How a Flush Toilet Works: Cistern, Siphon and Trap
A cistern stores a set amount of water, a valve releases it in seconds and a bent pipe keeps sewer gas out. Here is each part and how it fits together.
In this article
A flush toilet works by storing a fixed amount of water in a tank, releasing it quickly into the bowl so that waste is carried away, and then refilling the tank for next time. A bend in the pipe below the bowl keeps a plug of water in place, which stops gas from the sewer coming back into the room. Almost every part of the system is mechanical and driven by gravity, which is why the basic design has changed little in well over a century.
The cistern: a measured store of water
The tank behind or above the bowl is called the cistern. Its job is to hold enough water to flush in one go. According to Wikipedia's article on the flush toilet, modern tanks hold roughly 3 to 6 litres. Because the water is released from a height, it arrives in the bowl with force, and that force is what pushes waste through the outlet.
Pressing the button or lever opens a path from the tank to the bowl. Which kind of path depends on the design, and there are two common ones.
Two ways to start a flush
The first is the flapper (also called a drop valve). A rubber flap covers the outlet at the bottom of the tank, and the weight of the water above presses it shut. The handle lifts the flap on a chain, the tank empties into the bowl, and the flap drops back down as the level falls. Wikipedia notes this design is common in North America and continental Europe, and that it became legal in UK homes in 2001.
The second is the siphon. Here there is no flap that has to seal against the water. The handle lifts a disc that forces water over the top of a bent tube. Once the tube is full, siphonic flow takes over and pulls the rest of the tank through it. The flush ends when air enters the top of the tube and breaks the flow. The same article credits Albert Giblin with this valveless system, and says UK rules required siphon cisterns until 1 January 2001.
- Flapper: a rubber seal opens and closes; simple, but the seal can wear.
- Siphon: no sealing washer is needed; flow stops when air enters the tube.
Refilling: the float valve
After the flush, a fill valve lets fresh water back into the tank. A float sits on the surface and rises with the water. When it reaches the intended level, it closes the valve. If the float or valve fails, water can keep running into the tank, which is why tanks have an overflow pipe leading to the bowl or outside.
The same article says Joseph Bramah patented a float valve for flush tanks in 1778, and that a concentric-float design, where the float surrounds the valve, dates from 1957.
The trap: why the bowl smells of nothing
Under the bowl the pipe curves into a U or S shape. Water stays in the bend after every flush, forming a seal. Sewer gas cannot pass through standing water, so it stays in the drain. Wikipedia credits Alexander Cumming with patenting the S-trap in 1775, and says Bramah later improved on his design by replacing a sliding valve with a hinged flap.
A short history
The idea is older than the patents. Sir John Harington published a description of a forerunner of the modern toilet in 1596, with a flush valve and a wash-down bowl, and installed one for Queen Elizabeth I at Richmond Palace, according to Wikipedia.
A popular legend says Thomas Crapper invented the flush toilet. The same article says plainly that this is not true. It credits him with nine patents and with popularising the siphon system for emptying the tank.
Dual flush
Not every flush needs the same amount of water. The company Caroma developed the Duoset cistern in Australia in 1980, with two buttons: a small flush for liquid waste and a larger one for solid waste. Wikipedia says modern versions of this design can save the average household a large share of its normal toilet water use, with a figure of 67 per cent quoted there.
How much water toilets use
Toilets matter for household water use. The US Environmental Protection Agency says toilets are the largest source of indoor water use in a home, at nearly 30 per cent of the total. The current US federal standard allows 1.6 gallons per flush, while toilets with the EPA's WaterSense label use 1.28 gallons per flush or less, about 20 per cent less. Some older models use as much as 6 gallons per flush, according to the agency.
The EPA also says that household leaks can waste up to 180 gallons a week, and that toilets are often the cause. It names worn flappers as the usual culprit. A leaking flapper lets water trickle from the tank into the bowl, so the fill valve keeps topping up the tank.
Putting it together
- You press the button, and the flapper lifts or the siphon starts.
- The tank water rushes into the bowl and pushes its contents through the trap.
- The flush stops when the tank empties, or when air breaks the siphon.
- The fill valve refills the tank until the float shuts it off.
- Water left in the trap seals the drain until next time.
That sequence of store, release, seal and refill is the whole mechanism. The differences between toilet models are mostly in how the release step is done and how much water it uses.
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