MUNDANE MYSTERIES: Why Pouring Extra Water Into A Toilet Causes It To Flush

If you’ve ever been stuck at home in the midst of a water outage, then you’ve most likely had to find the biggest pot, bowl, bucket, or other container that you could get your hands on in order to answer nature’s call as normally as possible. Did you notice, though, that pouring a large quantity of water into the toilet will trigger an automatic flush? But why does it do that? And how does it work?

For most toilets, the handle or lever is connected to a chain that’s attached to what’s called the “flapper”. It’s essentially the lid on top of the flush valve. Whenever you press the handle down, that chain raises the flapper, and water from the tank flows through the valve & into the toilet bowl. The opening at the bowl’s bottom leads to a tube called a “siphon”, which curves up above the water’s resting line in the bowl before twisting down sharply. As the bowl gets quickly washed with water, everything’s forced through the siphon until there isn’t enough water to fill the whole tube. Then, when there’s no longer enough pressure to force the fresh leftover water up over the tube’s highest point, it settles back in the bowl.

Pouring a cup or two of water into your toilet bowl won’t generate a flush, because it’s not enough to fill the entire siphon, so it won’t cause what’s in the bowl to get flooded down. A gallon or two of extra water, on the other hand, will definitely do the trick. Basically, whenever you pour a bunch of extra water into the toilet bowl, you’re effectively doing the job of the toilet tank.

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