The toilet cistern
How does it refill itself, and know when to stop?
The question
Flush a toilet and the tank empties, then fills back up and stops, all by itself. There's no pump and no motor. What turns the water off?
Have a guess, then show the answer
The water does. A float rides up with the rising water on an arm, and when it gets high enough it pushes the inlet valve shut.
Play
Flush it, and watch the float
This is a cutaway of a tank. Press Flush and follow the flush, the float and the valve. Tap a part, or use the pickers, to swap in another real design. Then set the float lower and see how much less water the tank keeps.
Side-entry ballcock: a ball float on a long arm levers a piston valve near the top of the tank.
Flapper: the lever lifts a rubber flap off the drain; it drops shut once the water falls below it.
Full and still. The float has pushed the valve shut. This setting holds about 5.1 litres.
What's happening
- 1
Pressing the lever pulls a chain that lifts a rubber flap, the flapper, off the drain hole in the bottom of the tank.
- 2
Gravity does the rest. The water rushes out into the bowl. In the UK, one flush is capped at 6 litres.
- 3
As the water drops, the flap sinks back down and shuts the drain.
- 4
The float has dropped with the water. Its arm pulls the inlet valve open, and fresh water flows in.
- 5
The rising water lifts the float, and the arm pushes the valve shut. No sensor, no switch. The water level turns itself off.
Simplified, and sources
The steps describe one common design, a flapper and a float on an arm. The widget also shows a siphon flush, common in UK cisterns, and a newer fill valve whose float cup slides up the valve's tower. Real valves come in more variations than these two pairings. The drawing is simplified, the refill is sped up (a real one takes much longer), and the litres assume a straight-sided tank that holds 6 litres at the highest setting.