Running Toilet: How to Fix a Cistern That Won't Stop
Why a toilet keeps running or refilling, how to check the float valve, flush valve and washer, and which cistern parts to replace.
A toilet that keeps trickling or refilling is wasting water and, if you are on a meter, money. The cause is nearly always a worn or misadjusted part inside the cistern, and most of these parts are cheap and designed to be swapped by a householder.
Understand what is happening
A cistern holds water, a flush valve releases it into the bowl, and a fill valve (often called a float valve or ball valve) refills it, shutting off at a set level. If either part fails to seal, water leaks or keeps flowing.
Symptoms and likely causes:
| Symptom | Likely cause |
|---|---|
| Water trickles into the bowl constantly | Worn flush valve washer or flapper |
| Water runs into the overflow pipe | Fill valve not closing, float set too high |
| Cistern keeps refilling every few minutes | Small leak into the bowl past the flush valve |
| Hissing after flushing | Fill valve worn or debris stuck in it |
Check the basics
- Take off the cistern lid and rest it somewhere safe.
- Look at the water level against any mark on the inside, which is normally a few centimetres below the overflow.
- Put a few drops of food colouring into the cistern and wait twenty minutes without flushing. If the colour shows in the bowl, the flush valve is leaking.
Fixing a float that is set too high
If water runs into the overflow:
- On a traditional arm and ball, bend the arm gently down or turn the adjusting screw so the ball sits lower.
- On a modern plastic float, use the adjustment clip or screw on the valve to lower the water level.
Flush and see whether the level stops below the overflow.
Replacing the fill valve
If adjusting fails and water keeps running:
- Isolate the cistern using the small valve near the pipe or the main stopcock.
- Flush to empty the cistern.
- Disconnect the supply pipe from the underside, with a bowl and towel ready.
- Undo the backnut holding the valve in place.
- Fit the new valve, matching whether it is a side or bottom entry type.
- Reconnect the pipe, restore water and adjust the float.
Replacing the flush valve or seal
Older cisterns have a siphon, and newer ones a flapper or dual-flush valve. These wear out. If the colour test showed a leak into the bowl:
- A siphon diaphragm washer is a flat disc on the piston and needs replacing. The siphon lifts out after the cistern is drained and disconnected.
- A flap valve seal can be swapped if you match the model.
Take photos and take the old part to a plumber's merchant or DIY store to match it. Many parts are universal; some are not.
When it is not a simple fix
- The overflow is blocked or damaged.
- The cistern is cracked.
- The cistern is fixed to the wall in a way that makes access difficult.
- The flushing mechanism is concealed behind panelling.
In those cases a plumber is a sensible option. For general prices, see how much a plumber costs.
Preventing repeats
- Keep limescale at bay in hard water areas. It builds on seals and stops them seating.
- Do not use in-cistern block cleaners that sit in the water, as chemicals can degrade rubber.
- Flush gently, not forcefully.
Worked example: the phantom refill
A family hears the cistern top up for ten seconds every half hour, even though nobody has flushed. A food colouring test shows coloured water in the bowl after twenty minutes, so the flush valve seal is letting water leak past. The fix is a replacement seal, often a small rubber part costing a few pounds, and perhaps half an hour including draining and refitting. A plumber would typically charge their normal minimum call-out for something this small, so many householders do it themselves if they are comfortable with the steps above.
A decision table
| What you notice | Probably | Do this first |
|---|---|---|
| Water audibly running into the overflow pipe | Float set too high or fill valve stuck open | Lower the float, then test |
| Silent leak, colour appears in the bowl | Flush valve seal worn | Replace seal or flap |
| Slow, noisy fill with whistling | Fill valve worn or filter blocked | Clean or replace fill valve |
| Cistern empties slowly after flush | Chain too slack or too tight | Adjust the chain length |
| Handle sticks or does not return | Lever or linkage worn | Replace lever assembly |
Hard water and older cisterns
In hard-water regions, limescale builds up on seals and float arms within a few years. A thorough clean with a descaler may be enough to restore a seal that is not damaged. High-level cisterns on a chain, typical of older houses, use different parts from close-coupled ones, so take careful photos before buying anything. If the make is unknown, a plumbers' merchant can often match a part from the old one.
Rentals and shared flats
In a rental, report a running toilet to the landlord or agent. Continuous running can inflate a water bill and in a flat can irritate neighbours if it causes noise in shared pipework. Keep a dated message so a repair is on record.
If you cannot find the cause
When none of the adjustments help, the problem might be a cracked cistern, a damaged overflow or a concealed cistern behind a wall panel. Call a plumber and describe what you have already tried. It saves time and is cheaper than guesswork.
Frequently asked questions
How much water does a running toilet waste?
Quite a lot over time, though the amount varies. Even a slow leak is worth fixing, particularly if you have a water meter.
Why does my cistern fill slowly?
A partly closed isolation valve, a blocked filter in the fill valve or low water pressure could be at fault. See low water pressure causes.
Do I need a plumber to replace a fill valve?
Not necessarily. It is a well-defined DIY job, but if you are unsure about isolating the water, a plumber can do it quickly.