How a Cistern Works

1. An Inverted U-pipe in the cistern is linked to the flush pipe into the pan, and at the other end opens out into a dome (siphon). When the flush is operated, a lift rod raises a plate in the dome and throws water into the crown of the U-bend.

2. Openings in the plate are covered by a plastic-flap valve held flat by the weight of the water. As water falls down the flush pipe, it creates a partial vacuum, causing water to be sucked up through the plate openings and to raise the flap valve.

3. The base of the dome is about 10mm above the cistern bottom. When the cistern water level falls below the dome base, air drawn in breaks the siphonic action and stops the flush.

On a dual – flush cistern there is a hole in the side of the dome, and if the flush control is released immediately the hole lets in air to break the siphonic action after a gallon of water has been siphoned. When the control is held down for a few seconds, it operates a device that temporarily plugs the air hole, allowing a double flush

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