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Question DetailsAsked on 6/2/2014

Why does the Water level in toilet BOWLs drop between flushes?

2 new Koehler toilets installed 3 weeks ago.....water level started dropping between flushes 3 days ago. Ran hose down roof vent..clear...ran snake 6ft thru both toilets and they are clear. When one toilet flushes...the other will also drop water level. Toilets are back to back. No leaks under house or in far! Any ideas??

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2 Answers


This is my question.....I wanted to add that all the other drains (tubs, bathroom and kitchen sinks) seem to be working just fine.

Answered 6 years ago by Guest_9070915


As I read it, the level drops only when other toilet is flushed - right ? If dropping without flushing in other bathroom, then has to be wicking over the trap due to a piece of string or yarn or hair or something (which the snaking should have solved), or there is a leak in the toilet. Unfortunately, I have seen a cople of blogs and heard of this from a plumber recently, about a possible manufacturing defect where there is a connection between the bowl and the outlet in the casting - bypassing the gooseneck, so draining the bowl almost dry through a casting void. Don't know how true or prevalent this is.

My primary guess, assuming draining down ONLY when other toilet is flushed (which should be easy to check by not using first one and then the other toilet for a day or two), is something got stuck at the vent so the toilet, when flushed, is creating a partial vacuum in the sewer pipe, resulting in it pulling the fluid levell down in the other toilet (probably both, actually) the same as will happen on real windy days.

Why it took 3 weeks to occur - my guess is maybe there was a small airgap at one of the wax seals that finally closed up as the toilet was used and the wax compressed more and more, eventually allowing a vacuum to be established when flushing. With new toilet, it may be there was always an airgap at the old toilets, or maybe the new ones flush more rapidly so the pipe is more full of water during flushing, so previously air came up in the sewer drop from the vent to relieve the vacuum but cannot do so now.

I would confirm they only draw down when other one is flushed by leaving unused for a day - that would answer leak versus vent issue. Then if turns out to be leak, water should show up pretty soon - sounds like in crawlspace or unfinished basement in your case. And if it never does, then internal leak bypassing the gooseneck but not leaking outside the toilet. If vent issue, solution would be to add a vent pipe connection to the sewer pipe close to the toilets - normally with a street or long sweep wye off the top of the pipe to reduce blockage possibilities.

This is by no means an uncommon problem, with the way plumbers cheat on venting these days. In the old days, you generally ran the sewer pipe past the last drain in the bathrooms, commonly to a cleanout wye, the other arm of which went up to the vent pipe - so there was air source to all the bathroom drains on the "uphill" side, like this -

These days, many plumbers run all the drains into one sewer pipe (generally against code), then drop that down to the next level, and connect the vent pipe there - meaning all of a bathroom's (and sometimes the entire upstairs or sometimes 2 stories worth of sources flow down to the bottom level before encountering a vent connection. Totally misses the point of keeping air to the drains so they do not get sucked dry.

Some even use the drain pipe as the "vent" - like this one would if there was another floor below it connected to the Main Vent (Stack) - which would be TOTALLY wrong but is commonly done -

Note that in most of these images the tub is not vented either - which can lead to drawdown of its trap causing sewer gas entrance - this is commonly seen.

Answered 6 years ago by LCD

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