Why does my toilet randomly start running?

Sometimes my toilet starts running for a few seconds at a time, like it’s just finishing filling up the tank.

Usually this happens when the bathtub is being drained, or when I turn on the facets in the sink, but I think it also happens sometimes when no water is being used anywhere else.

As far as I can tell the flapper in the tank is working fine, and I don’t see any leaks anywhere.

3 Responses to “Why does my toilet randomly start running?”

  1. pdd says:

    The typical toilet tank will have a flapper which seals the water in the tank. When you flush the toilet, the flapper is lifted and water is allowed to move from the tank into the bowl. Once a set amount of water has exited the tank, the flapper falls back down and re-seals the tank.

    The most common issue with toilets randomly flushing is that the flapper has become brittle or sediment has formed on the flapper/tank which prevents the flapper making a complete seal. This will allow water to slowly drip from the tank into the bowl. If enough water is permitted to leak out of the tank, the filling mechanism will be triggered and the tank will refill. If enough water enters the toilet bowl, the self-siphonage will be triggered and the bowl will drain into the sewage system.

    Here’s a picture of a typical assembly:
    enter image description here

  2. Rahul Ghosh says:

    It can happen for one of the following reasons

    1. Main tank cleaning of the apartment buliding
    2. The toilet water controller or stopper is damaged
    3. Jammed flush systems due to high water iron

    Generally the thing stops automatically unless there is an internal fault. Try draining all water and refilling the flush to see if there is any leak or anything.

  3. Steven says:

    I had the same problem with an upstairs toilet. A plumber came over and fixed a Pressure Reducing Valve (PRV) valve in the basement that adjusted the water pressure into the house and havent heard a thing since.

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