Toilet Keeps Running: Fix It in Order

Listen first: hiss, trickle or refill every few minutes

The sound narrows the fault before you lift the lid. A steady hiss from the cistern means water is still coming in: the fill valve has not shut off. A quiet trickle into the pan with no hiss usually means water is going out past the flush valve seal, though a fill valve creeping slowly into the overflow can be almost silent too. A short burst of refilling every so often, with silence in between, is known as a ghost flush. The cistern slowly loses water past a leaking seal until the float drops far enough to open the fill valve, which tops it up again.

One more place to look: outside. Older toilets, including many low level cisterns fixed to the wall, have an overflow pipe that runs through the wall. If there is a pipe dripping or running on the outside wall at bathroom height, the cistern is overfilling. That is the fill valve, whatever the sound inside suggests.

Seven fixes, cheapest first

Take the cistern lid off and lay it somewhere flat and soft, because it is heavy china and cracks easily. Then work down this list, flushing and waiting for the cistern to refill after each step.

• Press the flush button firmly and let it spring back. A sticky button, or a cable-operated button whose cable has kinked, can hold the flush valve slightly open. Pull the button out of the lid and check it moves freely.
• Watch the float as the cistern refills. If it catches on the cistern wall, the flush valve or its own arm, it never rises far enough to shut the valve. Free it or turn the valve slightly.
• Check the level. When the valve shuts, the water should sit roughly 2 to 3 cm below the top of the overflow tube or at the moulded water line. Higher than that, lower the float using its adjusting screw or clip.
• Turn off the isolation valve and see whether the level holds. If it holds, the fault is on the fill side. If it drops, it is on the flush side.
• For a fill valve that hisses after adjustment, open its top cap and rinse the diaphragm washer and the small filter, if fitted, to clear grit.
• Replace the fill valve washer or the whole fill valve if rinsing does not stop it.
• For a level that drops, replace or descale the flush valve seal at the bottom of the cistern.

Why fill valves stop shutting off

A fill valve closes when the rising float pushes a rubber diaphragm or washer against a small hole. Anything that stops a perfect seal lets water through: a speck of grit from the mains, a build-up of limescale, or a washer that has hardened and taken a set. Grit problems often start just after work on the water mains in the street, when disturbed sediment travels up the service pipe.

Old brass ball valves with a metal arm and a plastic ball fail the same way, and can also fail when the ball splits and fills with water so it no longer floats. If you have one of those, replacing it with a modern fill valve is usually better than repairing it. Match the new valve to the supply: some valves come with a choice of seat or setting for high-pressure mains and low-pressure tank-fed supplies. Our guide to a leaking refill valve covers valves that also drip outside the cistern.

Why flush valves let water through

On a button toilet, the flush valve seal sits under water all the time and rubs on its seat every flush. Limescale from London's hard water and grit from the supply both shorten its life. The seal is a small, cheap part, but it is specific to the valve maker, so check the brand moulded on the valve before buying. A seal is typically a few pounds; a complete replacement flush valve runs roughly £15 to £35.

Some in-cistern toilet blocks and bleach tablets are known to make rubber seals swell or perish faster. If yours has one, take it out when you replace the seal. For the full test to confirm the seal is the problem, see a toilet leaking into the bowl.

Never wedge the button up or tie the float in place to stop the water. The next flush either will not refill or the valve stays open with nobody watching, and a blocked or undersized overflow then puts water on the floor.

When it cannot wait

A running toilet is rarely a flood risk on its own, because the overflow takes the extra water into the pan. It becomes urgent when the overflow cannot keep up and water is coming over the top of the cistern onto the floor, or when the outside overflow pipe is pouring onto a path or a neighbour's property. It also matters in winter, when a constantly running overflow outside can freeze and ice over a doorstep.

If you cannot stop it, turn off the isolation valve and use the toilet by filling the cistern or pan from a bucket until it is fixed. If the isolation valve is stuck or missing, turn the water off at the main stopcock. Our page on a cistern that keeps overflowing deals with water that is going over the top rather than down the pan.

What the repair usually costs

Parts are cheap. A fill valve is commonly £10 to £25 and a flush valve £15 to £35 at plumbers' merchants. Across London, a plumber replacing one of those on a close-coupled toilet typically charges in the region of £80 to £150 including the part. Concealed cisterns cost more because every part goes in through the flush plate opening.

With us, a toilet repair visit is a £60 call-out at any hour, and the repair price is agreed with you on site before any work starts. The engineer finds which valve is at fault, replaces it with a matched part, sets the water level and flushes several times to prove the running has stopped.