How a Flush Toilet Works
Until recently, gravity flush toilets were the most common types of toilets found in residences. In public restrooms, commercial buildings and apartments, we had the tankless Flushometer type operated toilets. But today, water conservation is the driving force behind a search for the most efficient flushing toilet, and we now have a variety of mechanisms operating our toilets. While the designs may vary considerably, the basic operation of a toilet is to have water that is stored in a tank above the bowl to clean the bowl of waste using the minimum amount of water.
Early gravity flush toilets used a tank that was mounted on the wall above the bowl and the user pulled a chain to release the water into the bowl. The weight and the pull of gravity permitted the water to clean and clear the bowl of wastes. With improvememts in the tank and bowl, the tank was moved down to sit on top of the bowl and in some cases even appears to be almost level with the bowl or an intregal part of the bowl.
According to Flex Your Power's website, "The United States uses about 5.8 billion gallons of water every day to flush waste." They report that toilets and urinals account for nearly one-third of building water consumption. Today, some toilets are so well-engineered they can flush solid waste with just 1.4 gallons of water per flush rather than 3.5 to 7 gallons of water used by older designs. The industry standard is 1.6 gallons of water.
There are newer designs that pressurize the water inside a container in the toilet tank.
"Not uncommon in the United States, this system (invented by Bruce Martin) uses the water pressure within a structure to compress air within a closed vessel located within the vitreous enclosure. When flushed, the compressed air pushes into the bowl at a velocity (flow rate in gallons per minute or liters per second) significantly higher than gravity flow. This system is more water efficient than a tank type and can be installed into the same fittings as the latter. However it costs 10% less than the new 3" (75 mm) gravity flapper equipped tank-type toilets. Pressure assist toilets are used in both private (single and multiple and lodging) bathrooms as well as light commercial installations (offices, etc.) They hardly ever clog and so require less maintenance, but tend to be noisier - a concern for residential settings." Wikipedia.
While gravity toilets will work with very low water pressure from the water supply system, the newer pressurized low-flow toilets need a minimum of 20-25 psi of pressure at the toilet to operate.
What is a Standard Toilet?
There are two toilet bowl rim sizes in the world that are considered "standard". The small one is called "round" and the larger is called "elongated". The only other variant from this is the height of the toilet. Most toilets are between 14 and 16-inches from the floor. The other height that is available is called handicap or ADA compliant height. This height is mandatory for handicap toilets in the U.S. (Mandated in the American Disabilities Act or ADA). The height of this toilet is between 16 ˝-inches and 18-inches.
The typical rough-in dimensions of the toilet is a measurement from the back of the toilet to the center of the waste outlet hole on the underside of the toilet bowl. Or put another way, the distance from the wall to the two bolts on either side of the toilet bowl. The most common dimension today is 12-inches, but in older homes 14-inches were common. But many toilet manufacturers also offer two other sizes in case mistakes are made during construction or a pesky floor joist gets in the way. It is not unusual to find toilets with a 10 or 14 inch rough-in dimension. Be sure you purchase a toilet bowl that has the correct rough-in dimension to match your current drainage pipe location.
Marvel of Physics
The Science Channel has a really good three minute video illustrating the flushing mechanism of a gravity flush toilet.
"How a Toilet Works" continues on