A continuously running toilet is nearly always one of two things: water leaking past the flapper into the bowl, or the fill valve letting water into the overflow tube. A few drops of food colouring in the cistern will tell you which within ten minutes, and both parts are inexpensive to replace.
Why it matters
Beyond the noise, a running toilet is quietly expensive. A worn flapper can pass a substantial volume of water continuously, day and night — this is one of the largest single sources of wasted water in a typical home, and it shows up on metered bills long before anyone investigates the sound.
The dye test
This single test splits the problem in half:
- Add colour to the cistern
Take the lid off the cistern and add a few drops of food colouring, or a dye tablet, to the water. Do not flush.
- Wait ten to fifteen minutes
Leave the toilet completely alone. Do not flush it and do not use it.
- Look in the bowl
Coloured water in the bowl means water is leaking past the flapper or flush valve seal. Clear water in the bowl, with the cistern still filling, means the fill valve is letting water through into the overflow tube instead.
Common causes
| Cause | How likely | What you would see |
|---|---|---|
| Worn or warped flapper | Very common | Dye passes into the bowl; flapper feels soft or distorted |
| Flapper chain wrong length | Common | Chain too short holds the flapper open; too long snags underneath |
| Debris or scale on the flush valve seat | Common | Dye passes even though the flapper looks fine |
| Water level set too high | Common | Water spilling into the overflow tube |
| Failed fill valve | Common | Valve never shuts off, or hisses continuously |
| Float damaged or waterlogged | Occasional | Float sits low; valve never senses a full cistern |
| Cracked overflow tube | Uncommon | Continuous drain with no obvious leak point |
How to fix it
Close the isolating valve on the supply pipe behind or beneath the cistern before removing any part. If there is no isolating valve, turn off the water at the main stopcock. Flush to empty the cistern before you start.
If the dye reached the bowl
- Inspect the flapper
Lift it and run a finger over the sealing face. If it feels soft, slimy or has hardened and warped, it has reached the end of its life. Rubber flappers typically last around four to five years, less in hard water or where in-cistern cleaning blocks are used.
- Clean the valve seat
Wipe the rim the flapper seals against. Mineral scale or grit on the seat will hold the flapper fractionally open. A worn or pitted plastic seat may need the whole flush valve replaced.
- Check the chain
Aim for about 10 mm of slack when the flapper is closed. Too tight and it never fully seats; too loose and it can trap under the flapper. Trim excess links rather than leaving them dangling.
- Replace the flapper if in doubt
Take the old one with you to match the size and style. It is an inexpensive part and rarely worth nursing along.
If the water was clear
- Set the water level correctly
The level should sit roughly 25 mm below the top of the overflow tube. Most modern fill valves adjust with a screw or a sliding clip on the float rod; older ball-float arms can be bent gently downwards.
- Check the float
A ball float that has taken on water will sit too low and never tell the valve to close. Unscrew it, shake it, and replace it if there is water inside.
- Clean or replace the fill valve
Grit lodged in the valve seal is a common cause of a valve that hisses continuously. Many valves can be dismantled and rinsed. If cleaning does not settle it, replacing the whole fill valve is straightforward and inexpensive.
Drop-in bleach and cleaning blocks sit in constant contact with the rubber components and shorten flapper and seal life considerably. If you have been using them, expect to replace the flapper more often.
When to call a plumber
Call a plumber if the cistern itself is cracked, if water is leaking from the joint between cistern and pan, if the isolating valve will not close or is seized, or if the supply pipework is corroded. Any leak that has reached a floor or ceiling below should be looked at promptly, whatever its source.
