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Septic gas coming in via b/rm sinks

Posted by Heater963 (My Page) on
Mon, Aug 19, 13 at 21:10

Hello everybody,

I am new to this forum, but saw "gwilson2" talking about several complex plumbing issues. I have one that I believe is complex, because I simply cannot figure out what is happening. We purchased a "new" home that sat vacant for 5 years. Among other things, we are smelling a sewage gas smell from two separate bathroom sinks. ( The sole thing in common is the 3" main drain to the septic tank, which is flowing freely) I checked the venting on one of the two b/rms and found the toilet, lav sink and shower all have independent drains to a 3 inch main drain. The sink and shower have P traps installed and they are all vented, via a common 21/2" WDV. The "plumber" connected the fixtures, via a 2" horizontal pipe approx 16" above the sink drainpipe. He went vertical into the attic and placed a 90 elbow and ran a horizontal 2" pipe approx. 14 feet before he placed another 90 elbow and went vertical thru the roof.

My question is how can sewer gas be coming into the bathrm via the sink drain? I flushed the toilet and drained the tub and carefully removed the sink P trap. It was still full of water, so it is not being sucked out, as I initially believed.

I believe this problem on two separate DWV set-ups in the house is due to some "minor" plumbing error. I know the horizontal pipe in the attic should be sloped as sewer gas will condense in the colder months and I suspect the smell will be intolerable.

Any ideas that I can check into myself , without tearing walls open?

Thanks everybody.


Follow-Up Postings:

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RE: Septic gas coming in via b/rm sinks

There is no telling what went into the basins during the 5 years it was vacant. Clean the overflow and trap on tubs and basins.
I assume from the details you know about vent layouts that you also know that none are disconnected inside wall.
Are you certain the gas isn't coming from poorly sealed toilet floor flange?
Any chance the vanities once had dual basins and the spare line wasn't sealed during a remodel?


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RE: Septic gas coming in via b/rm sinks

A friend's boyfriend is a home inspector. Once a client had terrible smell coming in under the bedroom and bath in a two-year-old home. Turns out the plumbing from the toilet was never connected to the sewer, so all the waste was just dumping there under the floor... I know, ewwwwwwwwwwwwwww....


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RE: Septic gas coming in via b/rm sinks

Verify the angle of your p-traps. If the water level gets to low from to much slope, the gases might be able to get through if the water level in the trap is so low, that it's not a trap anymore. You may also need a plumber to take a camera and look for open pipes.

Can you smell it under the sink as well? If it's coming from behind the wall, it might be coming through the sinks water line openings.


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