How Gravity Toilets Work


Regardless of price or style, all gravity toilets depend on gravity to pull the water-and waste-through the system (drawing right). When the handle is pushed, a flush valve opens, and the water in the tank drains into the bowl, either through rim openings, through the large siphon-jet opening across from the drain at the bottom of the bowl or through a combination of both. The gravity-fed speed of the water pushes the waste through the trap and into the drain.

Old 5-gal. toilets and new 1.6-gal. gravity toilets work the same way. The tank holds the water above the bowl. The lever opens the flush valve, which lets the water rush out of the tank into the bowl, either through rim holes, a siphon hole or both. In the bowl, the pressure of the water rushing down the drain creates a vacuum or siphon effect that draws the waste with it. Meanwhile, an automatic valve refills the emptied tank.

But not all gravity toilets work the same way. For instance, there are different ways of ensuring that only 1.6-gal. of water is used in a single flush. A few toilets use tank dams that fit around the flapper and allow only 1.6 gal. of water to flow into the bowl. Some toilets rely on an adjustable ballcock to cut off the flow of water after 1.6-gal. has flowed. On others, the flapper valve closes early. And still others include a plastic bucket type device inside the tank that lets only the top 50% to 75% of water in the tank pass through during a flush. That height increases the head pressure of the water going into the bowl and results in a brisker flush.

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