Toilet keeps running? How to find the cause and fix it

If your toilet keeps running, one of two parts is almost always to blame: the fill valve, which lets water into the cistern, or the flush valve, which lets water out into the pan. Lift the cistern lid and you can usually tell which in a couple of minutes. Many fixes are simple adjustments you can do yourself; others need a new part.

A running toilet is easy to ignore, but it wastes clean water all day and night. If you are on a water meter, you pay for every litre of it. It can also mask a slow leak that eventually causes damage.

How a toilet cistern works

Knowing the two main parts makes the fault much easier to find.

• The fill valve (also called the inlet valve or ballcock) lets water in. A float rises with the water level and shuts the valve off when the cistern is full.
• The flush valve (or siphon on older toilets) lets water out when you press the button or handle. A rubber seal or washer at the bottom holds the water in the cistern between flushes.
• The overflow is a safety route for water if the level rises too high. On modern toilets it usually runs inside the flush valve and empties into the pan. On older ones it is a pipe to the outside wall.

Why does my toilet keep running?

Take the cistern lid off (on a concealed cistern, remove the flush plate; there is usually an access panel behind it) and watch what happens after a flush.

Water level too high: the fill valve

If the water rises right up and trickles into the overflow, the fill valve is not shutting off. Common causes:

• Float set too high. The float tells the valve when to stop. If it is set too high, the water reaches the overflow before the valve closes.
• Float stuck or rubbing against the side of the cistern or the flush mechanism.
• Worn diaphragm or washer inside the valve, often caused by grit or limescale. London has hard water, and scale is a regular cause of fill valves that will not seal.

Water level fine but still trickling: the flush valve

If the water level sits below the overflow but you can still hear the cistern refilling every so often, or see a constant thin ripple running down the back of the pan, water is slipping past the flush valve seal. The cistern slowly empties into the pan, the fill valve tops it up, and the cycle repeats. This is very common on push-button toilets, where the rubber seal at the base of the valve hardens or picks up scale and debris.

The food colouring test

This is a simple way to confirm a leaking flush valve.

1. Lift the cistern lid and add a few drops of food colouring to the water in the cistern. Do not flush.
2. Leave it for 20 to 30 minutes without using the toilet.
3. Look in the pan. If the water in the pan has turned the same colour, water is leaking from the cistern past the flush valve seal.

If the pan stays clear and the water level in the cistern is sitting at the overflow, the fill valve is your problem instead.

Toilet keeps running after flushing: how to fix it

Adjusting the float

1. Turn off the water to the toilet. Most have a small isolation valve on the supply pipe: turn the screw slot a quarter turn so it sits across the pipe.
2. Find the adjustment on the float. On modern valves it is usually a plastic screw or clip on a vertical rod; on older ball valves it is the arm itself.
3. Lower the float a little at a time.
4. Turn the water back on and let the cistern fill. The water should stop about 2 to 3cm below the top of the overflow.

If the float is set correctly and the water still keeps running, the valve itself needs a new washer or replacing.

Cleaning or replacing the flush valve seal

1. Turn off the water and flush to empty the cistern.
2. On most push-button valves, the valve body twists or unclips and lifts out.
3. Check the rubber seal at the bottom. Clean off any grit or scale, and look for splits or flattening.
4. If it is worn, fit a new seal of the same size. Hardware shops sell them, but take the old one with you to match it.
5. Refit the valve, turn the water back on and test.

If the seal is fine but water still leaks, the valve may be cracked or the wrong size, and a new flush valve is the answer.

When it makes sense to call us

Some running toilets are fiddly, and a few are not good DIY jobs at all.

• Concealed cisterns behind a wall or in a vanity unit can be awkward to reach, and parts are often brand-specific.
• Close-coupled toilets sometimes need the cistern taken off the pan to replace the flush valve, which means remaking the seal between them.
• An older external overflow dripping outside can be a sign of a failing fill valve, and is worth sorting before it stains the wall or freezes in winter.
• The isolation valve will not turn or does not stop the water. You may need to use the main stopcock instead.

When to call an engineer

• You have replaced the seal or adjusted the float and it is still running
• Water is leaking onto the floor around the base of the toilet
• You cannot isolate the water
• Your water meter is still moving with everything turned off, which suggests a leak somewhere else, and our leak detection service can trace it

Most running toilets are a short job for one of our engineers, and the hourly labour rates are on our prices page, with materials at cost plus 20%. See our toilet repair service for more, or if your toilet is blocked rather than running, read how to unblock a toilet.

If you are not sure which part has failed, call Lee on 020 8050 5913. Describe what you see under the lid and he can often tell you over the phone, free of charge.