How Do Flush Toilets Work?

Toilets don't operate simply on the principle of gravity in sweeping water and waste materials down plumbing pipes. Toilets also use a siphoning system. Toilets are more functional than they are complex, and understanding the basic design behind a flush toilet is pretty simple.


Flush toilet

The flush toilet owes its existence to a long series of developments that date back more than 2,000 years. The flush toilet as we know it today is often attributed to Thomas Crapper, whose company, T. Crapper Brass and Co., Ltd., in England in the late 1800s, is believed to have popularized the siphon discharge system used in toilets to this day. This siphon discharge system was actually patented by another Englishman, Albert Giblin, who received a patent for his "Silent Valveless Water Waste Preventer" in 1819. Credit is also given to Alexander Cumming, who, in 1775, received a patent for the first "modern" toilet. Cumming patented the "S" trap, which had a sliding valve underneath to hold the water.

Siphon Function

The toilet siphon is a curved tube that is actually molded into the bowl. The siphon curves up and then back down, a sort of dam, where the bowl water level rests near the top of the curve without tipping over it. To test for yourself how the siphon system operates, pour a small amount of water into the toilet bowl. Pour cup after cup of water into the bowl. You will notice that the water level does not rise regardless of the number of cupfuls you pour in, nor does the toilet flush. The extra water simply spilled over the siphon tube edge and down the drain. Next, pour a 2 gallon bucket of water quickly into the bowl. The suction of the water flowing out of the toilet now pulls, or siphons, the water out of the bowl, and the toilet flushes and leaves a small amount of water behind in the bowl.

Inside the Tank

The handle itself rests on the front of the tank, but inside the tank are the guts of a toilet flush system. Connected to the handle is a flush lever, which is connected by a chain to the flapper valve. The flapper is seated in an opening between the tank and the bowl. The rubber flapper has ears that hook onto the overflow tube, a vertical pipe that stands in the middle of the tank. A thin refill tube connects the top rim of the overflow tube to the fill valve in the left corner of the tank. The bottom of the fill valve, whose threaded shank rests through a hole in the tank bottom, attaches to the water supply hose.

When You Flush

When you depress the handle, it raises the flush lever inside the tank, which in turn raises the chain to lift the flapper. This allows the tank water to quickly drain through the opened flapper seat down into the bowl. The float on the fill valve will also drop with the tank's declining water level until, after about 1 inch or so of drop, the fill valve will open to allow more water into the tank, arriving from the water supply hose connected beneath the toilet. The flapper will settle back into the seat, shutting off the water draining into the bowl, and the tank water level will rise again to a predetermined level. The fill valve also diverts a small amount of water into the overflow tube, via the refill tube, that helps replenish the water in the bowl below.

What Happens in the Bowl

Inside the bowl, under the lip of the rim, are a series of angled, small holes on many toilets. The water from the flapper seat enters the bowl through these holes, creating the swirling action. This action, coupled with the speed and volume of the incoming water, creates a curtain of water in the passageway leading to the drain pipe. It also creates a partial vacuum, preventing air from entering the passageway. As the incoming water from the tank into the bowl continues to accelerate, more air within the downward portion of the passageway is displaced until a good flush, or siphon action, is generated. As soon as the bowl's water level drops to the level where air is again allowed into the passageway, the siphon is broken.

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