Toilet keeps running: why and how to fix it

A toilet that keeps running is easy to ignore. It is also one of the most common jobs in Bristol homes, and the city’s hard water is a big reason why. This guide explains the two causes, how to tell which one you have, and what fixing it involves.

Two ways a toilet keeps running

Water has two ways to escape from a cistern.

What you see	Cause	Common fix
A trickle or ripple in the bowl long after flushing	Flush valve seal or siphon washer leaking	New seal or new flush valve
Cistern refilling by itself every so often	Flush valve seal leaking slowly	New seal
Water running down the internal overflow into the bowl	Fill valve not shutting off, water level too high	Adjust float, clean or replace fill valve
Water dripping from a pipe outside the house	Older cistern with an external overflow, fill valve not shutting	Replace fill valve
Hissing that never stops	Fill valve diaphragm worn or blocked	Replace fill valve or diaphragm

Older Bristol houses often still have external overflow pipes poking out of the back wall. If you see one dripping onto a rear yard, the toilet or the loft tank is overfilling.

Why hard water makes it worse in Bristol

Bristol Water says all water in its area is moderately hard to very hard. It lists limescale in and around toilets among the problems it causes. Inside a cistern, that scale does damage you cannot see:

• it builds up on the rubber seal at the bottom of push button flush valves, so the seal no longer sits flat
• it hardens rubber washers on older siphon flushes
• it furs the small hole in a fill valve diaphragm, so the valve shuts slowly or not at all
• it stiffens the float arm so it sticks

This is why a modern dual flush toilet in a Bristol bathroom can start running after a few years, even when nobody has touched it.

Simple checks you can do

First, stop the waste. Turn the isolating valve on the pipe that feeds the cistern. It usually needs a flat screwdriver turned a quarter. If there is no valve, turn off the stop tap.

Then lift the cistern lid and look.

1. Is the water above the overflow? If water is spilling into the overflow tube, the fill valve is the problem. Many modern valves have an adjuster to lower the water level.
2. Does the float move freely? Lift it gently. If the water stops, the float was sticking.
3. Food colouring test. Put a few drops of food colouring in the cistern. Do not flush. If colour shows in the bowl after half an hour, the flush seal is leaking.
4. Look at the seal. On a push button flush, the seal is a rubber ring at the base of the valve. Scale or grit on it stops a proper seal. A wipe can sometimes fix it for a while.

If you are not confident, leave the isolating valve off and book a repair. A running toilet is not usually an emergency once the water is isolated.

What a plumber does

On a visit a plumber will usually:

• identify the exact flush and fill valve fitted, as there are many types
• replace the seal, or the whole valve if it is scaled or brittle
• fit or replace an isolating valve if there is none
• set the water level correctly
• check the cistern to pan seal and the pan connector for leaks

Concealed cisterns behind boxing or wall panels are common in newer Bristol bathrooms and in flats. These need access through the flush plate or a panel. If the panel is tiled over, access takes longer.

Our tap and toilet repairs page explains what a repair visit involves.

The 2026 hosepipe ban

Bristol Water brought in a hosepipe ban from 27 August 2026 to protect the River Severn during drought. The ban covers hosepipes, not toilets, but Bristol Water is asking customers to save water wherever they can. A running toilet can waste water all day and all night. If you are on a meter, you are paying for every litre. Our guide to the Bristol hosepipe ban has more ways to cut waste at home.

When a running toilet means something bigger

• A toilet running and the meter still moving with it off. The leak may be elsewhere. Bristol Water’s help with leaks page explains a meter test.
• A loft tank overflowing too. In older homes with a cold water tank in the loft, a stuck float valve in the tank is a separate fault. Common in interwar semis in Henleaze and Westbury on Trym.
• A toilet in a basement flat that runs and then stops working. If it is a macerator toilet, see our guide to macerator toilet problems.

Keep it from coming back

• when parts are replaced, choose good quality valves that can be serviced
• clean visible scale from the flush seal when you clean the bathroom
• avoid blue in cistern blocks, which can harm some seals
• consider scale protection for the whole home if limescale is a constant battle