Fix a toilet cistern that's slow to fill

Diagnose and fix a slow-filling toilet cistern by checking the valve, filter and float, replacing the fill valve if needed.

CF-01Turn off the water and empty the cisternFind the isolation valve on the pipe feeding the cistern, usually behind or beside the toilet, and turn it clockwise until it stops. If there isn't one, turn off the stopcock instead. Flush the toilet to empty the cistern, then hold the flush handle down to clear as much water as you can.

CF-02Dry out and lift the cistern lidSoak up the last of the water with an old towel or sponge so you can see clearly. Lift the cistern lid off carefully and set it somewhere safe. Have a torch handy, as the inside of the cistern is often dim.

1 each · Old towel or sponge

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1 each · Torch

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CF-03Check the isolation valve is fully openA part-closed isolation valve is the most common cause of a slow fill. Turn the valve fully anticlockwise and make sure it stops against a hard limit. Turn the water back on briefly and check the fill speed before going further.

CF-04Identify the fill valve typeModern cisterns use a bottom-entry or side-entry fill valve with a plastic float, not the old brass ballcock with a ball on an arm. Note which type you have, as it decides how you strip it down. Turn the water off again at the isolation valve before touching it.

CF-05Clean the fill valve inlet filterMost slow fills are caused by grit blocking the small filter inside the fill valve. Unscrew the cap or top of the valve by hand, lift out the filter or mesh, and rinse it under a running tap. Refit it the same way it came out and hand-tighten the cap.

CF-06Check the supply pipe for kinksFollow the flexible tap connector from the isolation valve to the fill valve and check it isn't kinked, twisted or squashed behind the cistern. A kinked pipe restricts flow even with everything else working. Straighten it gently by hand if you find a kink.

CF-07Turn the water back on and test the fillOpen the isolation valve fully and watch the cistern fill. It should reach the marked water level within about two minutes. Check around the fill valve and connector for drips while it fills.

CF-08Adjust the float if the level or speed is still offOn most modern valves, turn the adjuster screw on top or slide the float up or down its arm to set the water level about 20mm below the overflow pipe. Setting the float too low can make the fill seem slow even when the valve is fine, so check this before assuming the valve is faulty.

CF-09Replace the fill valve if it's still slow or worn outIf cleaning and adjusting hasn't fixed it, the valve itself is likely worn and needs replacing. Turn off the water, disconnect the flexible connector and the old valve, and fit a new one, wrapping the threaded tail with PTFE tape before connecting. Any fill valve fitted must be WRAS-approved, as this is a legal requirement under the Water Supply (Water Fittings) Regulations 1999.

RULES

1 each · WRAS-approved cistern fill valve

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1 roll · PTFE tape

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1 each · Adjustable spanner

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KNOW THE RULES

The Water Supply (Water Fittings) Regulations 1999 require any fill valve or fitting connected to the mains water supply to be WRAS-approved, to stop contaminated water being drawn back into the supply.

CF-10Refit the lid and check the flush cycleWipe down the inside of the cistern and refit the lid carefully so it doesn't press on the flush handle or float arm. Flush the toilet once fully and time the refill, it should be quiet and finish within about two minutes. Check underneath and behind the cistern for any drips before you finish.

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