A toilet that keeps running almost always has one of two faults. Either the fill valve isn’t shutting off, so the cistern keeps filling until water escapes through the overflow, or the flush valve isn’t sealing, so water seeps into the pan and the fill valve keeps topping the cistern back up. Lifting the cistern lid and watching for a minute or two usually shows which one you’re dealing with — and that decides whether it’s a simple adjustment or a part that needs replacing.
If it will not stop with a simple adjustment, our toilet repair service covers fill valves, flush valves and concealed cisterns across East and South-East London.
How a Cistern Is Meant to Work
Two valves do all the work inside a toilet cistern. The fill valve (also called an inlet valve or ball valve) lets water in from the supply pipe and closes when a float rises to the set water level. The flush valve at the bottom of the cistern lets water out into the pan when you press the button or lever, then reseals so the cistern can refill.
An overflow is the safety route if the fill valve doesn’t close. In most modern cisterns it’s a tube inside the flush valve that sends excess water straight into the pan. In many older cisterns it’s a separate pipe that runs through the wall and drips or pours outside.
A running toilet means one of those three parts isn’t doing its job.
Lift the Lid: Five Safe Checks
These checks involve looking, not taking anything apart. Cistern lids are heavy ceramic, so lift it with both hands and put it down flat on a towel where it can’t be knocked.
- Look at the water level. If water is trickling over the top of the overflow tube, or the level sits right at its rim, the fill valve isn’t closing.
- If the level is below the overflow, watch the pan. A constant ripple at the back of the bowl, or the cistern quietly refilling every so often without anyone flushing, points to the flush valve leaking.
- Check the float. See whether it’s rubbing against the side of the cistern, caught on the flush mechanism, or sitting lower than it should because it has filled with water.
- Check the button or lever. A push button that hasn’t fully popped back up, or a lever arm or link that is catching, can hold the flush valve partly open.
- On older toilets, look outside. Water running or dripping from a small pipe on an outside wall usually comes from the cistern’s external overflow.
| What you see | Most likely culprit | Next step |
|---|---|---|
| Water at or over the top of the overflow tube | Fill valve not closing | Check the water level setting and the float; see below |
| Level below the overflow, but water trickles into the pan | Flush valve seal | Usually a seal washer or a new flush valve |
| Cistern refills by itself every few minutes or hours | Flush valve seal leaking slowly | As above; worth fixing even if it’s quiet |
| Button stays down or lever doesn’t return | Flush mechanism sticking | Often the button, cable or lever linkage |
| Pipe outside dripping or running | Fill valve on a cistern with an external overflow | Check level and float; the fill valve may need replacing |
| Slow, hissing refill that never quite stops | Worn fill valve washer or diaphragm, or debris | Clean or replace the fill valve |
If the bowl shows no movement at all but you still suspect a leak, our toilet repair and installation page describes a simple dye test that shows whether water is getting past the flush valve.
When the Fill Valve Is the Problem
The water level is set too high
If the float only shuts the valve once the water has reached the overflow, the cistern will run every time it refills. Most modern fill valves have an adjustment for this — a clip or screw on a float that slides up and down the valve body, or an adjusting screw where the float arm meets the valve on older ball valves. Lowering the level slightly, so it settles a little below the top of the overflow (many cisterns have a marked water line), is a reasonable first adjustment to try. If the level won’t stay where you set it, the valve itself is worn.
Grit, scale or a worn washer
Fill valves close against a small rubber washer or diaphragm. Over time that washer wears or hardens, scale builds up around it, or a particle of grit lodges under it — especially after work on the water main. The symptom is usually a faint hiss and a cistern that never quite stops filling. Some valves can be cleaned or have the washer replaced; often it’s more practical to fit a new valve.
The float is damaged or catching
An old-style ball float that has split will fill with water and sit too low to close the valve. A modern float cup can snag on the flush valve if the two are fitted too close together. Freeing a float that’s catching is a simple fix; a damaged float means a new part.
When the Flush Valve Is the Problem
Push-button cisterns
Most toilets with a push button or dual-flush button use a drop valve: a plunger that lifts off a rubber seal to flush and drops back down to close it. That seal can perish, collect scale or trap debris, and even a slight gap lets water run continuously into the pan. Sometimes the seal alone can be replaced; on other models the whole flush valve is changed.
Lever cisterns with a siphon
Many older lever-flush toilets use a siphon instead. A siphon has no seal at the bottom of the cistern, so it doesn’t leak in the same way. If a siphon toilet is running, the cause is usually the fill valve sending water over the internal overflow. The usual siphon complaint is the opposite: you have to pump the lever several times to get a flush, which normally means a worn diaphragm inside the siphon.
Buttons and cables that stick
A dual-flush button that sticks, a cable on a remote button that has stretched, or a lever linkage that catches can stop the flush valve dropping back fully. These are often the cheapest faults to fix, though button mechanisms tend to be specific to one make.
Concealed Cisterns: Same Faults, Harder Access
Back-to-wall and wall-hung toilets hide the cistern behind boxing or a wall frame. The same valves fail in the same ways, but reaching them is different. Many concealed cisterns are accessed by carefully removing the flush plate on the wall; others need an access panel. Forcing a flush plate can crack it or break the clips, and if the boxing has been tiled over with no panel, reaching the cistern may mean lifting tiles. This is one of the situations where it is worth getting a plumber to look before attempting anything.
Why It’s Worth Fixing Even When It’s Quiet
A running toilet rarely damages anything on its own, because the water goes down the drain. The cost is the water. WaterSafe, the UK register of approved plumbers, estimates that around 1 in 20 toilets in the UK leaks, and that a leaking toilet wastes between 200 and 400 litres of water a day — which matters most if your water supply is metered (WaterSafe: the leaky loo challenge).
WaterSafe also suggests a simple overnight check for leaks you can’t see or hear: place a dry square of toilet paper on the back of the bowl, below the rim, last thing at night. If it’s wet in the morning and nobody has flushed, water is running into the pan.
Living With It Until It’s Fixed
Most toilets have a small isolation valve on the pipe that feeds the cistern — usually a slotted screw you turn a quarter turn with a flat screwdriver. Closing it between uses stops the waste; opening it again refills the cistern when you need it. If there’s no isolation valve, the only way to stop the supply is the main stopcock, which turns off water to the whole property, so that’s a temporary measure at most.
Avoid tying the float arm up or wedging the float in place. If it slips free when nobody is around, the cistern can overflow — and on a cistern with no working overflow, that water ends up on the floor.
When It Becomes a Plumber’s Job
Adjusting the water level, freeing a float and checking a sticking button are reasonable things to try yourself. Other repairs are better left to a plumber, particularly when:
- the flush valve can only be replaced by taking the cistern off the pan, which means disturbing the cistern-to-pan seal and the fixing bolts on many close-coupled toilets;
- the cistern is concealed and there’s no obvious way in;
- the parts are specific to the make and you can’t identify the model;
- water is appearing on the floor as well as running into the pan;
- the cistern or pan has a crack, however fine.
Ceramic cisterns crack easily if bolts are overtightened, and a poorly reseated cistern-to-pan seal can leak slowly onto the floor. When the refill is simply very slow rather than continuous, the cause may be a part-closed valve or the supply itself — our guide to what causes low water pressure explains how to tell. If the water rises in the bowl when you flush rather than draining away, that’s usually a blockage, not a cistern fault; see how to unblock a toilet safely.
For a running toilet that won’t stop with a simple adjustment, Voltify Contractors’ toilet repair service covers fill valves, flush valves and concealed cisterns, and explains when a repair or a replacement toilet makes more sense. A photo inside the cistern with the lid off, and one of the flush button or plate, usually helps identify the parts before the visit.
Running toilet questions
Why does my toilet keep running after I flush?
Usually the fill valve is not shutting off when the cistern is full, or the flush valve is not sealing so water keeps seeping into the pan and the cistern keeps refilling. Lifting the lid and watching the water level for a minute or two shows which one it is.
Why does my toilet refill by itself when nobody has flushed?
That quiet, repeated refill usually means the flush valve seal is leaking slowly. The level drops a little, the fill valve tops it up, and the cycle repeats. It is worth fixing even though it is easy to ignore.
Does a running toilet affect my water bill?
It can, particularly on a metered supply. WaterSafe estimates that a leaking toilet can waste between 200 and 400 litres of water a day, even when it seems quiet.
Can I fix a running toilet myself?
Adjusting the water level, freeing a float and checking a sticking button are reasonable first steps. Replacing a flush valve, working on a concealed cistern, or any repair where water is reaching the floor is better left to a plumber.
Voltify Contractors repairs running toilets for homes and landlords across East and South-East London. See our toilet repair and installation page, or contact us with a photo of the inside of the cistern.
