Signs to look for
- A thin trickle of water running down the back of the pan long after flushing
- The cistern refilling by itself every so often with no one using it
- A hissing sound from the cistern that does not stop
- Water dripping or running from an overflow pipe on an outside wall
What to do first
Try the paper test
WaterSafe’s check: wait 30 minutes after flushing, dry the back of the pan, then lay a dry sheet of toilet paper there and leave it for up to three hours, ideally overnight. Wet or torn paper means the toilet is leaking.
Look inside the cistern
Lift the lid carefully. It is heavy ceramic, so put it somewhere it cannot fall. See whether the water is up to the overflow, or the fill valve keeps hissing. Look, but do not start adjusting parts.
Turn it off if needed
Most toilets have an isolating valve on the pipe feeding the cistern. A quarter turn with a flat screwdriver shuts it off. You can still flush once by hand with a bucket of water if you need to.
Check the overflow outside
If a pipe on the outside wall is dripping, check which toilet or tank it comes from. An overflow running constantly needs attention.
Use only toilet paper for the test, and do not tip cleaning chemicals into the cistern while you are investigating.
Possible causes
These are common causes, not a diagnosis. The actual fault is confirmed on site.
Flush valve seal
In push-button and dual-flush toilets, a worn or dirty seal on the flush valve lets water trickle into the pan. WaterSafe identifies valve seals as the usual cause.
Fill valve not shutting off
If the inlet valve does not close fully, the cistern overfills and runs to the overflow or into the pan.
Water level set too high
If the float is set too high, water keeps reaching the overflow.
Debris or limescale
Grit or scale on a seal can stop it closing properly.
How WHP would assess it
- Confirm whether water is leaking past the flush valve, coming in through the fill valve, or both.
- Identify the cistern’s mechanism and get parts that suit it and meet water fittings rules.
- Check the isolating valve and the overflow route.
- Test several flushes after the repair and confirm the cistern settles at the right level.
We explain any call-out or diagnostic charge before booking, and agree the repair and its cost with you before carrying it out.
What affects the price
Every repair is different, so we do not publish fixed prices for this work. These are the things that make the most difference:
- Type of cistern and whether it is exposed or concealed behind panels or furniture
- Whether parts are standard or specific to the manufacturer
- Whether the isolating valve also needs replacing
- Access, for example boxed-in or wall-hung toilets
Before you call
Useful to send us
- A photo inside the cistern with the lid off
- A short video of the water running into the pan or overflow
- Any brand name on the toilet, cistern or flush button
- Whether the cistern is concealed behind a panel
Get professional help when
- The paper test shows a leak or the overflow keeps running
- The isolating valve will not turn or is leaking
- The cistern is concealed behind panels or a wall-hung frame
- Water is leaking onto the floor rather than into the pan
Common questions
Is a running toilet urgent?
It will not flood the bathroom on its own, because the water goes down the pan or the overflow. It does waste a lot of water, so fix it soon, especially on a meter.
Why does my toilet refill on its own every few minutes?
Usually water is slowly leaking from the cistern into the pan through the flush valve. When the level drops far enough, the fill valve tops it up again.
Can any replacement part be used?
Parts need to suit your toilet and comply with water fittings regulations, which WaterSafe highlights. We match parts to the cistern rather than fitting a generic part that will not seal.
Sources
Safety and responsibility details checked against these sources on 12 October 2026. Water supplier guidance can change. Check your own supplier’s current advice.