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Refill valve

Short definition

A refill valve (or fill valve) is the toilet tank’s inlet valve — the modern replacement for the older brass ballcock. It senses tank water level and shuts off supply when the level reaches setpoint. Most are float-cup style (Fluidmaster 400A, Korky 528MP); a few are pressure-assist cartridges. Code-required to include integrated anti-siphon protection.

What it is

Three things happen at the refill valve during a flush cycle:

  1. Tank refill. Most of the incoming water flows up through the valve body and out into the tank, raising the water level.
  2. Bowl refill. A small portion is diverted through a flexible refill tube into the overflow tube, replenishing the bowl trap (which would otherwise dry out from siphoning).
  3. Shutoff. When the float reaches setpoint, the valve closes and water stops.

The shank passes through a hole at the bottom of the tank, sealed with a spud washer above and a spud nut below. Universal-fit shank dimensions (7/8-inch outside diameter) allow most modern fill valves to drop into any standard tank.

UPC 603 requires anti-siphon protection on toilet fill valves to prevent contaminated tank water from siphoning back into the supply if pressure drops. Modern fill valves include this; older ballcocks may not.

Why it matters to a homeowner

After the flapper, the fill valve is the most-replaced toilet part. Symptoms — slow filling, won’t shut off cleanly, noisy filling, leak at the tank-bottom shank — usually clear up faster and cheaper by replacing the valve than by trying to rebuild it. A $12 Fluidmaster 400A and 30 minutes is the standard DIY upgrade.

How to identify what you have: look at the inlet side of the tank. Vertical riser with a sliding cup = modern fill valve. Horizontal arm with a brass ball at the end = older ballcock, likely overdue for replacement.

Common variants

  • Refill valve (this entry, modern term) ≈ fill valve ≈ ballcock (legacy).
  • Float-cup fill valve. The dominant modern sub-type.
  • Pressure-assist refill cartridge. Different mechanism, used in pressure-assist toilets.

RELATED GLOSSARY TERMS

Float-cup fill valve Ballcock valve Ball float Spud nut Overflow tube Dual-flush toilet Flapper Float adjustment screw