Stupid Q? Toilet Bowl Dynamics

Ok, call me dumb here: What keeps the water in the toilet bowl prior to a flush? If it’s a valve (some flapper-type mechanism?), what triggers it to close at the precise moment? EXTRA CREDIT: What triggers the supplemental stream of water at the base of the bowl (as some toilets do) to kick in and shut off?


  • Jinx

Look on the toilet right under the tank. There should be a part that sticks out on the side in the shape of a goose neck.

There is no valve. (this is hard to describe). The flush causes water to drain from the tank into the bowl. When the water level gets higher than the higest point in the gooseneck, water flows over the hump in teh neck. On teh other side of the hump is the sewer line. As water pours down into the sewer line, it pulls the remaining water out of teh bowl through cohesion and pressure differential.

While this is happening, the water from teh tank continues to drain into the bowl at a slow rate. Once the bowl is drained, the drainage pipe has no more suction /low pressure/cohesion to drain the bowl so it continues to fill up to a certain level (through the ducts built along the rim.)

I imagine you understand the floater shutoff so I wont drone on.

Mr. “Z”, I think I follow, but let me take a closer look tonight. I’m sure I’ll have a follow-up question or two to post here! Overall, I believe I follow what you’re saying.


  • Jinx

Oh, on the trickling: In the center of the tank, there is a tube about 12" long and 1/2" wide. Next to it is an apparatus with a small tube coming out (about 1/4" in dia). also attached to it is a stiff wire with a hollow ball at the end. The small tube from the apparatus goes into the larger 12" tube. Water goes from the small tube, into the larger tube, then into the duct around the rim.

The ball at the end of the wire is attached to a valve in teh apparatus. WHen the tank drains, the ball goes down. This opens the valve. When the valve is open, water flows out of the small tube. Water also begins to fill up the tank. when the tank reaches a pre-set level, it lifts the ball to a certtain level. At this level, it closes the valve in the apparatus. This shuts off the water flow to the tank and bowl.

Really a beautiful and elegant system.

I know I am not making this very clear, but it is tough to explain without pictures. Check out your toilet tonight. you will see what I mean.

Here is an incredibly detailed explanation of how a toilet works, in terms a layman should be able to understand:

It even has pictures!

You know Jinx, with a properly aligned spy satellite they could tell whether or not you flushed :eek: !