Why your toilet keeps running

Why it matters more than it sounds

A toilet that never quite shuts off is easy to live with and easy to ignore. It shouldn't be. It is one of the few household faults that costs money continuously and silently, and because the water goes straight down the drain there is no puddle to remind you.

The good news is that it is nearly always one of three parts, all of them under twenty dollars, and all of them replaceable in under half an hour with no specialist tools.

The dye test tells you which part it is

Before buying anything, spend fifteen minutes narrowing it down. This is what a plumber would do first and it costs nothing.

1. 1Take the tank lid off and set it somewhere safe — they crack easily.
2. 2Put a few drops of food colouring, or a dye tablet, into the tank water.
3. 3Wait fifteen minutes and do not flush.
4. 4Look in the bowl. Colour in the bowl means water is escaping past the flapper.
5. 5No colour, but still hissing or refilling, points at the fill valve.
6. 6Water visibly running into the open vertical tube in the middle of the tank means the level is set too high.

If colour reached the bowl: the flapper

The flapper is the rubber disc at the bottom of the tank that lifts when you flush and drops back to seal. Rubber hardens with age and minerals build up on the seat it lands on, and eventually it stops sealing. This is the single most common cause of a running toilet.

The one thing to get right is matching the part. Flappers are not universal — the two-inch and three-inch types look almost identical until you try to fit the wrong one. Take the old one to the store with you.

1. 1Shut the water off at the valve behind the toilet and flush to empty the tank.
2. 2Unclip the old flapper from the two pegs and unhook the chain.
3. 3Wipe the seat it lands on with a cloth — grit there will defeat a brand-new flapper.
4. 4Clip the new one on and set the chain with about half an inch of slack.
5. 5Turn the water back on, let it fill, and run the dye test again to confirm.

If no colour reached the bowl: the fill valve

The fill valve is the tall assembly, usually on the left, that refills the tank after each flush. When its internal seal wears, it never fully shuts off, so you get a persistent hiss or a toilet that cycles every few minutes for no reason.

Unlike flappers, fill valves genuinely are close to universal, so replacement is straightforward.

1. 1Shut off the supply and flush, then sponge out the last inch of water so it doesn't end up on the floor.
2. 2Disconnect the supply line and undo the lock nut underneath the tank.
3. 3Lift the old valve out and fit the new one, setting its height so shut-off lands about an inch below the top of the overflow tube.
4. 4Reconnect, turn the water on slowly, and watch one full cycle.

If water is pouring into the overflow tube

Nothing has failed. The tank is simply filling past the point where it can hold, so the excess spills into the central overflow tube and the toilet runs forever trying to reach a level it can never keep.

Find the adjustment on your fill valve — a plastic screw on top on modern ones, a clip on a metal rod on older ball-float designs — and lower the shut-off until the water stops about an inch below the top of that tube. If it will not hold any setting, the valve itself is worn and should be replaced.

The one thing that should make you stop

All of this depends on the shut-off valve behind the toilet actually closing. Old angle stops seize, and a seized valve that you force can snap or start weeping in a way that will not stop.

If the valve won't turn, or starts dripping once you've moved it, stop there and call someone. The alternative is finding your house main in a hurry.