We bought a nine year old house four years ago and there has always been a terrible septic gas smell that originates from the small half-bath on the ground floor and is so terrible it literally fills our two-story house. The toilet is not loose and was replaced by a plumber two years ago (along with some of the vent pipes along that wall, but neither did anything to help the smell). The odor only appears in cold weather, and does not exist during the summer months at all. It is worse when we use any plumbing/water in the house during cold weather.

The smell also permeates our yard from the vent stacks on the roof - and we plan to try the odor hogs recommended on this site. I'm afraid however that that's using the odor hogs at this juncture is only going to make the smell inside that much worse.

If it's not the wax seal on the toilet causing the problem - what's the best suggestion on how to begin to find the leak in the vent stack itself that could be causing the smell? (It's hard to imagine on such a new house that there is a clog in the line which seems like it would be the other issue). As the smell is so profound in this one bathroom - should we just replace the vent pipes from the toilet/sink and those running up to the roof as well?
Has anyone tried a smoke test?

Much thanks for any help!

Smells get worse at night and in cold weather because the air's heavier and the odor drops closer to the ground level. The first place I would began to check is the attic for a open vent that hasn't been run out through the roof. Try the odor hogs and let me know. Good luck, Tom

We did check for open/uncapped vents in the attic and there are none (there is also no smell in the attic whatsoever). The smell permeates the house just from the 1/2 bath and then rises up - but it's extremely potent. I'm not sure if we should remove the toilet again and reseal or how to look for a vent leak. I just wasn't sure if we were going to start ripping things out where the best place was to start looking for the problem. There is a bathroom directly below the half bath but there is no smell in the basement or in that bathroom.

I do hope the odor hogs will work for the outside draft problem. Thanks. (I know there are a lot of people with this issue on this forum, and I appreciate the assistance - it's a frustrating problem to have)

I should have asked earlier, but do any of the fixtures in that smelly bathroon make a noise when you drain or flush? When the plumber worked on the vent is there a chance he left something open inside the wall? Are you on city sewer or a septic tank? Regards, Tom

We're on a private septic and just had it pumped this summer.

The toilet and sink don't make any strange noises, and don't bubble. I don't believe the plumber left anything unconnected pipes as the smell predated what he changed (we renovated this bath from a full bath off our front entrance into a powder room when we moved in). The smell had always been there and we had hoped when we changed the fixtures venting for the new 1/2 bath it would have corrected the sewer gas odor, but it didn't.

Smells and noises are the most difficult to track down if you're not there.
While we can't pinpoint the source we can point you to the most likely offenders. Ask the plumber if he's familiar with the smoke test for vents? You drop a smoke bomb down each roof vent and look for any smoke inside the house. You sound like you've tried everything else. Regards, Tom