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Old 10-27-2017, 02:41 PM   #1
naguy
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Default Sewer Gas from Washer drain

We recently bought a 4.8 cu ft washer to replace our old 3.7 cu ft washer. Our laundry is located on the second floor. About a month or so after the new washer was installed, we started noticing a sewer gas smell coming from the washer drain pipe. This doesn't happen right after a wash but rather happens after using the shower (following a wash). If I simply put some water down the washer drain pipe and fill the trap, the smell is gone. So it seems that the shower use (after the wash) is draining the trap connected to the washer drain.

We have 2 full baths on the second floor. Initially, the smell was triggered after using the first shower which is closer to the washer. My thinking was that this shower's drain was perhaps clogged and so I unclogged it with Drano, hair removal etc. This seemed to take care of the trigger from the first shower but then the smell started to trigger after using the second shower. So I proceeded to thoroughly unclog the second shower drain as well. But the problem hasn't gone away. Now it is consistently triggering only after the second shower.

It's interesting that once I manually fill the washer drain trap, the problem doesn't resurface until the next time we use the second shower - but only following a wash.

I have checked visually checked that the dry vent from the washer drain and the two showers, isn't cracked etc in the attic and it seems to be connected to the main roof vent.

I'm not sure what to try next. I'm looking for suggestions and would really appreciate any guidance/suggestions.


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Old 10-28-2017, 12:14 AM   #2
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Drano or liquid drain cleaners are NOT an effective way to clean a drain.
Also the vent on the laundry drain might be blocked. Using the shower may be siphoning out the trap seal. you need to mechanically clean the drain with a cable/snake. Go down the vent if possible.


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Old 10-30-2017, 01:51 PM   #3
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Thanks for the suggestions. I will try to clean the laundry drain with a mechanical snake - see if that fixes the problem.
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Old 11-09-2017, 04:55 PM   #4
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I am not a plumber, but we had the same problem with our island sink vent. We are in a rural area with a septic system and I studied the problem and concluded that the air compressor on the septic system was blowing sewer gas back through the drainage system. I replaced the NPT cap at the outside access with a large vent and the problem went away.
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Old 12-06-2017, 04:21 PM   #5
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I cleaned the laundry drain vent with a mechanical snake and it was clean.

So I called a plumber over and we discussed this issue. He believes that the new high capacity washer is drying the drain trap. So he thinks we have 2 options
1) Increase the size of thee drain trap.
The laundry drain pipe is 1 1/2 inch and the dry vent pipe that goes up to the attic is 2 inch. We cant see the size of the laundry trap (because its hidden behind drywall), and he suspects that its 1 1/2 inch. Putting in a 2 inch trap might maintain more water in the trap.
He said that there is also a chance that its already a 2 inch trap - not sure what we'll do then.
2) Install a "self priming valve" to keep the laundry trap dry at regular intervals
Essentially tap the shower water line (which is close by) and use that as a source to get some water down the laundry trap to keep it full every time the shower is used. He was going to ask around and find out if this is feasible and can be used.

I wanted to post this here to see if you guys can weigh in on the options or suggest something else.


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