How to Fix a Running Toilet, Which Is Probably Wasting More Than Your Shower

A toilet that runs continuously can waste several hundred litres a day. Many leak silently, with water trickling down the back of the pan where you will never see it.

Test for a silent leak

Put a few drops of food colouring in the cistern and do not flush for thirty minutes. If colour appears in the bowl, the flush valve is leaking.

It takes two minutes and identifies a fault that most households have somewhere and are unaware of.

Understand the two parts

Inside every cistern there are exactly two mechanisms and one of them is at fault.

The fill valve (the float mechanism) lets water in and shuts off at the right level. If it fails, water keeps entering and pours out through the overflow.

The flush valve (the large valve at the bottom, with a rubber seal or a flapper) holds water in until you flush. If its seal fails, water leaks continuously into the pan.

Which is it? Look at the water level.

• Water at or above the overflow level — fill valve.
• Water below the overflow, but the pan is wet or the dye test is positive — flush valve.

Fixing the fill valve

1. Turn off the isolation valve on the pipe feeding the cistern, and flush to empty it.
2. Try adjusting first. Every fill valve has an adjustment — a screw, a clip on a rod, or a sliding collar. Lower the shut-off point so water stops about 25mm below the overflow.
3. If adjusting does not stop it, the valve washer has perished or grit is holding it open. Dismantle the top of the valve, clean the diaphragm and seat, and reassemble.
4. If it still fails, replace the whole valve. They are inexpensive, universal, and fit in about fifteen minutes. This is usually the better option — rebuilding an old valve rarely lasts.

Fixing the flush valve

1. Turn the water off and empty the cistern, sponging out the last of it.
2. Look at the seal — on most modern siphon-less valves it is a large rubber washer or a flat diaphragm.
3. Clean it and the seat it presses on. A single grain of grit or a build-up of limescale is enough to hold it fractionally open, which is exactly the silent leak.
4. If the rubber is hard, cracked or distorted, replace it. The washer alone costs very little; take the old one to the shop.
5. On older siphon-type cisterns, the failure is usually a split plastic diaphragm inside the siphon. It is a cheap part but the whole siphon has to come out, which usually means removing the cistern from the pan.

While you are in there

Check the overflow arrangement. Older cisterns overflow through a pipe to the outside wall; modern ones overflow internally into the pan. If yours is external and dripping outside, that is your diagnosis already.

Also check the cistern is not sweating with condensation, which people sometimes mistake for a leak. Condensation forms evenly over the whole outside of the cistern rather than trickling from one point.