A toilet cistern is the tank or reservoir that stores the water used to flush a toilet. The cistern typically houses the flush mechanism and a float valve set to fill the cistern to a predetermined level after each flush. The toilet cistern may be an exposed unit attached to the wall above the toilet bowl or a concealed unit mounted within the wall cavity. Exposed cisterns are typically either independent units separate from the bowl assembly or part of a one piece integral unit. Cistern flush mechanisms may be exposed lever types mounted on the front of the tank, push button controls fitted flush on the tank lid or, in the case of concealed cisterns, mounted on the wall above the bowl.
The flush action in a conventional cistern toilet is achieved by releasing a gravity fed charge of water into the toilet bowl and out of the closet bend. The water charge is housed in a tank or reservoir known as a toilet cistern mounted above the toilet bowl that gives the exiting water body the gravitational acceleration to effectively flush the contents out of the bowl. The cistern is supplied with a water feed pipe that passes through a float valve positioned inside the cistern. With each flush, the float descends on the diminishing water level, thereby opening the valve and allowing the cistern to refill. When the water level in the cistern reaches a preset level, the float valve closes and the flush system is once again ready for use.
Cisterns may be mounted in several ways depending on the toilet design; the two main mounting types are exposed and concealed. Exposed cisterns are generally mounted on the wall above the toilet bowl or on the toilet body itself in the case of one piece units. In each case, though, the cistern is installed above the bowl to give the necessary head, or gravitational impetus, to the flush charge. Most modern toilet designs feature cisterns mounted only a foot or so above the bowl while many older designs placed the cistern far higher, often just below ceiling level. Standalone cisterns are connected to the bowl by a flush pipe which runs from the flush valve to the bowls water entry ports.
Concealed cisterns are ideal for smaller toilets areas where space is an issue. These types feature a cistern that mounts within the wall cavity on a specially designed frame. The refill water feed and flush pipes are all routed inside the cavity with the toilet bowl mounted flush against the wall. The flush control for the concealed toilet cistern is usually mounted on the wall above the bowl. These systems are great space savers but require a lot more work when toilet cistern parts require replacement.