Evidence of flush toilets has been found dating as far back as 2500 BC, but the technology was lost when the Roman Empire fell. Modern flush toilets have a porcelain bowl with a siphon built in.
The bowl is flooded quickly with enough water to fill the siphon tube and the weight of the water in the tube causes the bowl to empty. When the level falls below a bend in the siphon, air enters and breaks the siphon and flushing stops.
Float Ball Fill Valves
Typical tank-style toilets have a fill valve and a flush valve. Some toilets, especially older ones, employ a float ball fill valve.
The valve is opened and closed by a float on the end of a float arm. When the tank is empty, the weight of the float on the end of the arm causes the valve to open, filling the tank.
When the water reaches a high enough level, the float lifts causing the valve to close.
Float Cup Fill Valves
Float cup fill valves operate much the same way as float ball valves. Instead of a float on the end of an arm, the float cup is toroidal (doughnut-shaped) and slides up and down around a cylinder.
Float cup fill valves are used in new toilets and are often chosen to replace float ball valves. Besides being more modern, they take up less space and are easier to adjust.
Flapper Flush Valves
The other valve in tank-style toilets is the flush valve. The flapper flush valve consists of a flapper or flapper ball that lies atop a large hole in the bottom of the tank.
It is connected to the flush handle by a chain, and when the flush handle is pressed, the flapper lifts allowing water to rush into the bowl and siphon. This valve has the advantage of simplicity, but it is prone to leak.
Siphon Flush Valves
Siphon flush valves are not new technology. They were invented by Thomas Crapper in the 1880s and have been continuously in use since.
It has no sealing washers that can wear out and leak. When the flush handle is pulled, water is forced into a siphon.
The siphoning action empties the tank into the bowl where the bowl’s siphon continues the work. Its low potential for leaks is important for water conversation.
Flushometer or Sloan Valves
Commercial or industrial toilets and urinals make use of the flushometer or Sloan valve, introduced in 1906 by the Sloan Valve Company. This valve uses the pressure of the water supply line instead of gravity to fill the bowl.
When the handle is pulled, the valve is open. The water flows past a diaphragm containing a small hole that allows water in, and when the water pressure on both sides of the diaphragm equalize, the valve closes.