Quote Originally Posted by Paul Alciatore View Post
Thanks for that. I had not heard of this kind of siphon so this video enlarged my knowledge.

I don't know what country you are in/from, but in the US most home toilets do not use a siphon to empty the tank. Instead there is just a large diameter hole in the bottom and a flapper valve on it which is held closed by the water pressure. When the handle is pushed down, the chain pulls that flapper valve up, starting the flush. The water exits somewhat rapidly through the large opening and passage below it. The flapper valve has an air chamber on the bottom. This air chamber is in air when the valve is closed but when it is pulled up the air chamber is now in the tank's water so it tries to float. It is held in the full open position until the water level reaches it and then it closes as the water continues to flow. Finally, when there is only about 1 inch (2.5 cm) of water left the flapper valve closes and, just as you illustrated, the float activated valve is filling the tank again.

A bit of this fresh water flow is diverted through a tube to the overflow tube so that it refills the bowl which now is almost empty due to the siphon action of the S trap between it and the sewer pipe. That flow probably continues until the water level in the bowl slowly reaches the downward bend in that trap and probably trickles over it until the tank is full and the water supply is shut off. So both the tank and the bowl are refilled with fresh water.

The more recent mechanisms also have a feature that turns the water off if there is a slow leak. The overflow tube and S curve trap below the bowl prevents any floods, but this feature stops water from being wasted.

It is all very clever engineering. And we just take it for granted.

The toilets in public rest rooms here most often use an entirely different flushing mechanism; one that seems to rely on a timed flow or measured amount of water. But I have never had to repair one so I am not sure.
Things that aren't appreciated, stem from those without interest; that's a given. But what causes it, harder to detect. Personally, I find admiration for those who deliver a concise description, that literally forces reader to VISUALIZE. Graphics are fine, but not sure they teach in same manner as technical writing. On the other hand, patent applications go overboard in wordiness. I'll speculate the related attorneys are paid by the paragraph.

Another "taken for granted" feature is the physical air gap maintained between bowl and tank. This is a positive backflow prevention device of the simplest order.