A friends toilet bowl which in is a 2 year old home, drains without flushing. No water from the tank is added. Things flush well (toilet paper was used to test). The vent seems to be okay, as it's not visibly clogged. I've plunger'ed it and nothing seems to change. I'm stymied.

How low does the water level go in the bowl? A crack in the bowl or something caught in the trap siphoning the water out. How long does it take to lower? Does it matter if water is used elsewhere in the house?

No where near enough information to even make an educated guess.

After you flush and the tank fills and stops running, does it drain immediately? You say water isn't added, but I would suspect that it is slowly adding and when it reaches a certain level it causes the draining.

Have you tried turning off the water supply as soon as it fills to see if it still empties itself?

Assuming it's not leaking, of the two times this has happened to me, they were partial clogs.

When you flush does the bowl "burp" some air before flushing begins? Slow draining past a partial clog seems to create a siphon effect that continues to drain the bowl past its normal water level. A partially clogged vent may cause the toilet trap to be siphoned only when large volumes of water are discharged upstream, like from a washing machine. May be the washer's vent that's clogged.

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Not a plumber by trade but a fierce DIYer



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