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
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.
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