How to Replace a Toilet Fill Valve

01Spot fill valve failure first

The fill valve is the tall tower on the left side of the tank that refills it after each flush. You know it is failing when the toilet hisses or trickles long after the flush should have stopped, fills painfully slowly, runs continuously, or pulses on and off (a "ghost" cycle) on its own. The common thread is that the valve no longer shuts off cleanly, either because internal seals are worn or because grit from the supply line is holding it open a hair.

Before you replace it, rule out the simpler culprits. If the toilet is overfilling, the float may just be set too high, which is a no-cost adjustment. If the bowl is draining and a dye test bleeds color, the flapper is the leak, not the fill valve. The fill valve is the answer when the water level is correct, the flapper passes the dye test, and the valve still hisses, runs slow, or will not stop. If the tank will not fill at all, work through the broader causes on our toilet will not fill page first, since a closed shutoff or a kinked supply line mimics a dead valve.

• ·Hissing or trickling long after the flush: worn valve seals
• ·Slow fill: debris in the valve or a partly closed shutoff
• ·Continuous run with correct water level: valve will not shut off
• ·Ghost cycling on its own: usually a flapper leak, confirm with a dye test before blaming the valve

02Shut off the water and drain the tank

Turn the shutoff valve on the wall behind the toilet clockwise until it stops. Flush and hold the lever to drain as much water from the tank as you can. Sponge or towel out the remaining inch in the bottom so you can disconnect the supply line without a flood. Keep a small bucket and rag under the connections; even a drained tank holds a little water in the valve body.

Disconnect the flexible supply line from the bottom of the fill valve, then unscrew the plastic lock nut that holds the valve to the underside of the tank. This is the nut you will fight if the toilet is old, so if it is corroded, a pair of channel-lock pliers and steady pressure beat brute force. Once the nut is off, lift the old valve straight up and out of the tank.

03Set the new valve and connect it

Modern universal fill valves (Fluidmaster, Korky and similar) are height-adjustable: you twist or unlock the body and extend it so the top sits about an inch above the overflow tube, then lock it. Set the height before you drop it in. Insert the threaded shank through the hole in the bottom of the tank, hand-tighten the new lock nut underneath, then snug it about a half turn more with pliers. Do not overtighten, since the tank is porcelain and the nut is plastic; cracking either is a real risk with a wrench and a heavy hand.

Reconnect the supply line to the bottom of the valve, hand-tight plus a slight snug. Clip the small refill tube from the valve onto the overflow tube (do not push it down inside the tube, or it can siphon). Turn the water back on slowly and watch every connection for drips before walking away. Most fill valve kits come with their own flapper, so if you are doing a fuller rebuild, our how to fix a running toilet guide covers tackling both at once with combo-kit logic.

04Adjust the water level and fix the hiss

With the water back on, let the tank fill and check the level. It should rest about an inch below the top of the overflow tube. Too high and water spills into the overflow tube and the toilet runs forever; too low and the flush is weak. Adjust by turning the screw on top of the valve or sliding the float clip up or down, depending on the model, until the level is right.

If a brand-new valve hisses or fills slowly, the usual cause is debris that got dislodged during the swap and is now stuck in the valve seat. Shut off the water, pop the valve cap off (most twist off), hold a cup over it, and briefly turn the water on to flush grit through, then reassemble. A persistent slow fill on a correctly set valve can also be a partly closed shutoff or a low-flow restriction in the supply line. If the valve still misbehaves after that, and you have ruled out the flapper, the issue may be upstream: our guide to low water pressure covers the supply-side and pressure causes. A running, hissing tank that you cannot quiet at all is covered end to end on our toilet keeps running page.