Explainer #3
How does a toilet flush?
When you press the button, the flush valve in the cistern opens and the stored water rushes down into the bowl faster than the trap can carry it away. The bowl fills past the top of the trap, and the water spills through it and takes the contents with it. Then the valve closes, the fill valve refills the cistern until a float shuts it off, and the water left in the trap seals the pipe against sewer gas.
The principle
Stored water does the work, and gravity moves it. The cistern is a tank of water held above the bowl. Opening the flush valve lets it all out in a few seconds, a much higher flow than the tap that fills it. The surge raises the water level in the bowl above the high point of the trap (the “weir”), so water flows over it and down the drain, carrying the waste along.
The trap, the S-shaped bend below the bowl, is the clever part. After the flush, the water that runs back stops at the level of the weir, so a plug of water always stays in the bend. That water seal stops sewer gas from coming up into the room, and it is why a toilet that has not been used for months can start to smell as the seal dries out.
Refilling is automatic. As the water level falls, a float drops and opens the fill valve. As the cistern fills, the float rises with the water and, at the right height, closes the valve with its lever. No electricity is needed. Some toilets work slightly differently: older “siphon” models use a siphon in the trap to pull the bowl empty, which gives the typical gurgle at the end.
Common myth
Toilets flush in opposite directions in the northern and southern hemispheres
The Coriolis effect, which makes big weather systems rotate in opposite directions on the two sides of the equator, is far too weak to steer the water in something as small as a toilet bowl. The direction the water swirls is decided by the shape of the bowl, the angle of the water jets and any tiny movement already in the water. Two identical toilets in the same room can swirl in different directions.How this video was made: Rendered 3D cross-section of a simplified washdown toilet. The water is a simple animated shape, not a fluid simulation, and the sizes are exaggerated so the parts are easy to see.
