How things work ยท No. 5
How a toilet flushes
A working model of an ordinary tank toilet, cut in half. No motor and no power: the water levels you see are worked out from the flows between the tank, the bowl and one bent pipe.
Flush it
Skip the tank
The flush happens in the bowl. Pour water in fast and it starts by itself. Pour slowly and nothing happens.
What is simplified
- The water is three volumes (tank, bowl, rising leg of the trapway) joined by simple flow rules, with each surface drawn flat. Real water sloshes, swirls and carries air.
- The siphon here starts once the spill over the bend is large enough to fill the pipe, and stops the moment the bowl falls to the lip of the trapway. A real one sputters at both ends.
- The passage to the siphon jet is drawn in the front wall of the bowl so the cut shows it. On many bowls it runs around the sides instead.
- The seat, the bolts and the wax seal under the base are left out, and the flush valve is shown in brass so it reads against the porcelain. Most are plastic now.
- Speeds and sizes are typical, not taken from one model of toilet. Slide Speed down to slow the flush.
In short
A gravity-fed toilet needs no motor. Pressing the handle lifts a flapper, a rubber seal at the bottom of the tank, and the stored water drops rapidly into the bowl. That surge pushes the contents over the bend in the drain, and the flow can act as a siphon that pulls the bowl clear. The flapper then settles back onto its seat. A float-operated fill valve refills the tank, and an overflow tube guards against spills.