Fill Valve

A fill valve is a float-operated shutoff mechanism mounted inside a toilet tank, overhead cistern, or similar reservoir. Water enters through an inlet port; as the tank fills, a float arm or cup rises with the water level, gradually closing the valve until it shuts off completely at the desired height. The valve then stays closed until the tank drains (usually via a flush), the float drops, and the cycle repeats.

In the field, fill valves come in two main types: ballcock (traditional float arm with a ball) and modern float cup or diaphragm designs. Ballcocks are durable and easy to adjust—simply bend the float arm to raise or lower shutoff level—but they wear faster because the ball rubs inside a collar. Float cups and fill valves with diaphragms (like Fluidmaster types) last longer, rarely jam, and handle mineral-heavy water better. All fill valves must open fully during refill to avoid slow tank recovery.

Failure shows up as a constantly running toilet, hissing refill, or tank that won't fill. Most field fixes are simple: adjust the float, clean debris from the valve seat, or replace the diaphragm washer. Full valve replacement is fast and cheap—usually worth doing instead of chasing leaks on worn internals. Code typically requires an antisiphon fill valve in jurisdictions with backflow concerns.