. They sent a plumber in and he could find nothing wrong with the piping (it is all new 3 years ago) and he replaced the toilet seal (also new 3 years ago) and checked the traps. I contacted the plumber who did the renovation and he said all plumbing was tied in to the old roof vent.
There was never a problem with smell until sewer was hooked up. I know more about sewers now than I care to, but am at a loss as to what to do next. Someone suggested sealing off pipes near outside just to see if the smell goes away. I am concerned not only with the deterioration this is causing my new renovation, but about our health as well.
Has anyone had this problem? Please give me suggestions.

We had a problem like this on a project in Saudi. Essentially what you are smelling is called "Sewer Gas" which is heavier than air, and will seek out the lowest place. With all the vents tied together, the gas in the vent from upstairs, is sinkng down the vent and coming out downstairs from a missing or dry "P" trap... most likely the downstairs sink or shower. In Saudi, it was coming out floor drains that had dry traps.

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I contacted the plumber who did the renovation and he said all plumbing was tied in to the old roof vent.

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This I would want to check out myself. If any vents were overlooked this could be the answer to your problem. In the meantime can you sniff around the fixtures and tell me where the smell's the strongest? Good luck, Tom

Thank you Tom and RSain for your responses. The plumber who came in said the traps were not dry. I believe the smell is coming from both the sink and the shower drain.

Why would this happen when changing from a septic hookup to a sewer hookup?

I cannot figure out how to respond to you by email address!