Replacing a toilet syphon
A faulty toilet syphon is one of the most common reasons a toilet flush stops working properly — you'll notice the cistern either won't fill, won't empty, or produces a weak or incomplete flush. It's a problem that affects countless UK homes, particularly older properties where syphons have simply worn out over time. The good news is that replacing a syphon is a straightforward DIY job that most homeowners can tackle in under an hour with basic tools, and it'll save you a decent call-out fee. This guide walks you through the process step by step, from isolating the water supply through to testing your repair. If you're not confident working inside the cistern, or if you discover other issues once you've opened it up, it's worth calling a qualified plumber to take a look.
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Turn off the water supply to the cistern using the isolation valve on the supply pipe, or the main stop tap.
Flush the toilet to empty the cistern, then soak up remaining water with a sponge.
Disconnect the cistern from the toilet pan (two wing nuts under the rim) and the water supply. Lift the cistern off.
Undo the large nut at the base of the syphon inside the cistern and remove the syphon.
Fit the new syphon — they're sold by height (9", 9½", or 10.5" are common). Reconnect the cistern and water supply.
Turn the water supply back on and check for leaks. Test flush several times.
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Frequently asked questions
How do I know if my toilet syphon is broken?
A failing syphon usually shows itself through a weak flush, the cistern not emptying fully, or water running continuously into the pan. You might also hear unusual gurgling or hissing sounds from the cistern. If the flush works intermittently or requires multiple attempts to clear the pan properly, the syphon is almost certainly at fault.
Can I repair a syphon or do I need to replace it?
Most syphons cannot be economically repaired — replacement is the standard solution and usually cheaper than calling an engineer. Replacement syphons are inexpensive (typically £15–£40) and widely available from plumber's merchants, DIY stores, and online retailers. Unless you're handy with seals and internal mechanisms, replacement is always the best option.
How do I know which syphon size to buy?
Syphons are measured by height, with 9 inches, 9.5 inches, and 10.5 inches being the most common sizes in the UK. The easiest way to get the right one is to take your old syphon with you to the plumber's merchant, or measure the height inside your cistern from the base to the overflow pipe — the syphon must be slightly shorter than this measurement.
What should I do if water leaks after I've refitted the cistern?
Check that the large nut securing the syphon base is tightened properly — hand-tight is usually sufficient, but you can use a spanner for a firm grip. Also ensure the rubber seal around the syphon base is seated correctly and isn't damaged or twisted. If leaking persists, turn off the water, remove the cistern again, and check that the seal is properly positioned before refitting.