Off Topic--Problem With Drain Vent On Lavatory Sink

npalen · Sep 1, 2015

Our home was built by a Vocational-Technical school class back in '74. I suspect that they forgot to vent the drain in one of the bathrooms as I get the sewer smell when draining the sink. The trap is in place so I'm guessing the only way it can vent is back up thru the sink drain. Anyone have suggestions as to how this might be remedied without tearing into the wall? (You may wonder why I haven't fixed this after 40 years

Thank you in advance.

boggen · Sep 1, 2015

make a longer P trap. further it goes down and then comes back up same distance. the more pressure gasses / air will need to come back through the sink. ((most likely will not fully work if "siphon / suction" gets strong enough to pull water out of the P trap)) have seen some pretty ugly P traps at local hardware stores. that i could hardly call a P trap.

most likely not to code. run 1/2" or so hose and tee into drain just after P trap. so the Tee is pointing up. run the 1/2" or so hose up through ceiling and tie it into the drain vent stack in attic.

you might try enlarging pipe at or just after P trap. to point it connects to a vent. so waste water drains in lower portion of pipe and air can flow in upper half of the pipe.

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with above without proper venting. anything above more likely crud shot at best.

did someone leave a poo a tad to big and plug up vent for sink? *blames kids trying to flush stuff down toilet*

ozley · Sep 1, 2015

npalen said:
Our home was built by a Vocational-Technical school class back in '74. I suspect that they forgot to vent the drain in one of the bathrooms as I get the sewer smell when draining the sink. The trap is in place so I'm guessing the only way it can vent is back up thru the sink drain. Anyone have suggestions as to how this might be remedied without tearing into the wall? (You may wonder why I haven't fixed this after 40 years Thank you in advance.

If you can see the venting in the attic you may fine the pipe has a belly in it which can fill with water from condensation or rain blocking the vent.

npalen · Sep 1, 2015

Thanks for the feedback. guys. I'll check the venting in the attic also. Any and all suggestions appreciated.

soilsailor · Sep 1, 2015

If the house has a vent, which it sounds like it does, then I believe you may be able to add an air admittance valve. Note: I am not a plumber nor did I sleep at a Holiday Inn last night. Just watched lots of videos on You Tube.

farmer2009 · Sep 1, 2015

You can add a valve under the sink. I don't know the name of it but it was designed for use in islands with a sink. Don't want the vent pipe running to ceiling. But you tee it in on sewer side of trap and run it as high under the cabinet as possible.

MossRoad · Sep 1, 2015

Funny I was just dealing with this today. You should check out an AAV, Air Admittance Valve.

I'm buying one tomorrow.

Shop Keeney Mfg. Co. 1-1/2-in Plastic Mini Air Admittance Vent at Lowes.com

kenmac · Sep 1, 2015

If it's been draining water and you have had no problems from 74 until now.. I doubt it's a vent problem..

Soap scum & hair will gather in the drain lines over the yrs and tend to smell just like sewage..
Take your trap apart and inspect the drain pipe

npalen · Sep 1, 2015

OP here. I hate to admit it but I've been putting up with it for longer than I can remember. I did clean the trap recently so I don't think that is the problem.
Thanks for the link to the air admittance valve. May have to give that a try.