A toilet tank is a cistern that accumulates and stores water used in the process of flushing a toilet. Depending on the type of toilet, the tank may be mounted directly to the bowl or to the wall behind it. In low-flow or pressure-assisted toilets, the tank may contain a pressure vessel. In other types of toilets, the tank is typically just a large vessel that contains the flushing and fill mechanisms, in addition to holding the water necessary to flush the unit.
Both flapper and siphon-type toilets use a toilet tank as part of their design. The former is prevalent across the US and in many other countries around the world, while the latter is typically found in the United Kingdom (UK). In flapper-type toilets, the toilet tank is generally mounted directly to the toilet bowl. They are joined by a large outlet that allows water to flow from the tank into the bowl, thus causing it to flush due to a siphoning effect. In this type of toilet, the outlet between the tank and the bowl is obstructed by what is known as a flapper valve, which can be lifted in order to begin the flushing process.
In siphon-type models, a toilet tank is also utilized. Historically, these tanks were mounted on a wall near the ceiling, as this added height helped create additional water pressure when emptying the tank into the bowl. Other siphon-type toilets have a design that looks more like a flapper-type, with the tank joined directly to the bowl. While the method of moving water from the tank to the bowl is different, the basic purpose of the tank is the same in both designs.
Some toilets may use a high-pressure vessel within the toilet tank. These tank-within-a-tank designs are typically intended to save water with each flush. Rather than the cistern simply filling up and then emptying with each use, the pressure vessel can introduce a pressurized flow of water into the bowl, thus creating the siphoning action necessary for a successful flush with a lower volume of water.
In some toilets, such as flapper-type units, a leaky toilet tank can be one of the biggest wastes of water in an entire household. Internal leakage can allow water to seep past the flapper and into the bowl, resulting in a continuous, slow loss of water. External leakage can result in water emptying out of the tank and onto the floor or into the subfloor, which can result in serious damage. Shutting off the flow of water to the toilet tank may help determine if such a leak exists, as the level in the cistern should not lower unless it is flushed or a there is a leak present.