How a Toilet Tank Works (Fill Valve and Flapper)

• The flapper sits on the flush valve seat; when it lifts, tank water drops into the bowl.
• The fill valve refills the tank and stops when its float reaches the set water level.
• Excess water drains into the overflow tube, so a water level set too high runs continuously.
• A worn or warped flapper lets water seep into the bowl, which makes the fill valve cycle on its own ("phantom flush").

Most running-toilet complaints come from three parts: the flapper (seal wear or a chain that is too short), the fill valve (float set too high or a worn seal), and the flush valve seat (mineral buildup). The U.S. EPA WaterSense program suggests a simple dye test: put a few drops of food coloring in the tank, wait about 10 minutes without flushing, and check whether color appears in the bowl. Color in the bowl means a leak past the flapper.

EPA WaterSense estimates that household leaks can waste around 180 gallons per week in an average home, and toilets are one of the most common sources.