The siphonic toilet. An illustrated guide to the siphonic flush.
Siphonic toilets are the most popular toilets in the United States. Although easily identifiable through the gurgling sound at the end of the flush, how they exactly work remains a mystery to many. This article explains the exact working mechanism of a siphonic toilet and the characteristics of the siphonic flush (illustrated).
Here you see the first simple illustration, a siphonic toilet at baseline. The bowl is filled with water. Behind the bowl, invisible to the user, the siphon is partly filled with water. The water level in the trapway can not be higher in this resting state as excess water would flow down the siphon towards the outlet.
Now, the flush is actuated. Typically, at least a gallon of water is rapidly dropped in the bowl. The weight of the water is causing downward pressure.
The water needs to go somewhere and always chooses the path of the least resistance; the siphon is filling. Because of the rapid drop of water in the bowl and the resulting high downward pressure of the water, a solid column of water fills the siphon. All air is pushed downwards.
Now the siphonic mechanism is in full action. The siphon rapidly and completely fills with water.
The weight of the water (A) is sucking water from the bowl. As you can see, the trapway of siphonic toilets is very narrow. If the trapway was wider it would not completely fill with water as air pockets would start to connect resulting in loss of the vacuum. It acts like a straw in reverse, the suction comes from below.
It you would continue to fill the bowl at the same pace as the siphon flows there would be no end to this flush. But in this example (and in reality), the bowl is emptying.
The bowl contains almost no water anymore. Pockets of air are being sucked into the siphon. Soon the siphonic mechanism will be interrupted. At this point you start to hear gurgling sounds, typically of siphonic toilets.
Pockets of air are now connected in the siphon. There is no vacuum anymore, no negative pressure, and the flush is at its end. Remaining water in the siphon flows down back down into the bowl or flows down towards the outlet.
The bowl is empty now. The toilet slowly fills back up to baseline and will be ready for another flush.