A toilet that never quite stops running is easy to ignore. The water goes down the pan or out of a pipe on the outside wall, nothing gets wet indoors, and the sound fades into the background. But it can waste a lot of water over time, it can show up on a metered bill, and it can hide other problems. The good news is that the cause is almost always inside the cistern, and it is usually one of three parts.
This guide explains how a cistern works, how to tell which part is at fault, how to do a dye test, and what to check when the water is leaking out of the cistern rather than into the pan.
How does a toilet cistern work?
Every flushing cistern has the same basic parts:
- The fill valve, also called the inlet valve or ballvalve, lets water in. A float rises with the water and closes the valve when the cistern is full. The float may be a ball on an arm or a cup that slides up and down a tube.
- The flush mechanism releases the water into the pan. Many modern cisterns use a flush valve, operated by a push button, which lifts a seal off an outlet in the base. Older cisterns often use a siphon, operated by a lever.
- The overflow carries water away if the cistern overfills. The water fittings regulations require every flushing cistern, other than pressure flushing and urinal cisterns, to have a warning pipe or a no less effective device. On older toilets that is usually a pipe through the wall to the outside. On many modern ones, the overflow runs inside the cistern and discharges into the pan.
Knowing which type of overflow you have tells you where to look when water keeps running.
Why does my toilet overflow keep running?
| What you notice | Likely cause |
|---|---|
| Water level in the cistern at or above the overflow, water running out of the overflow | Fill valve not shutting off, or float set too high |
| Water level below the overflow, but a constant trickle into the pan | Flush valve seal not sealing |
| Cistern refills on its own every so often without being flushed | Flush valve seal letting water past slowly, so the fill valve tops it up |
| Fill valve hisses constantly but the level is normal | Water escaping somewhere, often into the pan, or a fill valve that is not closing fully |
| Button stays down or the flush keeps going after release | Sticking button or cable, or the flush valve not dropping back |
| Water outside the cistern or around the pan | A leak from the inlet connection, cistern seals or pan connector, not the overflow |
How do you check the fill valve and float?
- Lift the cistern lid and put it somewhere safe. On push-button toilets, the button usually unscrews or lifts out first.
- Look at the water level. Most cisterns have a marked water line on the inside wall or on the overflow tube. The water should settle below the top of the overflow.
- Lift the float gently. If the water stops flowing when you lift it, the valve works but the float is set too high, is catching on something, or has filled with water.
- Adjust the float. Many valves have an adjusting screw or clip that lowers the float. Lower it a little and let the cistern refill.
- If lifting the float does not stop the water, the valve itself is not sealing. Common causes are a worn washer or diaphragm, or scale or debris in the valve.
Close the isolation valve on the pipe to the cistern before working on the fill valve. It is usually a small valve with a slot, turned a quarter turn with a screwdriver. If there is no isolation valve, the main stopcock or the valve on the supply from a loft tank will do. Fill valves are inexpensive and many can be serviced or replaced by a competent householder, but a plumber can do it quickly if you are unsure.
How do you check the flush valve?
If the water level is below the overflow and water still trickles into the pan, the seal at the bottom of the flush valve is the usual suspect. Signs include:
- ripples or a thin stream of water running down the back of the pan;
- the cistern refilling briefly on its own at intervals;
- a flush that does not fully stop.
With the water isolated and the cistern flushed empty, the seal at the base of the flush valve can be inspected. Scale, grit or a hair on the seal can stop it sealing. A seal that is worn, distorted or hardened needs replacing. On many push-button valves, the seal and the valve can be removed without taking the cistern off the wall, but designs vary, so check what you have before buying parts.
Siphon cisterns work differently. Water passes a siphon only when it is flushed or when the level rises too high, so a lever toilet that keeps running into the pan is more likely to have a fill valve fault sending water into an internal overflow.
How do you do a dye test on a toilet?
A dye test shows whether water is leaving the cistern into the pan when it should not.
- Let the cistern fill and stop running.
- Add a few drops of food colouring or a dye tablet made for the purpose to the water in the cistern. Avoid anything that could stain the pan.
- Do not flush. Leave the toilet alone for about half an hour or more.
- Look in the pan. Coloured water in the pan means water is getting past the flush valve or into an internal overflow.
- Check the level. If the level is at the top of the overflow tube, the water is going into the overflow and the fill valve or float is at fault. If it is below the overflow, the flush valve seal is at fault.
- Flush afterwards to clear the dye.
How much water can a running toilet waste?
It depends on the size of the leak, and a trickle that is barely visible can still add up over days and weeks, because it never stops. On a metered supply, that water is billed like any other. A running toilet also keeps the fill valve working constantly, which wears it faster.
A meter test is a simple way to see whether a toilet is part of a high reading. With everything else off, read the meter, then close the isolation valve to the toilet and read it again after the same period. Our guide to unexplained high water usage explains the full routine.
What if the cistern is leaking rather than overflowing?
Water on the floor around a toilet is a different problem. Check:
- The inlet connection, where the supply pipe or flexible hose joins the fill valve under the cistern.
- The fill valve backnut, where the valve passes through the cistern.
- The close-coupling seal and bolts between the cistern and the pan, on close-coupled toilets.
- The flush pipe connections, on toilets with a separate or concealed cistern.
- The pan connector, which joins the pan outlet to the soil pipe and leaks only when the toilet is flushed.
- Condensation on the outside of the cistern, which can drip steadily in cold weather.
Dry everything and use kitchen towel under each connection, then check it after the cistern refills and again after a flush. A toilet with a concealed cistern behind a panel or wall can leak for a long time without showing. If the damp is appearing on a ceiling below, our guide to a damp ceiling patch with no obvious leak covers other possibilities.
If the overflow running outside comes from a loft tank rather than a toilet, the fault is in the tank's ballvalve. Our guide to loft tank and pipe leaks explains that.
When should you call a leak detection specialist?
A running toilet is a plumbing repair. A specialist is useful when:
- the toilet has been fixed and the meter still moves with everything off;
- a concealed cistern or pipework is leaking behind a wall or panel;
- damp is appearing below or beside a toilet and no connection you can see is wet;
- you need readings and a report for a landlord or insurer.
Our water leak detection uses meter and pressure testing, thermal imaging and moisture mapping. A combined detection package is £250 to £500. Call 020 3912 0200 to talk it through.
Frequently asked questions
Why is my toilet overflow pipe running outside?
Usually because the fill valve in the cistern is not shutting off, or the float is set too high, so the water rises past the overflow. Lifting the float shows which.
Why does my toilet keep running into the bowl?
Most often the flush valve seal is not sealing, or the cistern has an internal overflow and the fill valve is overfilling it. A dye test and a look at the water level separate the two.
How do I stop my toilet from running?
Close the isolation valve to the cistern. Then adjust or service the fill valve if the level is too high, or clean or replace the flush valve seal if water is passing into the pan.
How do I know if my toilet is leaking?
Add food colouring to the cistern, do not flush, and check the pan after about half an hour. Coloured water in the pan means the cistern is leaking internally.
Can a running toilet increase my water bill?
On a metered supply, yes. Water that runs continuously into the pan or out of the overflow is recorded by the meter like any other use.
