You can fix this
Likely cause: the fill valve keeps adding cold water
A tank that keeps taking in fresh cold water will sweat more. Lower the water level, and if it keeps running, the fill valve is likely worn.
Why this happens
Korky says that if water keeps flowing after the tank is full, the fill valve float may be set too high, clogged with debris or failing to shut off, and that adjusting, cleaning or replacing the fill valve fixes a constantly running toilet. This Old House says a float set too high lets water keep draining into the toilet through the overflow tube. This Old House's Richard Trethewey adds that new water coming in all the time would make any tank condense.
How to fix it
- Before you start, find the toilet's own supply stop, on the wall or floor near the toilet. If water ever runs over the top of the overflow tube or onto the floor, turn the stop off by hand and never force it. If it will not turn, shut off the main water supply, also without forcing it, and call a plumber.
- Lift the tank lid and find the float. This Old House says floats can be adjusted with a screwdriver.
- Lower the float a little and flush.
- Korky and This Old House both say the water should stop below the top of the overflow tube, because a level above it keeps draining down the tube. Watch the tank refill and repeat until it stops below the tube.
- If the water still runs, the fill valve may need cleaning or replacing, which this page does not walk you through. Turn off the toilet's own supply stop by hand to stop the water, and never force it. If it will not turn, shut off the main water supply, also without forcing it. Then call a licensed plumber.
- Once the running stops, dry the tank and check it again over the next few days.
When to stop. If the water will not stop or the supply stop will not turn, do not force it. If water leaks from the shutoff valve itself, do not force or tighten it either. Shut off the main water supply by hand, without forcing it, and call a plumber. If water reaches an outlet, plug, cord, switch or light, including a light in the ceiling below, do not touch it and call an electrician, since Ready.gov says not to touch electrical equipment if it is wet or if you are standing in water.
If this does not fix it, tell the plumber
- The tank hisses, refills or runs even when no one has flushed.
- The dye test showed no color in the bowl.
- Whether lowering the float changed anything.
- The tank also sweats on the outside.
Build the full list: what to tell the plumber.
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