How do I stop a running toilet?

A running toilet can waste more water than every other leak in a house combined — often hundreds of litres a day, silently. There are only two mechanisms involved, and the food-colouring test tells you which one has failed within ten minutes, which saves buying the wrong part.

Safety

What you'll need

• Replacement fill valve or flush valve seal (once diagnosed)
• Adjustable spanner
• Bucket and sponge
• Food colouring for the test
• Rubber gloves

Step by step

1. Take the cistern lid off carefully and set it somewhere safe.They're heavy ceramic and crack easily. Lay it flat, not on edge.
2. Add a few drops of food colouring to the cistern water and wait 20 minutes without flushing.This is the diagnostic. Don't skip it — it saves buying the wrong part.
3. If coloured water appears in the pan, the flush valve seal has failed.The rubber seal has hardened or a piece of grit is holding it off its seat.
4. If the water level rises until it runs into the overflow, the fill valve isn't shutting off.Either the float is set too high, or the valve washer has perished.
5. For a fill valve issue, first try lowering the float adjustment.The water should stop about 25mm below the overflow. Often this is the entire fix.
6. If adjusting doesn't work, turn off the supply, drain the cistern and replace the fill valve.Modern universal fill valves are inexpensive and fit most cisterns. Note the inlet position — bottom or side entry.
7. For a flush valve seal, turn off the supply, drain the cistern and lift out the flush mechanism.Usually a half-turn to release, though close-coupled cisterns may need to be unbolted from the pan first.
8. Clean the seat, fit the new seal, reassemble and test through several flushes.Grit on the seat will hold a brand-new seal open just as effectively as a worn one.

Tips

• Note the make and model on the cistern parts before shopping. Universal kits fit most, but not all, and a close-coupled cistern is much more work than a separate one.
• If the fill valve is noisy as well as slow to shut off, replace it rather than adjusting — it's near the end of its life.
• Limescale on the flush valve seat is common in hard water areas. Descale the seat while you have the cistern open.
• Take a photo of the mechanism before dismantling — reassembly order matters more than it looks.

Common mistakes

• Buying parts before diagnosing — Fill valve and flush valve failures look identical from outside the cistern. The dye test takes 20 minutes and removes the guesswork.
• Ignoring it because it's quiet — A slow leak past the flush seal is often silent and still wastes hundreds of litres a day.
• Forcing plastic fittings — Cistern components are brittle and cheap. Cracking the cistern itself is not cheap.

Questions people ask