Short definition

Continuous tank filling is when a toilet’s fill valve never shuts off — water runs into the tank constantly, often with a faint hiss. The cause is almost always a stuck float, a worn fill-valve diaphragm, or a tank water level set too high. A replacement fill valve costs about $15 and takes 30 minutes to install.

What it is

Inside a toilet tank, the fill valve is a float-controlled water inlet. When you flush, the tank empties, the float drops, the valve opens, and water refills the tank until the float rises and shuts the valve off. Continuous filling means that shutoff never happens — the valve stays open and the tank keeps cycling water, either over the overflow tube or through a flapper that won’t fully seat.

Lift the tank lid and look:

  • Water level above the overflow tube. Water siphons over the tube continuously and the fill valve never catches up. Adjust the float lower.
  • Water level normal but valve still running. The diaphragm or seal inside the fill valve is worn out. Replace the fill valve.
  • Float arm bent or hung up on tank wall. Reposition or replace.
  • Float won’t shut valve when lifted by hand. Diaphragm is the culprit, not the float.

In hard-water areas, mineral buildup at the fill-valve seat speeds this up — fill valves last 5–7 years in Spokane, 10–12 years in soft-water Seattle.

Why it matters to a homeowner

A constantly running toilet wastes far more water than a dripping faucet. The classic symptom is a water bill that creeps up for no reason; the second clue is a faint continuous hiss when the bathroom is otherwise quiet.

The fix is one of the cheapest in plumbing. A universal Fluidmaster or Korky fill valve costs about $15 at any hardware store and installs in 30 minutes with one wrench. If your toilet has continuous filling AND a flapper leak (both at once is common), buy the combined tank rebuild kit for $25 and replace everything while you’re in there.

When a plumber sells a “running toilet repair,” they’re doing the same job — most of the bill is labor.

Common failure modes

  • Hardened fill-valve diaphragm — 5–10 year life; faster with hard water.
  • Float stuck high — caught on tank wall, supply line, or flush lever.
  • Tank water level above overflow tube — water siphons over, valve compensates.
  • Mineral buildup at fill-valve inlet — Spokane and Eastside hard-water issue.
  • Cracked overflow tube — rare, but worth checking on old tanks.

Common variants

  • Continuous filling vs. phantom flush. Continuous filling is constant; a phantom flush refills the tank for 30 seconds every few minutes — that’s a flapper leak (silent toilet leak).
  • Continuous filling vs. silent toilet leak. Continuous filling is fill-valve side; silent leak is flapper side. A toilet can have both at once.
  • Hissing toilet vs. continuous filling. Hissing without active refill is usually a fill valve about to fail; continuous filling is fully failed.