Hourwiz Explains · video notes
How a Toilet Works: One Bend, Two Jobs
A toilet doesn't stink because of one bent pipe with two jobs. After every flush, a little water stays in the bend and plugs the way up from the sewer (the water seal). When you push the handle, the same bend fills with water and starts a siphon that pulls the bowl empty.
Job one: the bend holds a plug of water
- After a flush, a little water stays behind in the curve of the bend. That water sits between the sewer air and your bathroom.
- Alexander Cumming's S-shaped pipe worked this way: it trapped water in the pipe “to prevent any returning smells” (Science Museum).
- Thomas Crapper reshaped Cumming's S-bend into the U-bend (Science Museum).
Job two: the siphon flush
- The handle lifts the flapper, a flap at the bottom of the tank.
- Tank water rushes into the bowl and fills the curved pipe under it (the trapway).
- The change in pressure forms a siphon that pulls the bowl empty.
- The flapper closes. A float on the fill valve drops and the tank refills, and a little water settles back into the bend. The seal is restored.
This follows Kohler's own description of how a toilet flushes.
How we got here, one bad idea at a time
| Year | Step | What went wrong |
|---|---|---|
| Before plumbing | Cesspits, emptied by hand | Someone had to empty them: in London, “night-soil” collectors. |
| 1596 | John Harington's flushing device | It needed 7.5 gallons of water. He reportedly claimed it served up to 20 people before flushing. |
| 1775 | Alexander Cumming gets the first English patent for a flushing toilet | Flushing toilets later sent house waste straight to the river. His S-shaped pipe is the water seal, though. |
| 1819 | Albert Giblin patents the siphon flush | The flush got stronger, and older toilets went on to use as much as 6 gallons per flush (EPA). |
| 1800s | London's population doubles, 1800 to the 1850s, to over 2.5 million | More and more waste ended up in the Thames. |
| 1858 | The Great Stink: about 30 °C (86 °F) for weeks | Parliament soaked its curtains in chloride of lime. A bill proposed by Benjamin Disraeli then funded a new sewer system. |
| 1865–1875 | Joseph Bazalgette, Chief Engineer of the Metropolitan Board of Works, builds 1,100 miles of drains feeding 82 miles of brick sewers | Opened 1865, finished 1875. The tunnels still form the base of London's sewers today. |
| 1870–1910 | Sewers now connect homes | Americans feared “sewer gas” rising up the drains. |
Thomas Crapper did not invent the toilet. The Science Museum says he popularized the siphon flush and turned the S-bend into the U-bend. From Cumming's 1775 patent to the 1858 Great Stink is 83 years (our math).
Flush volumes
| Toilet | Gallons per flush | Source |
|---|---|---|
| Older toilets | As much as 6 | EPA WaterSense |
| US federal limit (Energy Policy Act of 1992) | 1.6 | EPA WaterSense |
| WaterSense-labeled toilets | 1.28 or less | EPA WaterSense |
The EPA says toilets are nearly 30% of an average home's indoor water use.
Our estimate. One family of 4, assuming 5 flushes per person per day (our assumption, not a measured fact): 4 × 5 × 365 = 7,300 flushes a year. At 6 gallons per flush that is 43,800 gallons a year. At 1.28 gallons it is 9,344 gallons. At the 1.6 federal limit it is 11,680 gallons. Our math, n=1, not a measurement of any real home.
What a toilet can flush
Flush toilet paper only. Everything else goes in the trash.
- The EPA says: “EPA urges Americans to only flush toilet paper.”
- Disinfecting wipes, baby wipes and paper towels should never be flushed (EPA).
- Wipes are made to stay in one piece, even when wet. Montana's environmental agency says they are “designed to be absorbent for cleaning and they stay intact even when wet.”
- This is 2020 EPA guidance.
“Flushable” on the pack
In 2015, the FTC alleged that one wipe maker's “flushable” wipes weren't safe to flush. Under the order, that claim needs proof from tests that replicate real-world conditions. An allegation is not a court finding, and we don't name the company.
Images in the video
No outside photos or footage. Doug and the scenes are drawn by Hourwiz.
Sources
Every fact on this page comes from these pages, opened on 22 October 2026. Things change: check the live page before you rely on it.
- Science Museum blog, “A flushing story” Harington's 7.5 gallons and the “up to 20 people” claim, Cumming's 1775 English patent and S-pipe, Giblin's 1819 siphon flush, and Crapper.
- London Museum, “The Great Stink of 1858” Night-soil collectors, flushing toilets sending waste to the river, London's population, the 1858 heat, the chloride of lime curtains and the Disraeli bill.
- London Museum, “How Bazalgette built London's first super sewer” Bazalgette's role, 1,100 miles of drains, 82 miles of sewers, 1865 and 1875.
- An, Yale Journal of Biology and Medicine, 2004;77:75–100 Americans' fear of sewer gas, 1870–1910.
- EPA WaterSense, residential toilets 6 gallons per flush, the 1.6 federal limit, 1.28 for WaterSense, nearly 30% of indoor use.
- EPA WaterSense, supporting statement on toilets (PDF) The 1992 Energy Policy Act and the 1.6 limit.
- Kohler, “How a toilet flushes” Flapper, trapway, siphon, fill valve.
- FTC press release, May 2015 What the FTC alleged and what the order requires.
- EPA, “Is it okay to flush disinfecting wipes?” (archived 2020) Flush only toilet paper.
- Montana DEQ, joining EPA's message Wipes stay intact even when wet.
Our water estimate is our own arithmetic on EPA's flush volumes with an assumed 5 flushes per person per day. Hourwiz is not affiliated with the EPA, the FTC, the Science Museum, the London Museum, Kohler or Montana DEQ, and none of them made, reviewed or endorsed this page or the video.
No sponsor and no affiliate links on this page. Vendors are named so you can check them yourself, not as recommendations.
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