Fix a toilet that won't flush properly

Diagnose why your toilet won't flush, fix the siphon or valve mechanism, adjust the water level, and test it works.

TF-01Turn off the water and drain the cisternFind the isolation valve on the pipe feeding the cistern, usually low down behind or beside the toilet, and turn it clockwise until it stops. If there's no isolation valve, turn off the mains stopcock, usually under the kitchen sink, and check it actually turns before you start. Flush the toilet to empty most of the water, then sponge out what's left in the cistern.

1 each · Old towel or sponge

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1 each · Small bucket

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TF-02Lift the lid and identify your cistern typeTake the cistern lid off carefully and set it somewhere safe. Older toilets with a single flush lever usually have a siphon unit — a tall plastic tube in the middle with a diaphragm or plate that pushes water over a bend. Modern toilets with push buttons usually have a flush valve instead — a shorter unit with a rubber seal (flapper) that lifts when you press the button. This matters because the two types are fixed differently.

TF-03Siphon type: check the diaphragm washerIf you have a siphon unit and the flush is weak or does nothing, the problem is almost always a worn or split diaphragm washer inside it. Undo the nut connecting the siphon to the flush handle or lever, then lift the siphon unit out of the cistern. Slide off the retaining plate at the bottom and check the plastic or rubber washer for splits, warping or a loose fit.

TF-04Fit a new siphon washer and reassembleBuy a replacement washer that matches your cistern make and size — take the old one to a plumbers' merchant if you're unsure. Fit the new washer onto the plate, push it back onto the siphon tube, and lower the whole unit back into the cistern. Reconnect the nut to the flush handle or lever, making sure the linkage moves freely.

1 each · Siphon diaphragm washer kit

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TF-05Valve type: check the flush valve seal and button linkageIf you have a push-button flush valve, press the button and watch what happens inside the cistern. If the valve doesn't lift properly, check the cable or plastic link running from the button down to the valve — it can slip off or snap. If it lifts fine but the toilet still won't flush well, or keeps running afterwards, the rubber seal on the valve is likely worn.

TF-06Replace the flush valve sealTurn the water off again if you haven't already, and drain the cistern. Unclip the old valve unit or just the rubber seal, depending on your toilet's make, and take it with you to buy an exact match. Fit the new seal or valve, reconnect the push-button linkage, and make sure it clips back together securely.

1 each · Flush valve seal or replacement valve unit

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TF-07Check and adjust the water levelMost cisterns have a water level mark moulded inside. If the level sits below this mark, the flush won't have enough water to work properly. Adjust the float — either bend the float arm slightly upwards on an old-style ballcock, or turn the adjustment screw on a modern float cup — to raise the level, and lower it the same way if it's overfilling near the overflow.

TF-08Check the rim jets and pan for limescaleA weak swirl even with a working mechanism often means limescale blocking the small holes under the rim of the pan. Use a small mirror and torch to check them, and clear any blockages with a limescale remover and an old toothbrush or wire, following the product's instructions. This is especially common in hard water areas.

1 bottle · Limescale remover

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TF-09Turn the water back on and testTurn the isolation valve or stopcock back on slowly and let the cistern refill. Flush the toilet two or three times, checking each time that it flushes fully and the water level settles at the mark without running on. Check around the base of the siphon or valve, and around the isolation valve, for any drips.

TF-10Tidy upWipe down the cistern, lid and surrounding floor with a dry cloth. Dispose of the old washer, seal or valve unit in general waste, not down the toilet. Dry your tools and wash your hands before putting everything away.

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