Toilet keeps running: find the leak and stop it

Water trickling into the bowl long after the flush
A hiss or refill noise from the tank when nobody has used the toilet
The tank refilling on its own every so often ("ghost flushing")
Water running down the overflow pipe inside the tank

Why it happens

Fluidmaster calls the flapper the most common problem, and the US EPA's Fix a Leak Week page warns that old or worn-out flappers, the valve seal at the bottom of the tank, can make a toilet flush on its own or leak silently, and the EPA says this can waste thousands of gallons a year.

Fluidmaster, which makes fill valves and flappers, lists the reasons a seal stops sealing on its flapper leak page:

• The flapper rubber has warped, which the maker says is often caused by in-tank chlorine tablets.
• Slime or debris sits on the underside of the flapper or on the flush valve seat it closes against.
• The chain is too short or too long. Fluidmaster recommends 1/4 to 1/2 inch of slack so the chain neither holds the flapper open nor gets trapped under it.
• The flush valve seat is damaged: nicks, corrosion or an uneven rim stop the flapper from closing flat.
• The refill tube siphons water when it is pushed too far into the overflow pipe. Fluidmaster says the tube should stay above the water line.

The second family of faults is in the fill valve. Fluidmaster's running toilet page explains that if the water keeps rising and drains into the overflow pipe, debris may be lodged inside the fill valve, so it never closes.

When the tank refills on its own at intervals, Fluidmaster's ghost flushing page says water is being lost somewhere: inside the tank (refill tube, flapper, flush valve) or outside it (the fill valve locknut under the tank, the supply line, or the seal between tank and bowl).

How to fix it

1. Lift the lid and watch the water level. Fluidmaster says the first thing to establish is whether the water is rising so high that it drains into the overflow pipe. If it is, the problem is the fill valve: go to step 6. If it is not, the water is escaping into the bowl past the seal: continue with step 2.
2. Run the dye test. The EPA's method is to put a few drops of food colouring in the tank. If colour shows up in the bowl after 10 minutes, you have a leak. Fluidmaster notes that dye is less reliable for slow leaks.
3. Run the water-level test. Turn the water off at the shut-off valve without flushing, mark the water level in the tank with a pencil and wait about 15 minutes. If the level drops, Fluidmaster says the flapper is the likely culprit. The level falls to the height of the leak and stops there, which also helps locate it.
4. Clean the seal. With the water off and the tank flushed empty, Fluidmaster advises wiping slime and debris from the underside of the flapper and from the flush valve seat with a clean cloth. Run a finger round the seat rim to feel for nicks or unevenness.
5. Adjust or replace. Set the chain slack to 1/4 to 1/2 inch and cut any excess chain that could catch under the flapper. If the flapper is more than five years old, Fluidmaster says to simply change it; the EPA calls replacing a flapper a quick and easy fix. Match the replacement to the size of your flush valve. If you feel nicks or unevenness on the flush valve seat, or a new flapper does not stop the leak, Fluidmaster says the flush valve itself needs replacing.
6. If water is going down the overflow pipe, clear the fill valve. Fluidmaster's method: turn off the water and flush. Lift the float cup, grip the grey shaft, press down and twist the cap and plastic arm 1/8 turn anticlockwise, then lift the cap off. Hold a cup upside down over the open valve and turn the water supply on full for 10 to 15 seconds to flush out debris, then put the cap back on. If cleaning does not stop the overflow, replace the fill valve.
7. Check the refill tube. If it is pushed down inside the overflow pipe, pull it out and clip it to the outside of the pipe so it stays above the water line, as Fluidmaster describes.
8. Avoid in-tank chlorine tablets. Fluidmaster warns against them because they can warp the flapper and damage tank components.

If the refill keeps happening after all this and you see water under the tank, Fluidmaster points to the fill valve locknut, the supply line and the tank-to-bowl seal. Those are outside-the-tank repairs; if you are not comfortable taking the tank apart, call a plumber.

Check that it worked

Repeat the tests once the repair is done. Turn the water back on, let the tank fill and confirm the level stops below the top of the overflow pipe. Then repeat the dye test or the pencil-mark test: no colour in the bowl after 10 minutes and no drop in the water level means the seal is holding. Listen for refill noises over the next hours, when nobody is using the toilet.

UK cisterns and push-button flushes

Many of the steps above come from a US maker and assume a flapper on a chain. United Utilities, a UK water company, explains on its leaky loos page that modern push-button cisterns tend to overflow into the bowl rather than through a pipe in the wall, which is why the leak is easy to miss. It estimates that a single leaky toilet wastes between 200 and 400 litres of clean water a day.

Its checks suit any cistern:

1. Look for trickling or rippling water at the back of the pan between flushes, and listen for hissing from the cistern.
2. Wait 30 minutes after the last flush, dry the back of the pan and lay a sheet of dry toilet paper there. Water running down will wet it.
3. Or use a LeakyLoo strip: dry the back of the pan 30 minutes after a flush, stick the strip on horizontally and check it after the toilet has gone unused overnight.

If you find a leak, United Utilities advises contacting your usual plumber or finding one through WaterSafe. This page does not give step-by-step repairs for push-button drop valves, because the sources above do not describe them.

A running toilet is rarely a mystery once you know whether the water leaves through the seal or over the overflow. Test first, fix the one part the test points to, and test again. The EPA describes a new flapper as a fix that saves water and money, and when the fault is outside the tank or out of reach, a plumber is the right call.