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Old 10-01-2011, 02:18 PM   #1
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Flushing one toilet drains another


Got a lot of plumbing questions today. The second floor of my house has a main bathroom and master bathroom that share a common wall. The toilets for each are basically back to back on opposite sides of the shared wall. When I flush the toilet in the main bathroom, it drains the water from the bowl of the toilet in the master bathroom. But flushing the toilet in the master bath has no effect on the toilet in the main bath. Any ideas? Could it be a venting problem?

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Old 10-01-2011, 05:50 PM   #2
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Flushing one toilet drains another


It may be the way they are tied into the drain pipe. If they both don't connect at the same level (a symmetric wye), that could explain what you are seeing.

If the main bathroom tie-in is higher than the master bathroom one, when you flush the main toilet, the water rushes past the master toilet connection and might pull some air and water with it. But it wouldn't occur the other way around. Your venting is probably OK as you are able to drain the bowls. It is just a quirk of how the drain flows interact.

Are you actually draining the master toilet, or is the water level just temporarily dipping while the main toilet is flushing? Is the wall open such that you can see how the piping is connected? Are these toilets exactly opposite of each other on the wall?

If this is what is happening, I don't know what to recommend as a fix. Maybe you can tout the self-flushing features of your master bathroom toilet?


Last edited by Thunder Chicken; 10-01-2011 at 06:00 PM.
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Old 10-01-2011, 06:08 PM   #3
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Flushing one toilet drains another


The water level in the master toilet drops and stays down after flushing the other. It never goes totally dry though, just takes it down to the bottom of the bowl or the beginning of the trap/siphon but no further. They seem to be directly opposite one another give or take a few inches.

Didn't think of the symmetric drains as a cause but that may be it. I've got to get into the wall to fix the master shower so I'll check then. It doesn't really bother me all that much, just wanted to make sure it wasn't a sign of a hidden problem.
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Old 10-01-2011, 06:44 PM   #4
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Flushing one toilet drains another


Sounds to me like you have (2) pots on one vent and the vent is downstream of the master pot. When you flush the one upstream of the master the water once past the master pot pulls a vacuum, drawing the water out of the MP.

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Old 10-02-2011, 04:28 AM   #5
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Flushing one toilet drains another


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Originally Posted by Jackofall1 View Post
Sounds to me like you have (2) pots on one vent and the vent is downstream of the master pot. When you flush the one upstream of the master the water once past the master pot pulls a vacuum, drawing the water out of the MP.

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I agree.......And.... maybe the vent is too small as well?
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Old 10-02-2011, 07:09 AM   #6
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Flushing one toilet drains another


I agree with the above!
If you're going to open up the wall -
consider re-venting the toilet/s (hope the existing "DWV" pipe is PVC!)

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