Why do toilets bowls most notably in America fill up during a flush before the water has appeared, and then almost empty before rising to the normal level?

Question

For context toilets in the UK just flush, the water level slightly increases then returns to the normal level. Also watched a Cody’s lab video about flushing mercury and seen the mercury level rise before the mercury appeared.

In: Engineering

Answer 1

Some toilets in the US flush like this but usually I see them in commercial buildings. Those usually dont have a basin and are connected to a direct water feed. There is piping, in the ceramic bowl, that feeds a portion of the high pressure water directly into the trap under the toilet bowl to help pull stuff down the drain.

Most residential toilets have a basin that release the water into the bowl until the weight of the water overcomes the pressure created by the air in the trap under the bowl, allowing it to finally flow through.

There are some residential toilets that function in manor like the commercials ones but do so in conjunction with a basin.

Answer 2

The water enters the bowl via two paths. The main flush comes from a bottom jet which shoves the waste towards the drain. The bowl also has holes around the rim which help clean the bowl. A proper flush requires the water to create a siphon in the U shaped channel. The U shape is a trap which keeps sewer gas from entering the home. That sucks the water level down before the bowl fills back up. The water level in the bowl ends up at the height of the U on the exit side.