A toilet cistern is an upper storage tank in a flush toilet that holds water for the flushing mechanism.
The cistern is typically either mounted on top of or just beside the toilet and can be hidden by the toilet pan (seat) lid, which covers it when not in use. If the cistern is mounted above, there will usually be a header tank to take high level/shower feed water down into it. Cisterns usually have a flip-top cover to prevent water spilling should the toilet be bumped. This is also where you will see the flush button and possibly the cistern outlet, which could either be a simple tap or something that refills the overflow tube back into the tank. In modern installations, particularly those with an "instantaneous" system, cold water from a combination of header and/or storage tanks enters the cistern through a filling loop or on some older installations directly from the incoming mains. This means that there is usually no separate tap for filling the cistern. It adds water to itself as required.
The storage capacity of modern cisterns in most toilets is usually between 7 and 15 litres which will normally last several flushes before refilling takes place.
As well as water, tanks may also carry a supply of sanitary chemical additives to improve the smell and basic hygiene, although these are generally not necessary in modern installations.
Any overflow from the cistern back into the pan is usually stopped by siphonage - automatic air vents stop unpleasant smells from occurring when this happens.
In an "instantaneous" system, there is usually a small motor-driven pump mounted in the cistern, which works in conjunction with valves in the valve chamber to refill it automatically after each flush - this is also where low level bath feeds enter.
This type of system is often employed on commercial premises due to its very efficient use of water and reliability but can be impractical for some domestic applications due to noise and the need for power.
If you are experiencing a leak in the cistern, you can simply replace it. A cistern can be replaced on a toilet without too much bother. Toilet cisterns don't cost that much, and it is not difficult to find one with the same dimensions as the current toilet installed. The new cistern just needs some basic plumbing skills to connect it up. If you are not a DIY person, contact a plumber. You need to connect up all of the pipes and fittings, including the overflow. This pipe should always terminate above ground or inside a waste pipe at least 100 mm above ground level so that if there is ever an over flow, it will drain back into the toilet bowl rather than cause flooding. It should also terminate within 150 mm of where your waste water enters the soil/drainage system (the pan). The flush pipe is the pipe that brings water into the cistern. The feeder is the pipe that takes water from the flush valve to the cistern. This may just be a simple push-fit on some systems, but others will have a fixing clip for easy connection. Some new toilets come with all of these flush mechanism parts already in place and just need connecting up.
When purchasing a replacement, ensure the dimensions and the set out of the new cistern fit the existing plumbing.
Before beginning any repair, turn off the water supply to both your toilet and home. Caution: Call a licensed plumber if you don't know how to turn off your water supply.Disconnect your old toilet cistern using channel-type pliers. Also, remove the clip that holds the water supply pipe to the base of the toilet bowl.
When doing this, ensure you keep the entire cistern upright; you don't want any dirty water spilling out on the floor or nearby fixtures.
Install the new cistern and connect the cold-water inlet pipe. Then attach a new clip to hold the toilet water supply line in place on top of your new toilet cistern. Turn on your home's water supply valve to refill your tank with clean water so you can use your newly installed toilet cistern immediately if needed.
Most household cisterns hold either 7, 9 & 13 litres of water. This volume is adequate for flushing most standard average-sized WCs. However, it will vary depending on how often the toilet is used whether this capacity will be sufficient. Obviously more people require more water to flush away their toilet paper and solid waste, and this may mean that you need a larger cistern fitted in your bathroom. Think of your toilet usage habits so that you buy the right toilet.
Dual flush cisterns allow you to choose between using half or full tank volumes at the push of a button - thus saving lots of money compared with conventional toilets, which use an average of 10 litres per flush (lpg). Dual flush technology means that there are two flush options for each toilet cistern. The first is often a 1 litre flush for liquid waste, and the second, a full flush which uses at least 5 litres, for solid matter.