Why your toilet keeps running, and how to stop it

A toilet that keeps running wastes water and, on a meter, money. In a UK cistern there are two parts that can cause it: the fill valve that lets water in, and the flush mechanism that lets water out into the pan. Much of the advice online is written for American toilets with a rubber "flapper". UK cisterns are usually different.

How a UK cistern works

Fill valve (also called a float valve or ballvalve). Water comes in through this valve until a float rises and shuts it off. Older cisterns have a float on an arm; modern ones use a compact float on the valve body.

Flush mechanism. There are two common types:

• Siphon. The traditional UK design. Pressing the lever lifts a plastic diaphragm inside a bent tube, starting a siphon that empties the cistern. A siphon does not normally let water through while it is idle, so when a siphon cistern keeps running the cause is usually the fill valve overfilling it. If a siphon toilet will not flush properly, the diaphragm (flap valve) is often worn.
• Drop valve, or dual-flush valve. Common in newer toilets with push buttons. The button lifts a valve off a rubber seal at the bottom of the cistern. If that seal is worn, dirty or misaligned, water trickles into the pan continuously.

Where is the water going?

Take the cistern lid off carefully. It is heavy and can crack.

• Water running into the overflow pipe, or out of an overflow outside, or into the pan via an internal overflow: the cistern is overfilling. Look at the fill valve.
• Water trickling into the pan with the level below the overflow: on a drop valve toilet, the flush valve seal is letting by.
• A hiss and constant refilling: either the fill valve is not shutting, or water is leaking out through the flush valve and the fill valve keeps topping up.

A simple test for a seal leak: when the cistern is full and quiet, put a few drops of food colouring in the cistern water and do not flush. If colour appears in the pan after a while, the flush valve is leaking.

Fixes to try

1. Isolate the cistern. Most have a small isolating valve on the supply pipe. Turn the slot a quarter turn so it is across the pipe. If there is none, use the stopcock. See finding your stopcock.
2. Check the water level. It should stop below the overflow, usually at a line marked inside the cistern or on the valve. Most modern fill valves have an adjustment screw or clip to lower the float. Older arm types can be adjusted at the float arm.
3. Check the float moves freely. A float rubbing on the cistern side, the flush mechanism, or a loose part can stop it rising.
4. Clean the fill valve. Grit or scale can stop it sealing. Many valves have a removable cap and filter. Follow the maker's instructions.
5. Clean or replace the flush valve seal. On a drop valve, the seal at the base can often be removed by twisting the valve out, then cleaned or replaced with the right part for that make.
6. Replace the fill valve if it still will not shut off. Match side entry or bottom entry to your cistern, and the thread size.

Restore the water slowly and watch for leaks at the connection under the cistern.

When to call a plumber

If the cistern is cracked, the leak is at the base of a close-coupled toilet, the parts are seized, or you cannot isolate the supply, call a plumber.

Cistern parts at Wickes

We haven't tested these. Match the part to your cistern before you buy.

We haven't tested this. Figures are as the maker states them (checked 13 November 2026).

Why does my toilet keep running after flushing?

Usually either the fill valve is set too high and water is going down the overflow, or a drop valve seal is letting water trickle into the pan. Lift the lid and see which.

Is a running toilet urgent?

Not usually dangerous, but it wastes water. Isolate the cistern at its valve until you can fix it. If water is coming out of the cistern or pipework onto the floor, turn off the supply at once.