I own an old cottage with partially updated plumbing. Off and on I have a sewage odor problem coming from the toilet area (one bathroom). All the bathroom plumbing has been redone except the toilet. The toilet is a relatively new Kholer fixture dropped into an old cast iron drain. New subflooring has been added and this has caused the toilet flange to not sit tightly against the "spout" of the drain that comes up to the floor. Initially I had noticed a sewage odor in the bedroom (next to the bathroom) coming up through the walls which were open at that time due to renovation. Upon pulling the toilet we noticed that there was actually a space between the toilet drain pipe and the sewage drain pipe it was dropped into. So the pipes were different sizes. The plumber's fix was to install a connector that tightened as you screw it and filled that gap. Solved the problem... or so I thought.

All winter there is no sewage odor problem. But last night it returned. It is just around the toilet and in the bedroom nextdoor. Like I said, its an old house (very tight crawl space back there). All the vents in kitchen and laundry have Studor vents on the and that has stopped any odor coming from those. I don't notice odor from tub or bathroom sink.

My neighbor says that the vent coming out of the roof is too short (it is also a 2" vent instead of 3") She thinks that if I add a longer pipe it will "draft" better... like a chimney.

Any ideas?

"Upon pulling the toilet we noticed that there was actually a space between the toilet drain pipe and the sewage drain pipe it was dropped into. So the pipes were different sizes. The plumber's fix was to install a connector that tightened as you screw it and filled that gap."
Is what you're telling me that the horn of the toilet bowl didn't go down into the closet bend so the plumber attached a rubber coupling with clamps on it so it would go down into the bend? Of course the horn's a different size then the closet bend. They're designed to fit inside each other. If the bowl set t6hat high off the flange what about the wax seal? That couldn't have a good seal. My advice is to pull the toilet and bring the flange up to floor level and set the toilet the correct way.
"My neighbor says that the vent coming out of the roof is too short (it is also a 2" vent instead of 3") She thinks that if I add a longer pipe it will "draft" better ...like a chimney."

A vent is not anything like a chimney. A chimney exhausts smoke while a vents function is to pull air into the drainage system to relieve the vacume set up by draining liquid. Good luck, Tom

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I wanted to thank you for your answer and possibly dig a bit deeper. The
Problem is a sewage odor coming from the toilet area when the air conditioner is
On... so its worse when the room is cooler than the crawlspace. Its taken me
Awhile to figure out when it is worse.

I've had the toilet pulled a couple of times. There is about 1/2 inch of space
Between the toilet pipe and the "closet bend" pipe that it seats into. It does
Go down into the drain (closet bend) pipe. The first plumber fixed it by using
An extra wax seal and pushing wax into the opening area all the way around. That
Fixed it for awhile. But as he told me... not permanently. The second plumber put
In the screw flange thing. It occurs to me that the toilet just needs to the
Pulled again and the thing screwed tighter since its probably expanded and
Contracted a couple of times since installed.

Like you suggest, the whole thing is probably caused by the toilet not sitting
Down tight against the seal because of the extra 3/4 inch subfloor above the
Level of the seal. Its just not right. So. Next question. Is there a
Relatively inexpensive way to bring the drain flange UP to the level of the
Floor? Some sort of connector? Or is this a major plumbing project. I've
Learned that if I understand the problem and the solution... I can either fix it
Myself or at least not get taken to the cleaners by the plumber that comes to
Fix it. I can't afford to replace the whole cast iron drain. I'm not strong
Enough to pull the toilet myself.

Thanks so much for your trouble.

" Next question. Is there a relatively inexpensive way to bring the drain flange UP to the level of the floor?"
Yes! They're called flange extenders,(see image) and it will bring the flange up to floor level. I'm surprised your plumber didn't do this instead of giving you a shoddy installation. Once the flange's brought to floor nlevel and the toilet reset the coirrect way I have a hunch yoiur [problem will disappear.
Good luck, Tom

Thanks Tom! I can't believe the plumber didn't offer this solution either. Makes me think twice about calling that company! I guess I need to find a new plumber! I have printed out the picture you were so helpful to provide and will let you know how it goes. That deeper flange extender looks like it will surely do the trick.