Flap flush, push button, lavatory cisterns
Not to be confused with the flap valve used in the syphon type of cistern, this flap flush valve directly controls the flow of water from the cistern.
The down pipe to the lavatory pan is attached to the outlet under the flap and mounted at the bottom of the cistern, the moving part of the valve simply sits in the outlet from the cistern, it is normally sealed by the pressure of water above it.
Some flap flush assemblies include an integral overflow which discharges any excess into the pan - (the illustration to the right shows an integral overflow, the illustration below shows a basic flap flush valve).
To operate a flap flush valve cistern, a button (normally fitted to the top of the cistern cover) is pushed to lift the valve by means of a chain. With the flap opened, the water flows down the pipe to the lavatory pan.
Release of the button, allows the flap to close and the water, as it fills the cistern, seals the flap against its seat.
Flap flush valves are also available which can be operated by a handle on the cistern (as a conventional syphon flush mechanism).
One 'disadvantage' of the flap flush system compared with the syphon style is that when the seal on the flap becomes worn, water will continually flow down into the pan - this does not happen when any parts of the syphon type become worn, the flush action just becomes harder to achieve.