How Does a Siphon Work? | Ask This Old House

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.

About Ask This Old House TV:
From the makers of This Old House, America’s first and most trusted home improvement show, Ask This Old House answers the steady stream of home improvement questions asked by viewers across the United States. Covering topics from landscaping to electrical to HVAC and plumbing to painting and more. Ask This Old House features the experts from This Old House, including general contractor Tom Silva, plumbing and heating expert Richard Trethewey, landscape contractor Jenn Nawada, master carpenter Norm Abram, and host Kevin O’Connor. Ask This Old House helps you protect and preserve your greatest investment—your home.

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How Does a Siphon Work? | Ask This Old House

[Stunbunny]

FINALLY! Thank You! I went through 5 other videos before I found yours that explained the siphon effect. Now I finally understand the force that empties a toilet bowl.

[CactusHugrAz]

Every single person needs to understand this to appreciate it. Great cutout as always! Seriously I'm going to force people to watch this now. Thank you!

[DanielL]

There's a bit more to siphoning; its not just the 'weight' of the water 'pulling' on the water in the upper bucket due to gravity; rather, the weight of the water in the tube does indeed cause the water to start to flow thru the tube, but here is the key - the evacuation of the tube would tend to create a vacuum behind it; rather than allow a vacuum to exist, the the laws of physics continually act to refill the tube via atmospheric pressure on the surface of the water in the upper bucket, to press upon and replace the water which is draining out of the tube (nature abhors a vacuum). If you sealed the upper bucket in an airtight fashion, the siphon would stop working because the force of gravity on the water in the tube is not able to produce a vacuum inside that upper (sealed) container. But in an unsealed container where air pressure can work with the weight of the water column, a siphon can work - up to about 10 meters in height depending on barometric pressure. Hope this helps - siphoning is gravity in partnership with atmospheric pressure (which is caused by gravity too) - gravity does all the work here, but it operates on the water itself AND CRITICALLY the column of air above it. All you have to do is make sure that there is no air-lock (air is compressible, water is not) inside the tube, to get things started. PS - if you don't believe science, believe a bartender who can turn a glass of water upside down and it won't run out because atmospheric pressure is holding it in against the force of gravity. Check it out and Cheers.

[Danny-fshk]

Another excellent video! They are such great teachers on this channel.

[suzannelaughlin]

Thank you this is a great video to show the parts of a toilet bowl and how they work.

[adamguymon]

I would love it if This Old House would make an entire episode in detail where they would talk about the workings of how American toilets and plumbing and how UK toilets and plumbing work and the dos and don'ts etc? As well it would be nice to know the differences about the things such as electrical and other things such?

[ShaikhNadeem]

Siphon effect is one of the greatest inventions of mankind : Says the Plumber... 😂

[ctran]

Best video explanation so far. Thank you!

[threebox-oo]

finally the information I have been looking for to close the gaps, thank you

[numskull]

They physics and math on this are really cool and simple too.

[Syrita]

Wonderfully explained and demonstrated!
Thanks so much 😊

[Oceaninmyshades]

Thank you so much for this explanation!

[H-Vox]

sounded like Tim Heidecker when he said "isn't that somethin" 0:59

[thedave-]

Question?
Does this go I line with the hot water heater I'm confused about where to install is it installed on the same cold water line before the water heater?
Please help

[SuperFastFlashSam]

does it the water starts at a uphill direction then to a downhill direction to make siphon work correctly.

[weirddoe]

How does the smaller hole near the top of the exit channel work? Try plugging it to see what it does or doesn't do, please. Is it a vacuum that helps pull or suck?

[ravindersingh-xnpb]

Why Water seal too high above bowl in syphonic type Water closet and upto what level on higher side water seal is acceptable

[sniffer]

How do you clean the jets and siphon tube.

[PetroCodmYT]

what if the bucket is on the ground and you want to fill something up above it

[joescheller]

is the jet actually a nozzel or just an opening to create the, siphon effect