Short definition
A float valve is a tank-fill valve closed by a rising float as the tank fills to a set level. Every residential toilet has one. Older designs use a brass arm and a hollow ball (“ballcock”); modern designs use a vertical float cup that rides on the central inlet stem. Functionally identical — they shut off the inflow when the water reaches the right height.
What it is
The classic ballcock is a brass valve with a horizontal arm that connects to a hollow plastic or copper ball. As water rises, the ball rises, and the arm pivots a lever that closes the inlet. Older toilets and some pressure-tank fill systems still use this design.
The modern cup-style fill valve replaces the brass arm with a vertical plastic cup that slides up and down on the central inlet stem. As the cup floats up, an internal mechanism shuts the inlet. Korky and Fluidmaster are the dominant brands. The cup design is more compact, less prone to leaking, and easier to adjust than the old arm-and-ball.
A separate but similar device is the float switch — an electrical switch operated by a float, used to turn pumps on and off (sump pumps, well pumps, sewage ejectors). Float valve = mechanical fluid shutoff. Float switch = electrical contact.
Why it matters to a homeowner
The float valve is the device responsible for a working toilet. When it fails, the symptoms are immediately recognizable:
- Running toilet. The valve doesn’t fully shut off; water flows continuously into the tank, over the overflow tube, and into the bowl. Often costs $20+ per month in extra water if not fixed.
- Slow tank refill. Diaphragm or cartridge clogged with debris. Cleaning sometimes works; replacement is more reliable.
- Tank fills past the overflow. Float arm bent or out of adjustment — water rises too high before the valve closes.
- Tank doesn’t fill. Float valve cartridge has failed entirely.
Replacement is among the easiest plumbing repairs: shut off the angle stop, flush the tank, disconnect the supply hose and the lock nut under the tank, lift the old valve out, drop the new one in, and reverse. A $15–$30 part, 30 minutes, no special tools beyond an adjustable wrench. Most homeowners can do it.
For older brass ballcocks, the standard upgrade is to replace with a modern cup-style fill valve at the next service — they’re cheaper, more reliable, and easier to adjust.
Common variants and what a float valve isn’t
- Ballcock (old style) vs. cup-style fill valve (modern). Ballcock has the classic brass arm and hollow ball; cup style has a vertical sliding cup. Cup style is the upgrade.
- Float valve (mechanical fluid shutoff) vs. float switch (electrical contact). Different devices for different jobs — float valve closes water flow, float switch energizes a pump.
Common failure modes
- Worn diaphragm or cartridge — slow fill or running toilet.
- Brass-arm ballcock float waterlogs — won’t rise to shut off.
- Float arm bent — water level too high or too low.
- Mineral or debris in the inlet — sticks the valve open or closed.