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Sewer Gas Source


Jerry Simon
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A bit of background. I inspected this house two years ago...brand new at the time. I opened the main sewer cleanout and saw a lot of standing water; the underground sewer line had a belly in it, and the builder had to replace. More on that later.

Today, Client calls and wants me to find the cause of sewer gases coming in the house. Comes in the first floor laundry room, and comes out of just one of the two master bath sinks, same in the level above the laundry room. He knows it's coming from the overflow in the master sink. Tapes up the overflow hole, runs water, and nothing...no tape, obvious stench out of the overflow hole when water is run. He thinks the smell is also coming out of the clothes washer purge line riser pipe. The bath and the laundry share the same plumbing vent stack. Stack blockage? If so, why does only one of two master bath sinks smell? Easiest route?

And, getting back to the bellied underground pipe, he opens it up today to check for perhaps another belly, and sewer gas comes spewing out under pressure. He said it was like opening a shaken bottle of soda pop. Ideas? Thanks so much.

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I'm not to sure what you mean when you say "Belly". Is that a flat spot in the lateral sewer?

If sewer gas puffed out of the clean out, toward the base of the stack, I would guess there is a venting problem. Either blocked stack vent or undersized, or maybe no venting at all. Sewer gas coming out of the overflow of a bathroom sink would lead me to believe the traps are getting siphoned out, allowing the gases to flow up. And if that is the case, it goes back to a venting issue.

Might check with the neighbors to see if they have an overpowering sewer gas problem, could be a municipal problem.

Frank

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A belly is a sagging area in the sewer; that's what we call them around here.

Vent inadequacy looms as culprit, but how to find the problem? I wouldn't rule out some foul form of excresence in the drain trap or overflow; after that, I'm looking for vent problems.

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Sounds like they need a smoke test to verify no leaks through the traps or anywhere inside the house (lack of smoke) and to prove the vent stacks are open (presence of smoke) above the roof.

If the house is vented properly, there can't be a build-up of positive (or negative) pressure in the line. If the vents are not working, the traps are almost useless.

Could be a squirrel, bird nest, most anything.

There is also the peppermint test, but the smoke test would give an easier test for the roof vents.

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Originally posted by homnspector

I'm sure you would have caught this the first time through, but I have run into a few new homes where they 'forgot' to remove the temporary rubber caps from the vents when they pressure tested the drain lines.

Sure sounds like it must be blocked vent(s).

I don't believe plumbers in greater Chicagoland pressure-test the waste lines. I've never seen it done, and as a super, I've seen hundreds & hundreds of houses built. I could be wrong about that, but sure would explain why at least half the new construction I inspect has at least one plumbing waste line leak...usually seen dripping from the ceiling drywall after running all fixtures for a bit.

I've also never seen a temporary vent cap, during or post construction. I agree, though...vent blockage. As Jim suggested, could even be a critter.

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