A flush valve is a mechanism used to flush a toilet or urinal. The valve and associated hardware are located within the toilet tank or in the body of the urinal. Most toilets have a lever or push button activator, which is used to operate the flush valve.
A standard flush valve uses a rubber flapper to hold fresh water within the toilet tank, though some older models may use a ball rather than a flapper. As the user flushes the toilet using the lever, a chain connected to the lever raises the flapper at an angle. When the flapper is raised, the fresh water from the tank rushes down into the toilet bowl. This forces the existing water and waste material down through the S-shaped pipe at the base of the toilet.
Once the tank is empty, the flapper falls back into position over the opening between the tank and the toilet bowl. A separate refill valve allows fresh water back into the tank, where it will be stored until the flush valve is activated once again. While there is no limit to the number of times the flush valve can be activated in a given time period, the toilet will not flush unless there is enough water in the tank to fill the bowl.
Many problems that occur with these valves are very simple to fix, and can be easily repaired by the average homeowner. If the valve doesn't activate when you press the lever, it's possible the chain has become disconnected from the flapper. Remove the lid of the tank and reconnect the chain to remedy this issue. If the toilet seems to run continuously, the flapper is likely not closing all the way. Try shortening the chain slightly so the flapper can lay flat, or replace the flapper entirely if the edges are frayed or damaged.
Many modern commercial toilets use a flushometer instead of a standard flush valve. These toilets do not require a tank, so water enters the bowl directly from building's water supply piping. When the toilet is flushed, a special one-way valve is opened to bring fresh water into the toilet bowl. There is no way for waste to exit back through the water piping, so it is flushed out through the standard drain pipe. While these units take up less space than standard toilets, they are also fairly inefficient due to the large amount of water required for flushing.