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In my duplex, when Unit B flushing water down the drain, the bathroom sink and bathtub in Unit A would have sewage overflow up.

I researched and I think about using Air Admittance Valve under the sink:

https://www.homedepot.com/p/STUDOR-1-1-2-in-or-2-in-PVC-Mini-Vent-Adapter-20341/202275238

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BUT my question is would this solve the problem of overflow in the bathtub? Or just the sink itself? If not, how to fix overflow in tub without tearing into wall?

Thanks.

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    Are these units side by side or on top of each other? This sounds like some blockage in the drain pipes. – JACK Oct 29 '19 at 12:57
  • I would agree with blockage blocked vent usually only cause slow draining not backing up especially as high as a sink, the tub is usually the first place to back up but tossing parts at it may be a bad idea, I have seen AAV’s on sinks fail because they were two low as yours is so it will get a bit of gunk each backup until it starts sticking then water drips in the cabinet. – Ed Beal Oct 29 '19 at 13:40
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How much sewage overflow occurs? If it's only a small amount, like a cup or so, then a blocked vent could be the cause.

As water runs down a drain pipe the air ahead of the water must be pushed out. If there's inadequate venting then the pressure build up in the drain can cause water to be expelled from the trap below the sink and tub. It could cause bubbling or splashing in the toilet, too.

To answer the title question "Can Air Admittance Valve under bathroom sink fix bathtub drain overflow?" -- no. An AAV allows air into the drain plumbing but does not let air out. An AAV could resolve a problem where the trap at a fixture is getting the water sucked out of it. As water runs down a drain pipe it creates suction in its wake. If there's inadequate vent or AAV then the suction literally vacuums the water out of traps.

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