A Toilet Cistern: How It Works

The toilet cistern, or the tank, is the part of your toilet that you may never notice, or think about, until a problem occurs.

If one of the parts inside the cistern malfunctions, the toilet will not flush properly.

The toilet cistern, or the tank, is the part of your toilet that you may never notice, or think about, until a problem occurs.  Typically the cistern, which stores water to be used when you flush your toilet, is mounted directly onto the toilet bowl, but it may also be concealed or connected by a pipe and mounted on the wall.

Internal Parts

A cistern has several parts inside, each with a specific task.  The flushing mechanism, which is usually a lever or a push-button, is located on the outside of the cistern and is connected to a chain inside.

The other end of this chain is connected to a flush valve, which is typically located on the bottom of the cistern.  There is also a float, which is a ball-shaped device connected to an arm.

The float is attached to a fill valve which is connected to the water supply line.  When you flush your toilet, all of these parts act to send the waste out of your toilet and refill it with fresh water.


When you press the flushing mechanism, the chain attached to it lifts the flush valve, which opens a drain that allows the water to empty from the tank into the toilet bowl and into the siphon hole which is located in the toilet bowl drain.  This creates a drawing effect, sucking both the water and the waste down the drain and out of your toilet bowl.


Once the cistern has emptied its store of water to flush away the waste in your toilet, the flush valve falls over the drain hole once more.  As this happens, the loss of water from the initial flush makes the float drop down, activating the filler valve it is attached to.

Water then flows through the supply line and back into the tank, raising the float.  When the float, which rests on top of the water, reaches the top of the tank, the filler valve shuts off and your toilet is ready for the next flush.

Dual-Flush Cisterns

A variation on the standard cistern is a dual-flushing cistern, which is designed to save water by giving you the option of flushing either all or part of the water stored in the toilet cistern.  Dual flush systems have two flushing mechanisms.

One is used to flush the toilet fully, as you would with a standard toilet, and the other flushes only half the water from the cistern.  The half flush is ideal when flushing fluid waste, where the full tank of water isn’t needed to ensure it goes down into the drain.

About the Author

Renee Miller began writing professionally in 2008, contributing to websites and the "Community Press" newspaper. She is co-founder of On Fiction Writing, a website for writers. Miller holds a diploma in social services from Clarke College in Belleville, Ontario.

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