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Introduction


Often considered the most overworked part of the plumbing system, all of us have at least�one cistern in our homes as a part of the wc suite (toilet) in the bathroom. Apart from toilet cisterns, modern cold water storage tanks, often located in the loft space, are technically a cistern too.

The mechanisms in a cistern are probably working at least once a day, therefore over time some servicing is invariably required to keep it functioning correctly.





Nearly all types of wc cistern are relatively easy to service, the only exceptions are concealed cisterns, where access may prove tricky, and high level cisterns as a step ladder is needed.

To all intents and purposes, cold water storage tanks, located in the loft space, are a cistern too. Short of a leak (or a puncture) the only occasional form of maintenance is to ensure the float valve is working correctly - if not then the tank will overfill and leak, or not fill at all meaning no cold water from the bathroom taps.

Before commencing work on the cistern or storage tank, make sure you have all the tools you need to successfully complete the job. Use the following link for a list of basic plumbing tools:


Repairing & replacing cistern float valves


A Float Valve Diaphram

faulty float/ball valve is more often than not the cause of the majority of problems associated with cisterns and storage tanks. Older valves use a washer to stop the flow of water, whereas modern versions use a diaphram instead which is less susceptable to scale deposits.


The float valve allows water to enter the cistern, as the water level rises so does the hollow float/ball, at a predefined point the float reaches it's highest level and closes the valve which shuts of the water supply.

If your float valve is old, the best advice is to replace it with a modern diaphram valve. These are quite inexpensive and readily available from a plumbers merchants.

Installing a new diaphram valve

  • Isolate the cistern in the plumbing system, or if there is no isolating valve turn off the mains water supply

  • Drain a toilet cistern by flushing the toilet

  • For a water storage tank, lower the water level by running a cold bathroom tap until the water has dropped at least by 6 inches

  • Disconnect the wall inlet pipe (or rising main in the loft) from the valve, and move a few inches to one side

  • Using two spanners, disconnect the entire float valve assembly from the cistern

  • Remove the float/ball from the end of the old valve and connect to the new one

  • Take one of the fixing nuts off the threaded shaft of the new valve assembly

  • Push the assembly through hole in the side of the cistern, and screw on the fixing nut to the protruding threaded shaft

  • Secure the float valve assembly in place using two spanners, and reconnect the inlet pipe/rising main

  • Turn on the water supply and watch closely for leaks as the cistern fills


Replacing a diaphram valve

  • Isolate the cistern in the plumbing system, or if there is no isolating valve turn off the mains water supply

  • Unscrew the large retaining cap on the valve, located inside the cistern at the end of the float arm

  • Depending on the particular model the retaining nut will be infront of or behind the valve

  • If the retaining nut is at the front then you will see the diaphram assembly immediately behind the cap

  • The the retaining nut is at the rear then slide out the cartridge assembly to locate the diaphram behind it

  • Before installing a new diaphram and assembling the valve, clean all the parts in cold water to remove any scale

  • Reassemble the valve and reconnect the water supply


Adjusting the water level in the cistern


A Float Valve

The water level in a wc cistern/water tank is controlled by adjusting the float/ball on the float valve assembly. The lower the float when the valve is closed the less water is allowed in the cistern.


  • Ideally the water level should be approximately 25mm (1in) below the overflow outlet.


This can be adjusted in a number of ways depending on the type of float valve present.

  • If the float arm involves a 90� kink at the end, and the float/ball is attached with a thumb screw to the arm, the height of the float can be adjusted by loosening the thumb screw and lowering/raising the float on the arm.

  • Diaphram valves often contain an adjustment screw where the float arm is attached to the valve assembly. The further the screw is in, the lower the float.

  • If the float/ball is attached directly a solid metal rod, the float can be lowered/raised by bending the arm down or up


Faulty flap valves


If a wc cistern will not flush, or requires the lever to be depressed a few times, then the chances are that the flush valve needs replacing.


A Flap Valve Assembly

The flap valve sits at the base of the cistern, and is operated by depressing the handle on the front of the toilet. When depressed, the lever lifts a flexible plastic diaphram (the flap valve) which allows the water in the cistern to flow through the siphon trap into the pan. As the water level lowers in the cistern, so does the flap valve until it closes over the plate, blocking the siphon and thus stopping the flow of water. As the water level rises again in the cistern the flap valve stays in place.


Replacing the flap valve

  • Isolate the cistern in the plumbing system, or if there is no isolating valve turn off the mains water supply

  • Drain a toilet cistern by flushing the toilet

  • Disconnect the flushing arm inside the cistern

  • Using a wrench, unscrew the large nut holding the flush pipe onto the base of the cistern

  • Move the flush pipe slightly to one side, place a bucket underneath and unscrew the retaining nut which holds the siphon to the base of the cistern

  • Lift the siphon assembly out of the cistern and take the diaphram off of the metal plate

  • Replace the worn diaphram with an identical sized new one

  • Reassemble the flusing mechanism and attach the flush pipe

  • Turn on the water supply and watch closely for leaks as the cistern fills


Curing a noisy cistern


A Bottom Entry Float Valve Assembly

A cistern which makes a lot of noise when filling can be particularly annoying when located in a bathroom next to a bedroom. The noise is often simply caused by the flow of water, however in some instances, if the water pressure entering the cistern is particularly high, the float/ball can bounce on the surging water and send banging noises along the plumbing system.


The noise can be cured in two ways:

  • Install a float valve assembly which has bottom entry. This means the water in the cistern fills from the bottom rather than pouring from the top.

  • Install a flexible silencer tube to the outlet pipe from the float valve. This tube is just short of the length of the cistern, and means the water fills silently from below.


  • If installing a silencer tube, make sure the tube is made from soft rubber/plastic. This means that it will collapse back on itself if siphoning with the plumbing system should occur. Solid tubing is not permitted by the water board.