Every so often, the valve lets the water run.

To check the flapper, I shut the water off overnight. The water in the tank didn't go down so it may not be the flapper.

What else could it be?

  • Can you check to see if the chain that goes from the flush handle to the flapper has any excess links that can get sucked under the flapper during a flush? This would prevent a seal and cause the water to stay running. Mar 9, 2016 at 20:10
  • On the flip side...Does the water level rise if you leave the supply on, but don't use the toilet? If the valve is letting water in occasionally, but it's not going down the drain. I think it's unlikely, but at least you'd know you're not losing water and can focus on the fill valve.
    – Mysterfxit
    Mar 9, 2016 at 20:33

3 Answers 3


Small rocks and scale can cause the fill valve to leak allowing more water in. Some can be fixed by turning the water off, then removing the top of the fill valve and slightly turning the water on Put a cup over the top so when it turns on you don't soak your bathroom. Push down on the float let the water run for a few seconds. turn water off and reassemble the valve. wait and see if that did the trick. Usually there will be rust and sediment in the bottom of the tank if it blows out a lot of debris. I like to clean this out with a wet vac so it wont leave rust stains in the bowl. The valves are inexpensive and not hard for a DIY person to change at worst case.

  • Is this more likely in an 'old' toilet, or equally possible in a new (~2-3 years old) one? (I have the same problem as OP, with a very new toilet.)
    – Joe
    Mar 11, 2016 at 20:59

Your fill valve may be set too high. Depending on the type of float it has it is allowing to much water in the reservoir and the water is being diverted through the over-flow tube. Sometimes it is difficult to see the flow of water from the tank to the bowl and if it is a very slow leak it might not be perceptible at all. A fast and accurate check for leaks is to shut the supply valve off and add some food coloring to the reservoir. If the bowl is colored your flapper or its' seat need attention.


If it's not the flapper, another common problem is siphoning. Check to ensure the bowl filler tube (the hose that comes off the top of the fill valve and sprays down the overflow tube) is not placed in the overflow tube where the end of it is below tank water level.

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