What Is a Fill Valve and Why Replace It?
The fill valve (sometimes called a ballcock) is the component inside your toilet tank responsible for refilling the tank after each flush. When it wears out, you'll notice symptoms like the toilet running constantly, slow tank refill, a hissing noise, or water spraying inside the tank.
Replacing a fill valve is one of the most rewarding DIY plumbing repairs — it takes about 30 minutes, costs under $20, and immediately solves most running-water issues.
What You'll Need
- Replacement fill valve (universal fit models work for most toilets)
- Adjustable wrench or slip-joint pliers
- Sponge or towels
- Small bucket
- Rag or old towel for cleanup
Step 1: Turn Off the Water Supply
Locate the shut-off valve on the wall behind or below the toilet. Turn it clockwise until it stops. Then flush the toilet to drain most of the water from the tank.
Step 2: Empty the Tank
Use a sponge or small cup to remove remaining water from the bottom of the tank. Place a towel on the floor around the base to catch any drips.
Step 3: Disconnect the Water Supply Line
Underneath the tank, you'll see a supply line connecting the shut-off valve to the fill valve. Unscrew the nut connecting it to the tank by hand or with pliers. Have a small bucket ready — a little water will drain out.
Step 4: Remove the Old Fill Valve
Inside the tank, the fill valve is held in place by a locknut on the outside bottom of the tank. Unscrew this nut counterclockwise (lefty loosey). Once it's off, lift the old fill valve straight up and out of the tank.
Step 5: Set the Height on the New Fill Valve
Most modern fill valves are adjustable. Before installing, set the height so the critical level (CL) mark on the valve will sit at least one inch above the top of the overflow tube. This is a code requirement and prevents siphoning.
Tip: Hold the new valve next to the old one to match the height as a starting point.
Step 6: Install the New Fill Valve
- Insert the new fill valve shank through the hole in the bottom of the tank.
- Hand-tighten the locknut from underneath — do not overtighten with a wrench, as this can crack the tank porcelain. Just snug it up with a quarter turn past hand-tight.
- Reconnect the water supply line to the fill valve shank.
Step 7: Attach the Refill Tube
The new fill valve will come with a refill tube. Clip it to the top of the overflow tube (do not insert it inside the overflow tube — this can cause the toilet to run). This tube directs a small amount of water into the bowl to refill it after each flush.
Step 8: Turn on the Water and Adjust
- Slowly open the shut-off valve.
- Watch the tank fill. The water should stop about one inch below the top of the overflow tube.
- If it fills too high or too low, adjust the float by turning the adjustment screw or sliding the float clip on the valve shaft.
- Flush a few times to confirm everything works correctly and check under the tank for leaks.
You're Done!
A properly installed fill valve should be silent when not filling, refill the tank fully within 60–90 seconds, and hold the correct water level indefinitely. If problems persist after replacement, the issue may lie with the flapper or overflow tube instead.