Replacing a toilet fill valve (ballcock)

The fill valve (or ballcock) refills the cistern after flushing. Signs of failure: slow filling, constant running, water overflowing into the pan, or a noisy fill cycle. Replacement fill valves cost £8–£20 and the job takes under an hour.

Turn off the water at the isolation valve (a slot-head screw on the supply pipe below the cistern — turn 90° to close). Flush the toilet to empty the cistern, then use a sponge or towel to mop up any remaining water.

Disconnect the supply pipe from the base of the cistern: unscrew the plastic or brass nut connecting the supply pipe to the fill valve inlet (this is below the cistern, through the ceramic). Have a towel ready for any residual water.

Inside the cistern, unscrew the large locknut at the base of the fill valve (accessible from inside the cistern, turning anti-clockwise). The fill valve can then be lifted out.

Fit the new valve: adjust it to the correct height (most modern valves have a telescopic adjustment so the critical water line sits just below the overflow level — usually 25mm below the cistern overflow pipe). Insert through the cistern base hole, fit the sealing washer and locknut, and hand-tighten securely from below.

Reconnect the supply pipe, turn the water back on, and allow the cistern to fill. Check the float adjustment — the water level should stop about 25mm below the overflow pipe. Most modern valves have a float adjustment screw or clip. Test by flushing several times and checking for any drips at the supply connection.