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How Does a Siphon Work?

This Old House Plumbing & Heating Expert Richard Trethewey demonstrates how siphons work. Richard shows how siphons are the basic principle that make toilets work, even if the water supply in the house was turned off.

In this video, plumbing and heating expert Richard Tretheway demonstrates how siphons work.

What is a Siphon?

A siphon is a tube that allows liquid to travel upward, above the surface of the origin reservoir, then downwards to a lower level without using a pump.

When a certain amount of water moves over the bend in the siphon, gravity pulls it down on the longer leg lowers the atmospheric pressure in the bend of the siphon. So, the atmospheric pressure on the other side of the siphon becomes stronger and forces the rest of the water up and over the bend in the siphon.

How Does a Siphon Work in a Toilet?

Without electricity or any mechanicals, a toilet is able to flush away any wastewater into a sewer or septic system—proving to be quite a marvel for modern society. Richard shows how siphons are the basic principle that makes toilets work, even if the water supply in the house was turned off.

How to Operate a Toilet

  • Push the lever to pull the stopper in the tank
  • With the stopper open, all the water in the tank rushes into the bowl
  • The rush of water activates a siphon, caused by the shape of the drain here, which pulls the waste out of the toilet
  • When the tank is empty, the stopper falls back over the hole, allowing the water supply to refill the tank

Where to Find it?

Richard demonstrated the principle of a siphon jet on a Glacier Bay toilet, though every toilet from every manufacturer operates using a similar siphon jet.