- Time30 to 90 minutes
- DifficultyEasy to moderate
- Key stepIdentify where the water goes
You will need
- Replacement parts from tap, toilet and cistern repair: fill valve, float, flush valve seal or washer kit to suit your cistern
- Flat-head screwdriver for the isolation valve
- Adjustable spanner or grips, from plumbing tools and chemicals
- Sponge, towel and small bucket
- Phone camera to photograph the setup before you take it apart
Why toilets keep running
A toilet cistern has two main parts. The fill valve (also called an inlet valve or ballcock) lets water in after a flush and shuts off when a float rises to the right level. The flush valve (a drop valve in push-button toilets, or a siphon in traditional lever toilets) releases water into the pan when you flush. A constantly running toilet means one of these is not doing its job.
Step 1: Diagnose the fault
Lift the cistern lid carefully and set it somewhere safe, as ceramic lids break easily. Then watch what happens after a flush.
| What you see | Likely cause | Usual fix |
|---|---|---|
| Water keeps rising and runs into the overflow pipe, or drips outside from an overflow | Fill valve not shutting off, float set too high, or worn valve washer | Adjust the float; replace the washer or fill valve |
| Water trickles or ripples down the back of the pan constantly | Flush valve or siphon not sealing | Clean or replace the flush valve seal or washer |
| Cistern refills by itself every so often with a hiss | Slow leak through the flush valve | Replace the flush valve seal |
| Push button sticks or stays down | Button mechanism or cable fault | Free or replace the button or cable |
| Lever toilet needs several pumps to flush | Worn siphon diaphragm | Replace the diaphragm or siphon |
A useful check for a flush valve leak: once the cistern has filled and stopped, look at the water in the back of the pan. Constant rippling or a thin trickle shows water is passing the seal.
Step 2: Turn off the water
Find the isolation valve on the pipe feeding the cistern, usually a small fitting with a screw slot. Turn the slot a quarter turn so it sits across the pipe to shut off the water. If there is no isolation valve, or it does not work, turn off the water at the main stop tap, often found under the kitchen sink. Flush to empty the cistern and sponge out what is left.
Step 3a: Fix a fill valve that overflows
- Adjust the float. Most modern fill valves have an adjustment screw or clip that sets the float height. Lower it so the water stops at the fill line marked inside the cistern, or about 25mm below the top of the overflow.
- Clean or replace the washer. Grit and limescale can stop the valve sealing. With the water off, remove the valve cap, clean the parts and fit a new washer if it is worn.
- Replace the fill valve. If adjustment and a new washer do not help, fit a new fill valve. Note whether yours is bottom-entry or side-entry, and choose a matching replacement. Undo the supply connector and the back nut, swap the valve, and refit, tightening firmly without overtightening plastic threads.
Step 3b: Fix a flush valve that leaks into the pan
- Photograph the mechanism, then release the flush valve. Many push-button drop valves twist or unclip from their base without removing the cistern.
- Inspect the seal or washer at the bottom. Clean away limescale and debris from the seal and the seat it sits on.
- Fit a new seal of the correct type and size for your valve. Replacement seals are often specific to the valve make, so take the old one with you or check the markings.
- Refit the valve, making sure it seats squarely and the button or cable is correctly connected.
- If the seal is fine but the valve body is cracked or the mechanism is worn, replace the whole flush valve. Some types require the cistern to be removed; if so, consider calling a plumber.
Step 4: Restore water and test
- Open the isolation valve slowly and watch the cistern fill.
- Check all connections under the cistern for drips with a dry piece of tissue.
- Flush several times and confirm the water stops at the correct level and the pan stays still afterwards.
- Refit the cistern lid carefully.
When to call a plumber
Call a plumber if the isolation valve or stop tap will not turn, if fixing the fault means removing the cistern from the wall or pan and you are not confident, if there is a leak from the cistern-to-pan joint or the pan itself, or if the toilet is a concealed or wall-hung unit you cannot reach safely. Pipe fittings and spares can be found in pipe and fittings.
Safety: turn the water off before working on any valve. Handle ceramic lids and cisterns carefully, as they are heavy and break into sharp pieces. If your toilet has a pump or macerator, switch off and unplug it at the electrical supply before opening it and follow the manufacturer's instructions. Wash your hands after working on toilets and wear gloves if you prefer.
While the lid is off, it is a good time to check the toilet seat fixings too; replacements are in toilet seats.