For over a year, we've had occasional sewer smell come thru hvac vents. It does not smell anywhere in the house. There are absolutely no dry traps. It started after we redid our master bath but there was no moving of plumbing pipes. We had a smoke bomb test in the house and determined there were no leaks because no smoke was seen or smelled throughout the house and when the cap was removed from the stack, smoke came out meaning it didn't go anywhere.
We have turned off the hvac and the smell is still evident in the room with the open pipe in the basement that is in the same room as the hvac unit. Hvac checked and cleaned. The ac drain has a trap that is not dry. As soon as the smell occurs, it's very strong but disappears quickly. We've replaced the hot water tank. We've installed a vent on the open pipe (not dry). We've turned off the hot water tank for a few days. We've had 4 plumbing companies out and cannot produce the smell for them so they think we're crazy. Im going crazy trying to figure this out. Any ideas appreciated!
What would cause an occasional sewer smell for over a year?

If the smoke test revealed no leaks then the odor is likely coming from outside. Is there any appliances that have a fresh air intake ?

That's a tough one because it could be from various places. Since it started with the bath remodel I would pull the toilet and check the wax ring. If not compressing that would be a possibility but I know you did the smoke test so wouldn't say for certain. It's a cheap easy place to eliminate a possibility. Is the HVAC return ducted? If so make sure all the ductwork has mastic (and zip ties if flex). Use tape to seal around the air handler cabinet access panel to eliminate that from pulling in air in case it's coming from the basement.

Thanks for your response. I don't think any appliances pull air from outside. I will ask though.



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Thanks for your response. I can pull the toilet to check. Hvac is return ducted. I haven't done any ductwork cleaning or taping since the smell is sewer but I guess I can try to eliminate basement return and see if that eliminates the smell. Then that would narrow down the source to the basement, correct? If not then all options are still open.