Toilet Refill Valve Leaking

Is it the refill valve or the flush valve?

A toilet that trickles into the bowl, hisses, or fills itself up every few minutes has one of two faults. Either the refill valve is not shutting off, so water keeps coming in and escapes through the overflow, or the flush valve at the bottom of the cistern is not sealing, so water keeps leaking out into the pan and the refill valve tops it up. The symptoms look almost the same, but the parts are different.

The test takes five minutes. Take the cistern lid off and look at the water level. If it is right up at the top of the overflow tube, spilling into it, the refill valve is the problem. If the level is below the overflow but water is still running into the bowl, turn off the isolation valve on the supply pipe, mark the water level with a pencil and wait twenty minutes. If the level drops, the flush valve seal is letting water out, which is covered in a toilet leaking into the bowl.

On an older cistern with an overflow pipe through the outside wall, a refill valve that will not shut off shows as a drip or a steady run from that pipe. On most modern cisterns the overflow goes into the pan instead, so you see a constant ripple in the bowl.

First check: the float and the water level

Every refill valve shuts off when its float rises to a set point. If that point is above the top of the overflow, water reaches the overflow before the valve closes and it will run forever. Most cisterns have a fill line moulded or printed on the inside back wall. The water should stop at that line, which is always a little below the overflow.

On a modern vertical valve the float is a plastic cup that slides up and down the valve body. Its height is set by a screw on the top or a clip on the side. Turn the screw or move the clip down a notch to lower it (the direction is often marked on the valve, and it differs between makes), flush, and let the cistern refill. On an older side-entry ball valve with an arm and a round float, there is usually an adjusting screw on the arm. If there is not, the arm on an old brass valve can be bent down very slightly, but a plastic arm will snap.

While the lid is off, make sure the float moves freely. A float that rubs against the cistern wall, the flush valve or its own chain can stick part-way and hold the valve open. A float that is full of water, which happens with old ball floats, sits low and will never close the valve. Shake it: if it sloshes, replace it.

Second check: grit and a worn seal in the valve head

If the float is set correctly and moves freely but water still hisses in, the seal inside the valve head is not closing. On a diaphragm valve the seal is a rubber disc pressed onto a small nozzle. A speck of grit or scale on either surface is enough to let water past, and after a few years the rubber hardens and takes a groove.

Many modern refill valves have a top cap that twists or unclips off, giving access to the seal without taking the valve out of the cistern. With the isolation valve off, remove the cap and lift out the seal. Rinse the seal and look at it: a smooth, soft seal can go back; a grooved or hard one needs replacing. Before refitting, hold a cup upside down over the open valve and turn the isolation valve on for a couple of seconds. The burst of water flushes grit out of the inlet. Refit the seal and cap and test.

Spare seals and diaphragms are sold for the common valves, typically for a few pounds, but they must match the make and model. Look for the maker’s name moulded on the valve body. For an old brass ball valve, a new washer can help, but the honest answer is usually a new valve.

Pressure: why some valves never quite shut off

Refill valves are built for a range of pressures. A valve designed for low pressure, from a tank in the loft, can be forced open by full mains pressure and will hiss or creep. A high-pressure valve on a low-pressure tank feed fills painfully slowly. Some valves come with a small insert or setting for high or low pressure, and fitting the wrong one is a common reason a new valve still leaks.

If your mains pressure is high, you may also hear a bang in the pipes as the valve shuts. That is water hammer. Each bang shakes the pipework and its joints, and it wears the valve seal faster. A plumber can fit a slower-closing valve or check the pressure at the stopcock.

Replacing a refill valve that will not stop leaking

If adjusting and cleaning have not stopped it, replace the valve. A universal bottom-entry or side-entry refill valve typically costs £8 to £25 at a merchant. Check which entry type your cistern has, and whether the supply comes from the mains or a tank, before you buy.

The job is: turn off the isolation valve, flush and hold the handle to empty the cistern, sponge out what is left, undo the supply connector and the plastic back nut, lift the old valve out, fit the new one with its seal inside the cistern, hand-tighten the back nut, reconnect the supply and set the float to the fill line. If the new valve leaks at the joints below the cistern rather than inside it, see a toilet inlet valve leaking.

If you would rather not, our toilet repair visit starts with the £60 call-out at any hour, and the repair price is agreed with you on site before any work starts. As a typical market guide, a refill valve replacement including a standard valve usually costs in the region of £80 to £150. If the cistern is also leaking at the base, a leaking toilet cistern explains the other parts worth changing while it is empty.