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Question DetailsAsked on 12/17/2014

Why does water in toilet draw down below trap

I have a condo on the top floor of a 4 story building. When I am gone for 4 weeks or more the condo smells from sewer gases. I found the water level below the trap in my toilet. The tank is full and there are no leaks anywhere that I can see. Can something in the buildings plumbing be causing a vacuum and drawing the water out of my trap and bowl?

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Sounds to me like you have correctly diagnosed the problem - you can find a number of prior responses to questions about lower water level in the toilet trap in the Home > Plumbing link in Browse Projects, at lower left.

Unfortunately, most plumbers incorrectly vent bathrooms - they cheat and put one vent stack up through the building, tying each subsidiary drain line into it - but NOT providing venting at the most upstream end of each drain line. Something like the first image here, so any vacuum getting to the toilet line pulls from the bowl, without any vent pipe beyond (upstream of) the toilet to pull air from, so it can only pull air (and trap water) from the bowl -

Therefore, anything major draining downflow of your toilet can create a partial vacuum in the line until the tail end of the outflow reaches the vent line - which may be several feet or flow. Hence, pulling a little bit of water out of your trap each time a neighbor flushes, for instance.

Another cause which unfortunately vent stacking does not eliminate entirely, even if done correctly - if you have strong, and especially if gusty, winds the wind passing across the top of the vent stack can put a partial vacuum on the entire building stack system, drawing down the level in traps and making them gurgle - and usually toilets are the first to do this.

Your solution - if you have someone house watching while you are gone, have them run 10 seconds or so of water in each sink, and flush the toilet - being sure to wait till it is done refilling to guard against any intearnal workings failure during a time you are gone for a long time.

One thing which might help matters if no one can check house for you - though a nuisance - tightly stretch shrink wrap (wide enough pallet-wrapping size to fit without joint) across the bowl, sealing all around - will not stop the drawdown though might inhibit it a bit, but should reduce gas outflow. Remember when you come home that the bowl will be full of noxious and potentially explosive gas so turn fan on before removing the wrap, and of course don't smoke.

Obvious long-term solution is to add a direct connection from your toilet sewer pipe near the toilet to the building vent stack, or more commonly from the pipe leading from the sink which the toilet joins with to go to the building stack. Like these schematics/photos (near end of article) -

Calls for cutting into walls in almost all cases (and commonly floor or underlying ceiling) so typically probably $500 range once vent and drywall repair and painting are done, plus likely interference with neighbor's places.

Another long-term solution - though it can introduce its own minor additional risk of leakage if it fails AND there is a sewer pipe backup so should be mounted as high as possible - have a plumber install a large capacity "island vent" nearby on the same drain/sewer line as the toilet - say under your sink, to let air into the line. Look like this, designed for use under kitchen islands where connection to a higher-elevation vent stack is not feasible -

That is not a great article on it and a funny site to find it at, but clearly shows schematic and photo of where they typically look like. Or following - second image in article -

Cost about $150-250 probbly installed for normal configuration plumbing.

Depending on responsibilities in your condo, this might be a building combined maintenance fund item, not your individual responsibility - check condo association contract and bylaws.

Answered 5 years ago by LCD

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