A fill valve is the valve inside a toilet tank, usually on the left side, that refills the tank and bowl after each flush and shuts off when its float reaches the set water level. Older brass ballcocks use a float arm, while modern float-cup valves slide on a vertical shaft. A worn fill valve causes running or hissing toilets.
Why it matters to you
A toilet that runs or refills on its own can waste a surprising amount of water every day without any visible leak, and the first sign is often a higher water bill.
Practical guidance
Add a few drops of food coloring to the tank and wait 15 minutes without flushing: color in the bowl means the flapper leaks, while water spilling into the overflow tube means the fill valve or its float needs adjustment or replacement. Close the toilet's supply stop before working inside the tank.