In an earlier post I talked a bit about various flushing systems and tried to explain a little about how gravity flushing systems work. Here I would like to offer a lot more detail. I want to extend great thanks to Ted Howell of Western Sales Company who provided the following content. Full disclosure, Western Sales Company represents Toto.
Siphon Vortex: Old fashion Kohler Rialto & similar models flushing with 3.5 or more GPG. Low profile. Most of the water entering the bowl is through one or two sideways pointed openings (similar to the current TOTO Double Cyclone jets). Took lots of water to swirl around the bowl and fill the trap passage-way until a siphon was created. The little wiggle in the vertical portion of the trap helped back up the water to fill the trap completely and start the siphon.
Siphon Jet: This is how most US toilets have and are flushing. Everything from an old Kohler or American Standard, before “G-Max”, to the current G-Max, E-Max, and knock off products. Water falling from the tank travels in a channel to the “jet” opening under water that is pointed at the trap outlet. This fills the trap faster and starts the siphon. Before TOTO created a new design, they and others all used the wiggle in the vertical trap section to stack the water up and start the siphon. Some water is directed to the rim to fall down the sides of the bowl and wash it off.
Siphonic Wash Down: This has also been used by many models for years. Think of the original TOTO Promenade or the CST703/704. Or the Universal Rundle toilets. All the water is coming from the rim. There is no jet hole. The water falling from the tank comes out the rim (in the UR case the rim was an open slot, not a lot of little holes) and falls onto the water surface area. The water level in the bowl rapidly rises, flowing into the trap-way, backs up in the trap passage due to the wiggle and a siphon is started.
A siphon is like a chain. Water molecules stick together like the links of a chain. But the links are weak and only stick together a little bit. The water in the bowl is like a coil of chain. Pick up one end and pull it over the edge, hang it down below the bowl a ways and the chain will pull itself out of the bowl. Like a slinky perhaps. The water sucks itself out. Like a vacuum action. This creates a very effective way to clear a bowl.
The water chain will fall apart if it’s too large in diameter. A small tube can easily create a strong and sustainable siphon. As the tube increases in diameter there comes a point where the vertical column of water falls apart under it’s own weight and no longer sustains a siphon. It’s a Catch-22 for flushing objects. The larger the trap passageway the more likely large material will pass through. Yet there is an optimal diameter for a strong siphon. It appears to be somewhere around 2 3/8″ – 2 5/8″, with several variations on that measurement. Conventional instinct may be that bigger is better but in the case of creating a powerful siphonic flush, this is not necessarily true. A carefully engineered trap size designed to create the optimal siphon is what is the most effective. TOTO’s creative redesign of the trap passage appears to be an innovation that has improved flush performance by coming up with a new way to create the siphon.
Wash Down: There is no siphon. The trap passageway can be quite large and thus not tend to clog. All the water falls from the rim and simply pushes the contents of the bowl over the edge of the trap and down the drain. Because the trap is large in diameter it is holding more water. This means that the water in bowl may be the same actual volume of water as in a siphonic toilet but more of it is out of sight. Thus there is a small visible “water spot”.