how do I find source of sewer gas smell in wall?

Hello, I have a 100 year old three family townhouse. The cast iron waste stack runs up through a wall between the closet on each floor and the airshaft. Also running in this wall are the vents for the bathrooms and kitchens of each floor.
During renovation of the the top floor kitchen, a section of this wall was opened up to gain access to plumbing and to run new electric up from the basement.
Now that a section of that wall is open I can occassionally smell sewer gas.
I do not smell it in our bathroom (of course there is a bathroom and kitchen on each floor where the tenants live and I have not journey into their apts). I smell it in only in the wall.
Any suggestions on how to determine where the smell is coming from? How can I figure if it's coming from a hole/crack in the waste stack, or the vents?
Thanks for your help!
-Rogue Petunia
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Maybe someone has a better idea, but chances are you are going to need to start opening the wall up until you find it. With luck you will find it at a joint and can re-seal the joint. At 100 years you are getting towards the end of the lifetime of that pipe. It may have another 100 left in it or it may need total replacement. I suggest if you do need to replace it that you replace it with the same cast iron that is there. Good for the rest of your life anyway and it will be quiet. Using a plastic replacement (or really bad a plastic patch, as other parts will just fail soon) will result in noise you will not like.
--
Joseph E. Meehan

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Rogue Petunia wrote:

No real easy answers to this one. I would not automatically assume that the source of the leak is the stack. Remember that all of the waste water from top down flows inside the stack. A hole/crack or joint problem will *usually* leave some tell-tale sign of water leakage. Look in the wall with a bright light to check.
More common is a toilet bowl not tightly sealed to the Closet bend. A failed wax ring can allow ater to pass thru without leaking but still allow sewer gas to escape.
I have used a gas-leak detector instrument (normally used for natural gas piping) to find sewer gas leaks. May not be practical where the wall isn't open completely. You can inject wintergreen oil into the stack to generate an easily traceable odor or use a commercial smoke generator to visually locate small leaks. These tests begin to get beyond what a homeowner can do.
Jim
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My gut instinct is that it is in one of the old lead vents that connect to the stack.

There is no gas smell near the toilet. At least not the toilet on this floor.

yes indeedy, blowing smoke is what some of the plumbers I've met have done best ;)
The wintergreen oil idea is interesting. When you say "inject" it, do you mean pour some down the stack from up on the roof? How much would you use? a pint? a gallon?
Thanks, Rogue
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Rogue Petunia wrote:

Tablespoon, down the stack (if you can get up there). Treat it as an experiment which may or may not yield anything. Jim
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OK, a tablespoon down the stack (easy to get to, we have a flat roof). Then what? Go around sniffing near the stack on each floor of the building down to the basement?
Thanks, -Rogue
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Rogue Petunia wrote:

Yep, use your nose. Maybe enlist the help of a dog... As I said, treat it as a fun-filled experiment. Jim
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This problem is 99% solved.
I went up onto the roof and looked down the waste stack with a flash light. Completely blocked at the elbow just below where it goes through the roof! Used a piece of metal conduit to poke at the blockage, loosening it up and sucking it up with the shop vac - I didn't want to let lots of it go down, some did of course, but a lot got sucked up. It was a very fine, dark black, granular substance. What was that? Get shivers just thinking what it might be. Bleck.
Once I cleared the blockage I could smell sewer gas rising up through the top of the waste stack. Hallelujah! Finally coming out where it is supposed to! And inside the house, so far, I would say the smell has decreased by 99%. In the past 48 hours I only smelled it once eminating from the opened section of wall in the kitchen, and it was slight compared to the gagging amount that was there before.
What did I learn? Before complicated and expensive smoke tests and ripping out walls I will always check for blockage up at the top of the stack. :)
-Rogue Petunia

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Rogue Petunia wrote:

Good job!
Might have been decayed leaves, although I've had small raccoons get stuck in there...
Jim
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