Fix a toilet that flushes by itself

Find out why your toilet keeps refilling itself, then replace the worn syphon or valve seal causing the leak.

AB-01Confirm it's a phantom flushDry the inside of the pan with a cloth, then leave the toilet completely alone for 15-20 minutes. If water trickles into the pan on its own, or the cistern refills every so often with no one flushing, that's a phantom flush caused by a slow leak inside the cistern. Also check the overflow pipe outside the house isn't dripping - that's a different fault (a stuck float valve), not covered here.

AB-02Work out which flush mechanism you haveLift the cistern lid and look inside. A lever or handle on top usually means an older mechanical syphon, with a wide plastic tube standing in the middle of the cistern. A push button on top usually means a newer flush valve (sometimes called a drop valve or piston valve), a shorter unit with a rubber seal underneath. This tells you which spare part to buy before you start.

AB-03Isolate the water and empty the cisternFind the isolation valve on the pipe feeding the cistern - usually a small slotted screw or lever behind or beside the toilet - and turn it off. If there isn't one, turn off the stopcock instead, normally under the kitchen sink. Flush the toilet and hold the handle or button down to empty as much water as you can, then sponge out what's left.

1 each · Sponge or cloth

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AB-04Remove the lid and disconnect the inletLift the cistern lid off and put it somewhere safe - it cracks easily if knocked. Undo the tap connector nut on the water inlet pipe with an adjustable spanner and let any last water drain into a bucket. You don't need to take the whole cistern off the wall for this job.

1 each · Adjustable spanner

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1 each · Bucket

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AB-05Syphon type: replace the diaphragm washerUndo the large plastic nut underneath the cistern that holds the syphon in place, then lift the syphon unit up and out through the top. Turn it upside down and you'll see a thin rubber or plastic diaphragm washer clipped to the bottom plate - this is what perishes and lets water leak through into the pan. Fit the new diaphragm washer the same way the old one came off, lower the syphon back in, and tighten the nut by hand plus a small turn with the spanner. Don't overtighten - the plastic thread strips easily.

1 kit · Syphon diaphragm washer kit

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AB-06Push-button type: replace the flush valve sealLift the flush valve unit straight up out of its seating in the base of the cistern. Turn it over and check the rubber seal underneath - if it's stiff, split, or has grit stuck to it, that's your leak. Fit a new seal from a repair kit for your valve brand (Fluidmaster and Dudley are common), or replace the whole valve if the seal isn't sold on its own, then push it firmly back into place.

1 kit · Flush valve seal repair kit

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AB-07Reconnect and refit the lidReconnect the water inlet pipe and tighten the tap connector nut by hand plus a quarter turn with the spanner - don't force it. Refit the cistern lid, lining up the lever or button through its hole.

AB-08Turn the water back on and testOpen the isolation valve slowly and let the cistern fill. Check around the syphon or valve base and the inlet connector for drips while it fills. Flush once to check it works properly, then leave it dry and untouched for 20-30 minutes to confirm the phantom flush has stopped.

AB-09Tidy upDry up any spilt water around the cistern or floor to stop damage or a musty smell. Recycle or bin the old washer or valve. Make a note of the date - phantom flushes sometimes come back if grit gets under a new seal, so it's worth checking again in a year or two.

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