How a toilet tank works
When you flush, a rubber flapper at the bottom of the tank lifts and lets water rush into the bowl. When the tank empties, the flapper drops and seals, and the fill valve refills the tank until a float tells it to stop.
If the flapper does not seal or the float never tells the valve to stop, water keeps moving and the toilet keeps running.
Likely causes
- A flapper that has hardened, warped or collected mineral buildup
- A lift chain that is too short and holds the flapper open, or too long and catches under it
- A float set so high that water spills into the overflow tube
- A fill valve that is worn and no longer shuts off fully
A quick test
Put a few drops of food coloring in the tank and wait fifteen minutes without flushing. If color appears in the bowl, the flapper is leaking. If the water level in the tank is above the top of the overflow tube, the fill valve or float needs adjusting.
Why it is worth fixing
A running toilet can waste a large amount of water every day, and it is one of the most common reasons for a water bill that jumps without explanation. Most causes are inexpensive parts.