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Question DetailsAsked on 3/19/2015

What can cause an upstairs toilet to randomly lose water in the bowl?

30 year old split level home with bathrooms stacked on 2 different floors. Upstairs toilet will lose water in the bowl, not constantly, just when downstairs toilet is flushed or washing machine downstairs going or downstairs shower being used. Using downstairs bathroom or kitchen sinks or upstairs bathroom sink doesn't seem to drop water level in bowl. Downstairs toilet keeps normal level of bowl water, but seems to flush loudly and have quite a few tiny bubbles come up from the drain only when flushing. Started 2 days ago when upstairs toilet seemed partially clogged, plunged it, but still losing water. Both issues started same time. No noticeable water on any floors/walls/ceilings, none of the sink or shower drains are slow, and no odors. Both toilets are 2 to 3 yr old AquaSource low flows. Upstairs shower drain had a leak and isn't being used until we can afford a bathroom reno.

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1 Answer


Your key is saying it only happens when using large quantities of water downstairs - and your downstairs toilet is building up air bubbles in the syphon (the upper gooseneck curve inside the toilet casting) - that indicates that either:

1) your vent pipe is not working correctly, so when downstairs water is used in volume it is drawing water out of the upstairs traps because the vent pipe is blocked, causing a partial vacuum in the pipe above the blockage point. Unfortuntely, these days most plumbers seem to be either very lazy or never learned how vents are supposed to be piped, so they leave out the "upflow" portions from each fixture - or yours might have gotten plugged during the plunging, or

2) partial blockage in pipe downstairs is letting downstairs small flows (sink) to go by OK, but larger volumes are backing up in the sewer pipe, causing the air in the pipe to back up to your upstairs toilet, which is not correctly "upflow" vented, so the air comes up into the traps and displaces the water in them, so the water level drops as it is displaced - this is more likely the cause.

You might be able to clear it yourself - but be careful and takes 2 people to avoid making a mess - using a hose to flush out the sewer vent pipe(s) from the roof (and be sure to get right pipe, not a furnace vent or such) and if that does not work, use hose with rag wrapped around it to make a tight fit in the bowl to flush out upstairs toilet, while someone watches downstairs to be sure is not backing up in a fixture down there (or in floor drains, if you have any). Depending on where blockage is, could back up at any fixture - lowest elevation on if pipe to street is where blockage is, otherwise at lowest elevation drain upflow of the blockage.

Otherwise, plumber to snake vent from upstairs toilet down past the downstairs points where water use causes backup. Since downstairs ones are not showing this effect, I would guess the blockage is probably between the upstairs fixtures and shortly below the downstairs, because if it was in the main sewer pipe to the street I would expect the downstairs fixtures to show the same effect or backup when the upstairs toilet was flushed or full tub emptied.

Answered 5 years ago by LCD

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