We are having a problem with septic odor. We bought a 4" Odorhog for the vent on the roof but that hasn't resolved the smell. It is more in the full bath upstairs and the half a bath in the basement. Our septic was previously cleaned so that is not the issue, we have called plumbers who don't seem to offer much help saying it's a "sewage problem" We have read about intalling a trap. Question is, how do we know if there is a trap already installed (which is unlikely cause it's an old house) and if not where would we install it?

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We have read about intalling a trap.

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Why? The smell's inside your house not out at the tank and that's what fixture traps are there to prevent. I don't think a running trap on your septic line will solve your problem.
You are sure all your traps,( including floor drains) have water in them aren't you?
A few questions, Has the odor always been there or did start with cold weather? It is worse in cold weather, correct? Did the plumbers check up in 5the attic for a open vent or a bad connection allowing sewer gas to escape? Did they even mention running a smoke test on your drainage to detect any leakyn or open vents? These are things that I'd be looking into. Answer my questions and let me know of further develops. Good luck, Tom