How to Fix a Running Toilet: 5 Common Causes.

Toilets seem complex, but the mechanism inside the tank is actually quite simple. It relies on gravity, water pressure, and a few rubber or plastic components. Below are the five most common causes of a running toilet and how to address them.

1. A Worn-Out Toilet Flapper

The flapper is the rubber seal at the bottom of the tank that lifts when you flush. Over time, chlorine and minerals in water degrade the rubber, causing it to warp or rot, letting water slowly leak into the bowl.

Solution: Turn off the water supply valve below the toilet. Flush to drain the tank. Snap the old rubber flapper off the overflow tube hooks, unhook the chain, and snap a new, matching flapper in its place. Reconnect the chain.

2. Flapper Chain Length Issues

If the metal chain connecting the flush handle to the flapper is too short, it will hold the flapper slightly open. If it is too long, it can get caught underneath the flapper as it closes, breaking the seal.

Solution: Adjust the link clip on the chain so there is only about 1/4 to 1/2 inch of slack when the flapper is fully closed.

3. The Float is Set Too High

Water fills the tank until the float shuts off the fill valve. If the float is adjusted too high, the water level will rise past the top of the overflow tube, constantly spilling down into the toilet bowl.

Solution: Locate the adjustment screw on the float arm or cylinder. Turn it counterclockwise with a screwdriver to lower the float level. The water level should rest about 1 inch below the top of the open overflow tube.

4. The Refill Tube is Too Long

The thin rubber tube running from the fill valve to the overflow pipe can sometimes be pushed too far down inside the pipe. If the end of this tube rests below the tank water level, it will siphon water out of the tank continuously.

Solution: Pull the refill tube out of the overflow pipe. Trim it if necessary, and use a plastic clip to attach it securely to the top rim of the overflow pipe so it sits above the water line.

5. A Faulty Fill Valve

If you have adjusted the float and the flapper is sealing, but water continues to run or hiss inside the tank, the internal seals of the fill valve have likely failed.

Solution: You will need to replace the fill valve assembly. Turn off the water, drain the tank, sponge out the remaining water, unscrew the locknut underneath the tank, slide out the old valve, and install a new universal fill valve (such as a Fluidmaster 400A).