Replacing a toilet fill valve (ballcock)
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Most likely cause & what to check
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.
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Frequently asked questions
How do I know if my fill valve needs replacing?
Common signs: the cistern takes more than 5 minutes to refill after flushing, water runs continuously with a hissing sound, the cistern overflows into the pan, or the fill mechanism makes a loud noise. A fill valve replacement costs £8–£20 in parts and is a straightforward DIY task.
Can I replace a fill valve without a plumber?
Yes — replacing a fill valve is a standard DIY toilet repair. Turn off the supply at the isolation valve under the cistern, flush to empty, then disconnect the supply pipe and unscrew the locknut holding the valve in place. Fit the new valve, reconnect, and adjust the float to the correct water level.
What is the difference between a fill valve and a syphon?
The fill valve (ballcock) refills the cistern with water after flushing. The syphon (or flush valve) is the mechanism that releases the water when you press the handle. If the cistern is not refilling properly, replace the fill valve. If the flush is weak or the cistern drains without flushing, the syphon needs attention.