Find out why it runs
Lift the lid off the tank and lay it on a towel. Look at the water level. A stream into the overflow says the fill valve either sits too high or never closes. If the level is below the tube but the tank keeps refilling, the flapper is leaking.
Fill valve adjustment
- Find the adjustment: a screw on top of the valve, a clip on a rod, or a float arm.
- Lower the float so water stops a couple of finger-widths under the rim of the overflow.
- Flush and watch. If it still overfills, the valve may be worn and need replacement.
Flapper problems
- Check the chain. A taut chain props the flapper up; a slack one tucks under it.
- Run a finger around the flapper seat. Grit or mineral scale can stop it sealing.
- Look at the flapper itself. Warped or hardened rubber should be replaced.
Field note. Take the old flapper to the store. Toilets use different sizes and styles, and some manufacturers use their own seals.
Replacing a flapper
- Shut the stop at the wall, then flush so the tank drains.
- Slip the worn flapper from the overflow tube pegs and the chain.
- Clean the seat with a scrubbing pad.
- Attach the new flapper and adjust the chain with a little slack.
- Open the valve and test several flushes.
Replacing a fill valve
Fill valves are universal for most toilets. With the water off and the tank empty, unscrew the supply line and locknut under the tank, pull the tired valve and drop in its replacement at the correct height.
When to call
If water weeps at the tank bolts or into the bowl after new parts, or the toilet is old and cracked, see toilet repair and toilet installation.