Why does my toilet keep running or refuse to stop filling?

By far the most common reason a toilet keeps running is a worn or scaled fill valve (sometimes still called a ballcock, though most modern cisterns use a diaphragm-type valve on a vertical stem). Over years of use the washer or diaphragm inside hardens, and limescale builds up on the moving parts, so the valve does not seal fully once the cistern reaches its intended level. Water then keeps trickling in, usually escaping down the overflow pipe.

You will often hear this as a faint hiss or gurgle that never quite stops, sometimes only noticeable at night when the house is quiet. Lift the cistern lid and watch the water level: if it is at or above the overflow line and still rising, or if you can see water dribbling from the valve itself even at the correct level, the fill valve is the prime suspect.

The second most common cause is simpler still: the float arm or float cup is set too high, so the valve is being told to keep filling past the point where the overflow (the small pipe standing inside the cistern) starts taking water away. This can happen after someone adjusts the float, after a cistern is refitted, or just from vibration over time.

Most modern fill valves have a small adjustment screw or a float that slides up and down a threaded rod. Lowering it by a centimetre or two, then flushing and watching where the water settles relative to the overflow marking (usually printed inside the cistern), often solves the problem in minutes with no parts needed.

If the cistern fills to the correct level and stops, but you still hear intermittent refilling every few minutes, the fault is usually not the fill side at all but the flush valve at the bottom of the cistern (the part the flush handle or button operates). A worn seal, a perished diaphragm washer, or a bit of grit or limescale trapped under the seat lets a small amount of water leak past into the pan. As the cistern level drops, the fill valve senses it and tops up again, giving a repeating cycle rather than a constant hiss.

The classic way to confirm this is the dye test: add a few drops of food colouring or a dab of washable paint to the cistern water, wait fifteen minutes without flushing, and check the pan. If colour has appeared in the pan, water is passing the flush valve seal.

Toilets with a push-button dual flush mechanism have an extra failure point: the button or the plastic cable/rod that lifts the flush valve can jam very slightly out of its rest position, holding the valve open just a fraction. This gives a similar symptom to a worn flush valve seal but the fix is different, usually re-seating or replacing the button mechanism rather than the seal itself. Give the button a firm press and release to check it returns fully, and look for any sign of the cable linkage being kinked or trapped under the lid.

Isolate the water at the small service valve on the pipe feeding the cistern (or the main stopcock if there is no isolation valve), then flush to empty the cistern before doing any work. Fill valve and flush valve repair kits for standard UK cisterns are inexpensive and widely available, and swapping either is a reasonably straightforward DIY job if you are comfortable with basic plumbing: undo the tail nut, lift out the old valve, fit the new one and reconnect.

Where it gets trickier is older concealed or cistern designs, cisterns integrated into furniture, or valves with no readily available replacement part. If you are not confident isolating the water supply, or the cistern lid or mechanism will not come apart cleanly, it is worth calling in a plumber rather than forcing anything and risking a crack.

A toilet that runs continuously can waste a genuinely large amount of water over a year, which shows up on a metered water bill even though the leak itself feels minor. It is worth fixing promptly rather than living with it. If your water supply is unmetered this is less of a cost issue, but a running toilet is still worth sorting since it points to parts that will only get worse.

If the cistern is old, the plastic has become brittle, or you find yourself replacing the same worn parts repeatedly within a year or two, it is often more sensible to replace the toilet or cistern outright rather than keep patching it. This is particularly true for older low-level or high-level cisterns where spare parts are getting harder to source.