Updated
Toilet Keeps Running
Water trickles into the pan constantly, or the cistern never stops refilling after a flush.
Find a plumber
What's happening?
A cistern has two valves and a running toilet is one of them failing. The inlet valve — the float valve — should shut off the incoming water once the cistern reaches its fill line; if it does not, water keeps arriving and escapes down the overflow into the pan. The outlet valve — the flush valve or siphon — should seal the cistern from the pan between flushes; if its seal has worn or the flush mechanism is catching, water leaks past it continuously. Modern cisterns discharge the overflow internally into the pan, which is why the classic sign is a permanent thin stream down the back of the bowl rather than water outside.
What usually causes it?
- Worn flush valve seal (the rubber diaphragm on a drop valve) letting water past into the pan
- Float valve washer perished, so the inlet never fully shuts and the level keeps rising
- Float or float arm set too high, filling the cistern above the overflow line
- Limescale on the valve seat holding the seal fractionally open
- Flush button or lever sticking and keeping the valve partly lifted
- Grit from pipework fouling the inlet valve after other plumbing work
- Slow inlet valve on low mains pressure, which never reaches the shut-off point cleanly
Is it dangerous?
No, but it is the single most wasteful common fault in a UK home — a leaking loo can waste hundreds of litres a day, straight down the drain and unnoticed on an unmetered bill. If the toilet is on an old cistern with an external overflow pipe through the wall, the running water shows up outside instead and can soak the brickwork below it for months.
Can I fix it myself?
Confirm it first: wipe the back of the dry pan and watch for a thin wet line forming, or put a little food colouring in the cistern and see whether it appears in the pan without flushing. Take the lid off and check the water level against the marked fill line. Bending a plastic float arm is not the fix — most modern valves adjust on a clip or a screw. Replacing a float valve washer or a whole flush valve is achievable if you can isolate the supply and get the cistern off, but a close-coupled pan usually has to be separated from the cistern to reach the flush valve, which is where most DIY attempts stop.
When should you call a plumber?
Call a plumber when the toilet is close-coupled or concealed behind a frame, when the isolating valve does not hold, when replacing the valve has not stopped it, or when the toilet is wall-hung and the cistern sits inside a duct. Also call if the overflow is discharging outside and the wall below it is staying wet.
What will the plumber actually do?
- Prove which valve is passing rather than replacing both on spec
- Fit the correct flush valve seal or replace the drop valve or siphon for the pan type
- Replace or rebuild the float valve and set the fill level to the cistern's marked line
- Descale the valve seats and check the isolating valve holds properly
- Refit a close-coupled cistern with new doughnut and bolt seals so the join does not weep
- Check the pan connector and the overflow arrangement while the cistern is off
Frequently asked questions
We’re happy to answer all your questions.
Need a plumber?
Compare local plumbers on Tradehand — coverage areas, accreditations and phone numbers, no middleman.