How a Toilet Works
Interactive cutaway simulation of a gravity-fed siphonic flush toilet. Explore hydraulic head pressure, trapway priming, atmospheric siphon vacuum, and refill water seal equilibrium.
The Fluid Dynamics of Modern Siphonic Toilets
Unlike European washdown toilets that rely purely on cascading water volume, American siphonic toilets rely on negative pressure generated inside an inverted U-trapway.
1. Hydraulic Head Rush
When the handle is depressed, it pulls the flapper chain. Because the flapper has an air pocket, it stays buoyant until the tank water drains. The 12-inch water column creates hydrostatic head pressure ($P = \rho g h$) that forces water through the siphon jet at over 7 ft/sec.
Torricelli Velocity: v = â(2gh)2. The Bernoulli Siphon
Water rushes into the bowl faster than the 2-inch trapway can exit under gravity. As the trapway fills completely, cascading water entrains and evacuates trapped air. Atmospheric pressure (14.7 psi) pressing down on the open bowl pushes waste up and over the weir.
P_bowl + ½Ïvâ² = P_weir + ½Ïvâ² + Ïgh3. Siphon Break & Trap Reseal
When the water level in the bowl drops below the weir entrance, atmospheric air is violently sucked into the trapway with a distinctive "slurping" sound, instantaneously breaking the vacuum. The refill tube simultaneously squirts water down the overflow pipe to rebuild the sewer trap seal.
Gas Seal: Îh ⥠2.0 inches HâO