How to Fix a Running Toilet

Why a Running Toilet Is a Problem

A toilet that keeps running after flushing wastes a staggering amount of water. Depending on the severity, it can add significant amounts to your water bill and puts unnecessary strain on the water system. The good news is that most causes are straightforward to diagnose and fix.

How a Toilet Cistern Works

Understanding the basic mechanism helps with diagnosis. When you flush:

1. The flush valve opens, releasing water from the cistern into the bowl
2. As the cistern empties, the float drops
3. The dropping float opens the fill valve, letting fresh water in
4. As the cistern refills, the float rises
5. When the float reaches the correct level, the fill valve shuts off

A running toilet means one of these steps is not completing properly.

Common Causes and Fixes

1. The Flapper Valve Is Worn

The flapper (or flush valve seal) sits at the bottom of the cistern and creates a watertight seal. Over time, it warps, cracks or develops mineral deposits.

How to check: Add a few drops of food colouring to the cistern water. Wait 15 minutes without flushing. If coloured water appears in the bowl, the flapper is leaking.

How to fix:

1. Turn off the water supply to the toilet
2. Flush to empty the cistern
3. Remove the old flapper (it usually unclips or slides off)
4. Take it to a hardware shop to find a matching replacement
5. Fit the new flapper and turn the water back on

2. The Float Is Set Too High

If the water level in the cistern rises above the overflow tube, water continuously drains away.

How to fix:

• Ball float: Bend the arm slightly downward to lower the float position
• Cup float: Adjust the screw or clip on the fill valve to lower the float
• The water level should sit about 2-3cm below the top of the overflow tube

3. The Fill Valve Is Faulty

If adjusting the float does not stop the running, the fill valve itself may be worn and failing to shut off properly.

How to fix:

1. Turn off the water supply
2. Flush to empty the cistern
3. Remove the fill valve (unscrew the connection underneath the cistern)
4. Replace with a new universal fill valve from a hardware shop
5. Reconnect and turn the water back on

4. The Overflow Tube Is Cracked

Less common but possible, a crack in the overflow tube allows water to continuously drain into the bowl.

How to fix: This usually requires replacing the flush valve assembly, which is a more involved job.

When to Call a Plumber

While replacing a flapper or adjusting a float is manageable for most people, consider calling a professional if:

• You cannot identify which part is causing the problem
• The cistern mechanism is old and corroded
• You need to replace the flush valve assembly
• There are signs of leaking from the base of the toilet
• You have a concealed cistern behind wall panelling

Our plumbing team fixes running toilets and all other toilet issues across South London. We carry common replacement parts and can usually resolve the problem in a single visit. We serve Clapham, Borough, Kensington, Chelsea and all surrounding areas.