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Question DetailsAsked on 4/18/2016

Toilet Water Loss

I have back to back toilets on second floor. Every time I flush one toilet, the water level in the other one
dropped half to one inch. Over the course, if I am not using one of them and keep using the other one, the water on the unused one will drop to the barely minimum covering the hole. It has been like this for a few months. Vent problem?

Before when one toilet was flushed, the water moved in another one, but not dropping too much. That's why I like to hear some suggestions here. Thanks.

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1 Answer


Sounds like it - here is a link to a similar question with response -

Since it is probably well above freezing in your area at this time, frosting up of the vent pipe outlet seems unlikely. You could check the vent pipe (usually exposed plastic pipe, commonly but not always black ABS, generally wide open to the air but occasionally fitted with a candy-cane U-bend at the top to reduce drafting) for insect or bird nest blockage.

Otherwise, some solids probably backflushed from the toilet into a same-level drain line that the toilet goes into and plugged the vent up. Commonly running water from a hose into the vent pipe (of course, make sure it is the vent pipe, not a flue from a high-efficiency boiler/furnace/water heater) will flush it out. You need someone watching the different drains at all levels to be sure it is not backing up and coming out into a tub or shower or sink or such.

I would guess one of the below is what your plumbing and vent look like - most commonly occurs where there is no sink "upflow" from the toilet to flush out any solids that backed up "upflow" from the toilet. Worst when they run the drain line near horizontal "backwards" from the toilet before going vertical as a lot of solids can fill in the pipe. Used to HAVE to have basin/sink "upflow" or the toilet if plumbed that way, but they took it out of the code a couple of decades ago to be more flexible - ignoring that it causes problems like yours. The second picture shows the worst example of this - not only nothing "upflow" of the toilet (which would sit on top of the nearer largest diameter vertical pipe), but also went to smaller diameter pipe on the horizontal run rather than only after going vertical with the vent (the closest small diameter pipe) - and put the reduction real close to the toilet which is even worse. This is a classic example of not thinking of the consequences of the plumbing - about the only worse thing (which I have seen) would be putting the dsanitary wye in facing the wrong way, or using a Tee instead of a wye so the solids go both ways from the toilet riser. (the one labeled back drain toilet)

Otherwise, having it snaked from the roof vent should work.

One other thing - if it has been windy, this can pull the bowl water level down as the wind blows across the top fo the vent pipe and causes a partial vacuum - commonly you see movement in the bowl watrer at about 20 mph or so, drops in the 30-40 mph range, near total draining in the 40-60 mph plus range - very roughly. Or course, easy to see if that is the issue just by (when not windy) flushing one toilet a couple of times and then checking the other one.

Answered 2 years ago by LCD

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