How to Repair Toilet Cisterns


Niggling toilet problems, such as hit and miss flushing, have a habit of turning into something more serious. Even so, there are surprisingly few problems that can’t be fixed with just a few simple tools and the right spare part.

This section explains how flushing faults and leaks occur, and shows what can be done to fix them.


How toilets work

workings of a typical toilet cisternToilets are far from complicated, but knowing how they work makes it much easier to trace and rectify faults.

A modern toilet cistern is designed to deliver a measured quantity of water to flush the pan. This is done by storing the water in the cistern and then discharging it via a siphon unit operated by a lever or button. The siphon only works when the cistern is full, ensuring the right amount of water is delivered.

Flushing causes a lifting plate and diaphragm inside the siphon to rise, drawing water up and then down again into the flush pipe. From then on, the siphoning action takes over, bending the diaphragm upwards and drawing the rest of the water from the cistern into the flush pipe.

When the cistern empties, the siphoning action ceases, the diaphragm falls, and the ballvalve controlling the water inlet admits a fresh supply of water.


Variations on a theme

The description above applies to a washdown pan, in which the flushing water simply displaces the water in the pan trap. Modern washdown pans have a dual position siphon, to save water: pressing the lever or button and then releasing it delivers a part-flush; holding the lever down delivers a full flush.

More sophisticated toilets work on the siphonic principle and usually incorporate a double trap pan. In this case, a pressure reducing (‘puff’) pipe attached to the siphon sucks air out of the chamber between the traps as the diaphragm in the cistern rises. This causes the pan contents to empty before the flushing water enters, giving quieter operation.



washdown toilet panWashdown: the most common type, in which water from the cistern enters through the flush pipe and displaces the contents of the trap.

single trap siphonic toilet panSingle trap siphonic: the narrow trap outlet creates a partial vacuum which sucks out the contents as fresh water enters. Quieter in operation.

double trap siphonic toilet panDouble trap siphonic: flushing creates a vacuum in the chamber between the two traps, sucking out the contents silently as fresh water enters.

high level toilet cisternHigh level: the cistern and pan are connected by a long flush pipe – usually of metal, now sold as two-piece plastic.

low level toilet cistern

Low level: the plastic cistern is connected to a separate pan via a length of plastic or metal flush pipe.

close coupled toilet cisternClose coupled: the ceramic cistern is joined directly to the pan, both being sold as a matching set.



Shopping List

As with other plumbing repairs, it’s often difficult to buy replacement parts for toilets without first dismantling the mechanism concerned and then taking the part along to a plumbers’ merchant to be matched. Note the make (and if possible the model) of the cistern.

This means thinking ahead: carry out repairs when you know the shops are open, and when the rest of the family won’t be too upset by the water being turned off.

In an emergency, you can usually improvise something (ie. stiff wire for the lever mechanism, or plastic sheet for a torn diaphragm), but short-term remedies are unlikely to stand much wear and tear.

Tools checklist: Adjustable wrench, slip-joint pliers, PTFE tape.

24. May 2011 by admin
Categories: Plumbing, Toilets | Tags: | Comments Off on How to Repair Toilet Cisterns


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