If your fill valve keeps running, it’s usually failing to shut off because the float or the valve seat inside has worn out. The fix is almost always a full fill valve replacement, not an adjustment. Here’s how to confirm that’s the problem, and what to tell a licensed plumber when you call.
How a Fill Valve Works
The fill valve sits on the left side of your toilet tank and controls the water that refills after each flush. When the tank empties, the valve opens. As water rises, a float (either a ball on an arm or a cup that slides up the valve body) rises with it. Once the water hits the set level, the float signals the valve to close.
When it works, this takes 30 to 60 seconds. When it doesn’t, the valve never fully closes and runs continuously. That’s a fill valve problem. If your toilet cycles on and off on its own, that’s usually a flapper leak instead — the flapper isn’t sealing and water slowly drains from the tank, triggering the fill valve to top it off. Both waste water, but they’re separate issues.
The Most Likely Causes, in Order
Worn valve seat or diaphragm. Inside the fill valve is a rubber diaphragm or set of rubber seals that press against a seat to stop the flow. These harden and crack over time. Once the seal fails, water trickles past even when the float is fully raised. Most common on valves five or more years old.
Float set too high. If the float is adjusted so the water level sits above the overflow tube, water continuously drains into the bowl and the valve runs to keep up. You’ll hear water trickling into the bowl.
Debris in the valve. Sediment or pipe scale can lodge against the diaphragm and hold it open. This sometimes appears suddenly after supply line work or a brief water service interruption.
Cracked or corroded valve body. Older brass or plastic bodies can fail at the connection points. Mineral staining on the outside of the valve is a sign something is leaking.
Two Checks You Can Do Right Now
Confirm it’s the fill valve, not the flapper. Press down on the flapper while the tank is running. If the sound stops, the flapper is leaking. If nothing changes, the fill valve isn’t shutting off.
Check the water level. It should sit about an inch below the top of the overflow tube. If water is visibly above the tube, it’s draining into the bowl constantly. Tell a licensed plumber both what you heard and what you saw in the tank — that narrows the diagnosis before they arrive.
What a Licensed Plumber Will Do
If the float position isn’t the culprit, the valve needs to come out. A plumber will shut off the angle stop under the toilet, drain the tank, swap in a new fill valve (the part is inexpensive), set it to the correct height and float level for your tank, and check the supply connection for slow leaks before finishing.
The reason to call a pro rather than tackle it yourself: the angle stop under the toilet is often old and seized. Forcing it to close can cause it to fail or leak at the wall stub-out, turning a simple valve swap into a supply-line repair. A plumber knows what to look for before turning anything.
Also have a plumber take a look if you see corrosion on the supply stub-out, cracks in the tank porcelain near the fill valve hole, or any water damage around the toilet base. Those point to more than just a valve.
Call a Licensed Plumber
This is an information-only guide. This site is not run by a licensed plumber and does not offer plumbing services.
If you’re in California, verify any contractor’s license at cslb.ca.gov before authorizing work. Fill valve replacement is a quick job for a licensed plumber, and getting someone in early avoids the angle-stop surprise that can turn a small repair into a bigger one. Get a quote before anyone starts work.