Running Toilet Fill Valve: How to Fix or Replace It

The fill valve is the component in the toilet tank that lets water in after a flush and shuts off when the tank is full. When the fill valve fails — doesn’t shut off, stays partially open, or can’t maintain pressure — the toilet runs continuously or intermittently. This is a common and fixable problem. Here’s how to diagnose a fill valve issue and fix it.

How the Fill Valve Works

The fill valve opens to let water in and closes when the tank reaches the set level.

Modern fill valves use a float mechanism — when the tank water rises to the set level, the float lifts a component that shuts off water flow. When the toilet is flushed and the water level drops, the float falls and opens the valve again.

Two main fill valve types:
– Float ball (older style): A large ball float on an arm; the arm connected to the fill valve shuts off water when the ball rises to the set height
– Cup float (modern): A cylindrical float on the fill valve body; rises with the water level and shuts off at the set height

Signs the Fill Valve Is the Problem

Running water in tank after flushing:
After the toilet flushes, the fill valve runs and runs — well past the time needed to fill the tank — and water is flowing over the overflow tube.

Running sound that doesn’t stop:
The toilet continuously makes the sound of water running, even when the tank is full. Lift the tank lid: if the water level is above the overflow tube and water is trickling into the bowl, the fill valve isn’t shutting off.

Intermittent fill cycles without flushing:
The toilet randomly refills (sounds like a flush without anyone in the bathroom). This is usually the flapper, but if the food coloring test rules out the flapper, an intermittent fill valve that doesn’t fully close is possible.

Hissing sound from tank:
A hissing fill valve is passing water even when the tank is full. This is a sign the valve is worn and not closing properly.

Fill Valve Adjustment (Before Replacement)

If water is overflowing into the overflow tube, adjust the float level first.

For modern cup-type fill valves:
– Most have an adjustment screw or clip on the valve shaft
– Turning the screw or moving the clip down lowers the water level at which the valve shuts off
– The tank water level should be approximately 1 inch below the top of the overflow tube

For old float ball type:
– Bend the float arm down slightly, or adjust the adjustment screw at the valve end of the arm
– Lower float arm position = lower water level cutoff

After adjustment: Flush and watch whether the tank fills to the new (lower) level and shuts off without overflowing.

Fill Valve Replacement (DIY)

If adjustment doesn’t work or the valve is more than 7–10 years old, replace it.

Parts: Universal fill valve — Fluidmaster 400A, Korky 528, or equivalent. $15–$25 at any hardware store. These fit most standard toilets.

Steps:
1. Shut off the water supply valve (clockwise, under the tank)
2. Flush the toilet to empty the tank; bail or sponge remaining water
3. Disconnect the supply hose from the bottom of the fill valve (underneath the tank)
4. Unscrew the lock nut holding the fill valve to the tank (counterclockwise, from below the tank)
5. Lift the old fill valve out of the tank
6. Set the new fill valve height per the instructions (usually: set so the critical level mark is 1 inch above the top of the overflow tube)
7. Insert the new valve, tighten the lock nut from below (hand-tight plus 1/4 turn — don’t overtighten)
8. Connect the refill tube from the new valve to the top of the overflow tube
9. Reconnect the supply hose
10. Turn on the supply valve, let the tank fill, and verify the valve shuts off at the correct level

Total time: 20–30 minutes for most homeowners.

Common Fill Valve Installation Mistakes

Overtightening the lock nut: Cracks the toilet tank. Hand-tight plus a quarter turn is sufficient.

Refill tube too long or positioned wrong: The refill tube should direct water into the top of the overflow tube, not down the side. If it’s submerged in the tank water, it creates a siphon that can cause running.

Float set too high: If you set the float (water cutoff level) too high — above the overflow tube — water will run over the overflow tube continuously. Set the water level 1 inch below the overflow tube top.

How Long Does a Toilet Fill Valve Last?

5–10 years for most standard fill valves.

Signs it’s time to replace:
– Hissing when tank is full
– Intermittent running that isn’t the flapper
– Fill time has gradually slowed (diaphragm worn)
– Valve makes clicking or chattering sounds

At $15–$25 for a quality replacement valve, proactive replacement of an aging fill valve is reasonable maintenance.

FAQ

Q: How do I fix a toilet fill valve that won’t shut off?
A: First adjust the float height to lower the water cutoff level. If the valve doesn’t shut off even with the float adjusted down fully, replace the fill valve. Universal fill valves ($15–$25) replace most standard fill valves in 20–30 minutes.

Q: How do I replace a toilet fill valve myself?
A: Shut off supply valve, flush and empty tank, disconnect supply hose, unscrew the old valve lock nut from below the tank, lift out old valve, install new valve at correct height, reconnect supply hose, turn on water, adjust float to 1 inch below overflow tube top.

Q: Why does my toilet fill valve keep running after a flush?
A: Either the fill valve float is set too high (water overflows into overflow tube), the valve diaphragm is worn and won’t fully close, or the refill tube is creating a siphon. Adjust the float level first; replace the valve if adjustment doesn’t resolve it.

Q: How long does a toilet fill valve last?
A: 5–10 years for most standard valves. Signs of wear: hissing when tank is full, gradual slowing of fill time, intermittent running. At $15–$25 for a replacement, proactive replacement is reasonable maintenance on an aging valve.

Q: What is the best toilet fill valve to buy?
A: Fluidmaster 400A and Korky 528 are the most widely recommended for standard toilets. Both are available at hardware stores, work reliably, and include installation instructions. Avoid generic no-name valves from unknown sources.