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Why your toilet keeps running (and why it is worth fixing quickly)

A toilet that keeps refilling after it has already flushed is one of the most common plumbing complaints, and one of the cheapest to fix once you know which of two things is actually wrong.

Cause one: the flush valve is not sealing

Inside the cistern, a flush valve (sometimes called a flapper on older systems) sits over the outlet at the bottom and is meant to form a watertight seal between flushes. Over time these wear, warp slightly, or get coated in limescale, and stop sealing fully, letting a slow trickle of water leak from the cistern into the pan continuously. You can often confirm this is the cause by adding a few drops of food colouring to the cistern water and checking, without flushing, whether colour appears in the pan within 15 to 20 minutes.

Cause two: the fill valve is not shutting off

The fill valve controls water coming into the cistern and should shut off once the water reaches a set level. If it is not shutting off properly, water keeps trickling in, usually escaping down the overflow tube, which can sound like continuous running or a faint hissing. This is often a worn washer or diaphragm inside the valve, or a float that has become waterlogged and is no longer rising to the correct shut-off point.

Why it is worth fixing promptly

A running toilet can waste a genuinely significant amount of water over the course of a year, and on a meter that shows up directly on the bill. It is also one of the more common causes of a mysteriously higher water bill that people otherwise cannot explain.

When to call rather than DIY

Both parts are inexpensive and the repair is usually quick, but cisterns vary enough between manufacturers and ages that fitting the wrong replacement part is a common DIY trap. If you are comfortable with basic DIY it is a reasonable job to attempt yourself; if not, it is one of our most routine call-outs and rarely takes long to sort properly.