Toilet keeps running? The common causes and how they are fixed

A toilet that keeps trickling into the pan, or refills on its own every so often, is one of the most common plumbing jobs there is. It's rarely an emergency, but it wastes water all day and night, and on a water meter you pay for every drop.

How a modern cistern works

Most toilets in the UK now have two main parts inside the cistern. The fill valve lets water in and shuts off when the cistern is full. The flush valve lifts when you press the button and drops back down to seal the cistern once it's empty. A running toilet almost always comes down to one of those two parts.

Check one: is water going into the overflow?

Take the cistern lid off and look at the water level. If it's rising above the line marked inside, or running into the overflow tube, the fill valve isn't shutting off. Sometimes the float just needs adjusting. More often the valve's seal has worn or limescale has stopped it closing properly.

Hertfordshire has very hard water, so limescale is a regular cause of sticking valves in homes across the county.

Check two: is water trickling into the pan?

If the water level looks right but you can see a steady trickle down the back of the pan, the flush valve isn't sealing. The rubber washer at its base wears out over time, or bits of grit and scale stop it sitting flat. A new washer, or a new flush valve, usually sorts it.

A simple test: put a few drops of food colouring in the cistern and don't flush for half an hour. If colour appears in the pan, the flush valve is leaking.

Check three: does the button stick?

Push-button flushes can stick down, holding the valve open. Pressing it again or wiggling the button sometimes frees it. If it keeps happening, the button or cable needs replacing.

Can I fix it myself?

Adjusting a float is fine for most people. Replacing a fill valve or flush valve means turning off the water to the toilet, emptying the cistern and sometimes taking the cistern off the wall, depending on the model. Parts vary between makes, so getting the right one matters.

A plumber can normally replace the faulty part in a single visit. If your toilet is old and parts are hard to find, it may be worth talking about a new one.