ELi5 why does toilet water never rise above “set” level? You flush toilet? Water level inside of it doesn’t rise over the “set level” and etc.



ELi5 why does toilet water never rise above “set” level? You flush toilet? Water level inside of it doesn’t rise over the “set level” and etc.

In: Engineering

The water in the bowl comes from the tank. So it can only have as much in the bowl as there is in the tank. The tank has a float in it. This float, depending on how high or low you set it, determines how much water fills the tank. When water fills the tank to a certain level determined by the float, it triggers a valve to close so no more water fills the tank. When you flush, the water from the tank is released to the bowl.

Not sure I completely follow. Are you asking what shuts off the water supply so it doesn’t keep filling the bowl?

The bowl isn’t a closed system. It has a drain in the back. That drain pipe uses a bend to trap water to prevent gas coming from the sewer/septic system into your house. As you fill the bowl with water, the water level rises in the trap until it reaches the top of that bend and flows out the drain. That is why the toilet never rises above that certain level. That height is the level of the drain. The toilet bowl only overflows when that drain becomes blocked.

This wiki article gives some images for the trap:


Toilets work via siphon. There is a bend that traps water in the bowl, which is the “full” water height. Any water that comes in *slowly* will trickle over the top of the bend and drain out.

When you flush, the sudden rush of water fills the bowl and completely fills the drain. Instead of trickling over, the pipe becomes a siphon and pulls *all* of the water out until there’s a gap of air that breaks the siphon.

This works because the water falling down the drain pulls the water above it down along with it. If the water above it didn’t move, and no air can get in, what would fill the space left behind as the water falls down the drain? Nothing else can get into that space, which means it creates an empty vacuum. That vacuum sucks the water in the bowl up and over the bend, emptying the bowl.

Once the bowl is empty, the pipe isn’t completely full of water and air gets in, which fills the vacuum and breaks the siphon. More water trickles into the bowl until it’s full again, and with the siphon broken any extra water trickles over the bend and drains.

Unless the drain is clogged, the fullest the bowl will get is however much water has time to get in before the siphon forms, which depends on the size of the tank and the hole from the tank and the size of the drain.

Your toilet [has a trap in the back](https://upload.wikimedia.org/wikipedia/commons/thumb/b/b2/Diagram_of_a_jet-siphonic_WC_bowl.svg/1024px-Diagram_of_a_jet-siphonic_WC_bowl.svg.png) which is a little S shaped bend that is partially full of water

The trap ensures that there is an air tight seal on the end of the sewage pipe which is good, but its also laid out so that if the water level in the bowl rises, it just goes over the back end of the trap and the water level resets to the left of the back of the trap as shown in the image.

This is also why sometimes you flush and it seems the water all seems to get sucked out of the bowl, its because it has formed this air tight seal but also picked up enough speed that it is generating suction in the bowl and pulling all the water (and other bits) down with it