Isolate the water, swap the worn fill valve, then test and adjust it so the cistern refills correctly without leaks.
FV-01Turn off the water and empty the cisternClose the isolation valve on the pipe feeding the toilet. If there isn't one, turn off at the stopcock, usually under the kitchen sink. Flush the toilet and hold the handle down to empty as much water as you can, then soak up what's left with a sponge or old towel. Keep a bucket handy to catch drips as you work.
FV-02Disconnect the supply pipeUndo the nut connecting the supply pipe to the base of the fill valve, underneath the cistern. Hold the valve steady from inside the cistern with one hand while you turn the nut with an adjustable spanner. A little water will still drip out, so keep a bowl underneath.
FV-03Remove the old fill valveReach into the cistern and unscrew the large locknut holding the valve to the base, turning it anticlockwise. Lift the old valve out through the top. Check the rubber washer underneath and note how the old valve sat, so the new one goes in the same way.
FV-04Fit the new fill valvePush the new fill valve down through the hole in the cistern base, seating its washer as you go. Tighten the locknut by hand from underneath, then snug it up a little more with the spanner — don't overtighten, as cisterns crack easily. Reconnect the supply pipe to the base of the valve, using a new washer or a wrap of PTFE tape if the connection is threaded.
FV-05Set the water levelTurn the water back on slowly and watch both connections for leaks. Most fill valves have an adjustable float on an arm or a screw on top — follow the marking or instructions supplied to set the water level around 25mm below the top of the overflow pipe.
FV-06Test, flush and fine-tuneLet the cistern fill fully, then flush and check it refills properly and shuts off cleanly without hissing or dribbling. Check under the cistern again for any drips at the connections. If the water level is off, adjust the float and flush again to recheck.
FV-07Tidy up and final checkWipe down the cistern and floor with a dry cloth. Dispose of the old fill valve. Come back after a few hours and check the connections again — a slow drip often only shows up once everything has settled.
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