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A push-button flush valve inside a toilet cistern

How to Fix a Toilet Constantly Running: Simple Steps to fix the issue

Lee Devlin, Director of Homecure Plumbers

By Lee Devlin, Director, Homecure Plumbers
Last updated 13 October 2026

A toilet that keeps running is almost always one of three things: the fill valve is not shutting off, the flush valve seal is letting water into the pan, or the water level is set so high it runs into the overflow. Lift the cistern lid and watch where the water goes to tell which.

Most of these can be fixed with a new washer or valve. If you would rather not, or the cistern is boxed in, call us.

Find the cause in two minutes

What you see with the lid offCauseFix
Water level reaches the top of the overflow and keeps running into itFill valve not shutting off, or set too highLower the float, or clean or replace the fill valve.
Water level stays below the overflow, but water trickles into the panWorn flush valve seal (washer)Clean or replace the seal, or the whole flush valve.
Cistern keeps refilling every few minutes on its ownFlush valve seal leaking slowlyAs above.
Water runs out of a pipe on the outside wallOverflow from the cisternLower the water level, or fix the fill valve.

Note: modern toilets have an internal overflow system that dumps excess water straight into the toilet bowl rather than through a pipe to the outside wall.

Before you start: turn off the water

Find the isolation valve on the pipe feeding the cistern, usually a small valve with a screwdriver slot. Turn the slot across the pipe to shut it off, then flush to empty the cistern.

Fix 1: the fill valve

The fill valve lets water in and shuts off when the float rises.

  • Lower the water level: most modern fill valves have an adjusting screw or a clip on the float. Set it so the water stops about 2 to 3 cm below the top of the overflow.
  • Clean it: grit or limescale can stop the valve closing. Most valves have a cap you can remove to rinse the seal.
  • Replace it: if it still will not shut off, fit a new fill valve. Take the old one with you so you buy the right type (bottom entry or side entry).

Fix 2: the flush valve seal

The flush valve holds the water in the cistern until you flush.

  • Push-button (drop) valves: twist or unclip the valve from its base, then clean or replace the rubber seal on the bottom. Many makes sell replacement seals.
  • Lever flush (siphon): if water is constantly trickling into the pan on a traditional siphon toilet, the fill valve has failed and water is spilling down the siphon’s internal overflow. (If the toilet simply won’t flush when you press the lever, the thin plastic diaphragm inside the siphon has split and needs replacing.)

Follow the fitting instructions for your make of valve. Siamp publishes fitting instructions for its valves.

Engineer holding a toilet fill valve during replacement
A fill valve being replaced. If the water keeps running into the overflow, this is usually the part.

Safe for you to try

  • Turning off the isolation valve and emptying the cistern.
  • Adjusting the water level on the fill valve.
  • Cleaning or replacing the flush valve seal.
  • Replacing a fill valve in a standard close-coupled cistern.

Call us if

  • The isolation valve will not turn or is leaking.
  • The cistern is concealed behind a panel or wall.
  • Water is leaking from the cistern bolts or the pan connector.
  • The cistern or pan is cracked.

These need a plumber, and a leak between the cistern and pan can damage floors and ceilings below.

A recent job

From the job: a toilet trickling into the pan all day

Toilet trickling into the pan all day. The seal on the Siamp Optima 49 push-button flush valve had perished. Fitted a new seal, done in about 20 minutes.

Expert notes by Kevin Saban, Senior Plumbing & Heating Engineer, Homecure Plumbers

Get it fixed

Our plumbers fix running toilets across London. Repairs from £95 + VAT for the first hour, including diagnosis (or a fix if it can be done within the first hour with no parts needed), no callout charge. Prices apply during office hours; parking or the congestion charge may be added. See our plumbing services or call 0203 504 4000. For macerator toilets, see common Saniflo problems.

Questions about running toilets

Does a running toilet waste much water?

Yes. A constant trickle runs all day and night, which adds up quickly on a water meter. It is worth fixing straight away.

Why does my toilet refill by itself every so often?

The flush valve seal is letting water leak slowly into the pan, so the fill valve tops the cistern up. Replace the seal.

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