Water sucked out of tub trap when sink is emptied

[MasterPlumbersKeymaster]

I have a 90 yr old house with semi-original drain plumbing in the bath. The drain lines are all original, just the traps and feeds to the drain lines are new. The tub is about 2 ft from the stack and the lav is about 8 ft from the stack. I think the tub drain joins the sink drain about 1 ft prior to meeting the stack. The problem I have is that when the sink is filled w/ water and then drained, the water is sucked out of the sewer trap and then you can smell sewer gas come up out of the tub drain. I assume this is a venting problem, but i’ve run a snake down the stack from the roof and it seems clear. One other piece of info — I’ve also noticed that sometimes when the sink is drained, it backs up into the tub a little — this seems inconsistent w/ the problem I have, but figure I should mention it. Any ideas on what the issue could be? Are there easy ways to fix it (that don’t involve putting a new vent through the roof)? Thanks.

[Retired plbg1Participant]

.Sounds like the sink drain is partly stopped, and that long sink run causes a vacum and syphones out the tub trap. You need some kind of relief vent in that sink line. Is that all thata on the stack is the 2 fix. If so cut a tee in the sink line and connect pipe to the stack. You might try one of those studo vents, its a Mechanical vent you put in the pipe line. But they give you trouble later on so make sure you can get to it for future repair.Let me know.

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Art retired plbg

[Retired plbg1Participant]

The way you describe the pipe work is the way they did it in 1900, they had an S-Trap on the lav. and a drum are salem bath trap on tub, try having it cabled out to stack if that dont work you need a vent. Let me know. If you need drawing let me know.

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Art retired plbg

[hardycmParticipant]

Thanks for the replies, Art. So you think snaking out the drain could help? I sure hope so, as I’d hate to have to chip up tile to put a vent on the lav. Is there another way to provide a vent without doing that? I.e. what about something similar to what is done on a kitchen sink in an island?

[Retired plbg1Participant]

Thats called a loop vent, but it has to be installed right are it wont work. Did you check out those stuor vent, its a mechanical vent you buy at box store, some places they are not legal. If you have a S-Trap you can buy a vented S-trap to replace and run a vent up from there through roof.

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Art retired plbg

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