How to
Toilet keeps running? How to find the fault
Lift the cistern lid. If the water is up at the overflow, the fill valve is not shutting off; if it is below and water trickles into the pan, the flush valve seal is worn. Both are cheap parts.
Why it is worth fixing now
A toilet that runs continuously can waste 200 to 400 litres a day. On a water meter that adds to the bill every hour, and on a modern cistern you may not notice, because the overflow empties into the bowl rather than out through a pipe in the wall.
The quick test is food colouring. Put a few drops in the cistern, do not flush, and come back in twenty minutes. Colour in the pan means water is getting past the flush valve. A dry piece of toilet tissue held against the back of the bowl will also show a thin, steady trickle.
Checking the water meter
If you are not sure whether the toilet is the only leak, the water meter will tell you. Turn off every tap, make sure the washing machine and dishwasher are not running, and that nobody flushes. Read the meter, wait half an hour, and read it again. If it has moved, water is leaving somewhere. Turn off the isolating valve on the toilet and repeat. If the meter now stays still, the toilet was the leak. If it still moves, the leak is on the pipework, and that needs tracing.
Two valves, two different faults
Inside every cistern there are two working parts. The fill valve lets water in from the supply and shuts off when a float rises to the right level. The flush valve sits in the base of the cistern and lets water out into the pan when you press the button or pull the handle, then reseals.
Which one has failed decides the fix, and lifting the lid tells you:
| What you see | Likely fault | Usual fix |
|---|---|---|
| Water level up at the overflow tube | Fill valve not shutting off | Adjust the float, clean or replace the valve |
| Level below overflow, trickle into pan | Flush valve seal leaking | Replace the seal or the valve |
| Cistern refills briefly every so often | Slow flush valve leak | As above |
| Water running out of a pipe outside | Fill valve, on an older external overflow | Adjust or replace the fill valve |
Fixing a fill valve
Turn the water off first. Most toilets have a small isolating valve on the pipe into the cistern, turned with a flat screwdriver so the slot sits across the pipe. If there is none, use the stopcock.
On many modern fill valves, the water level is set by a screw or a clip on the float. Lower it a little so the valve shuts off below the overflow. If the valve still lets water through, grit from the supply may be holding it open: most can be opened, the small diaphragm washer rinsed, and the valve reassembled. If it is old, cracked or still passing, replace it. Note whether yours enters through the bottom or the side of the cistern before buying, because the two are not interchangeable.
Fixing a flush valve
Push button cisterns usually have a drop valve, a plastic column that lifts to flush. On many, the column twists or unclips off its base without tools once the water is off and the cistern emptied, and the rubber seal underneath can be replaced. A seal that is flattened, split or carrying limescale is the usual cause of a trickling pan.
Older lever cisterns use a siphon. The fault there is usually the flap valve, a thin plastic diaphragm that wears and lets water through, and the classic symptom is having to pump the handle to get a flush.
When to call a plumber
Some cisterns make a simple part change awkward. On a close coupled toilet, where the cistern sits directly on the back of the pan, replacing the whole flush valve often means lifting the cistern off and renewing the large rubber seal between the two. A concealed cistern behind a panel or wall needs the access plate removed and sometimes a specific make of part. A cracked cistern, a seized isolating valve, or a leak at the joint to the pan are all jobs for a plumber.
A cistern repair, either valve, typically costs £80 to £180. Replacing a close coupled WC like for like is £150 to £350 in labour.
Buying the right part
Take a photo of the inside of the cistern and the name moulded on the valves before you go to a merchant, or take the old part with you. Universal fill valves fit most cisterns. Flush valves are less forgiving, especially on dual flush buttons, where the button has to match the valve. A part that matches exactly keeps a running toilet from turning into a leaking one.
Common questions
Why does my toilet refill on its own every few hours?
Water is seeping past the flush valve seal into the pan. When the level falls far enough, the fill valve opens to top it up. Replacing the seal stops it.
How much water does a running toilet waste?
Commonly 200 to 400 litres a day, which is six to twelve cubic metres a month. Metered water and sewerage are both charged by the cubic metre, so the cost shows up on the next bill.
Is a running toilet an emergency?
No. Turn off the isolating valve on the supply pipe and the toilet will stop running; you can still flush it by filling the cistern with a bucket until it is repaired.