In the U.S. and some Canadian locations, the maximum flush volume of a toilet is set at 1.6 gallons – 6.0 litres. Generally speaking, these toilets are called "low-flow", "ultra-low-flow" or some similar term. Up until now, the 1.6 gallon/6.0 litre toilet has dominated the marketplace.
Beginning in 1999, however, manufacturers began introducing what are known as "high-efficiency" toilets, or HETs. HETs are defined as having an effective flush volume of 1.28 gallons/4.8 litres or less. The HET category includes both single flush and dual-flush fixtures.
Single flush HETs are available that flush as little as 0.8 gallons (3.0 litres) and as much as 1.28 gallons (4.8 litres). Today, there are 66 manufacturers offering 1,215 models of single-flush HETs!
Dual-flush HETs provide the user with the option of a "full" flush for normal operations or a "reduced" flush for liquids only. They are subject by the national codes and standards to a maximum flush volume of 1.6 gallons (6.0 litres) in the "full" flush mode and 1.1 gallons (4.2 litres) in the "reduced" flush mode. (Most dual-flush toilets, however, use about 0.8 gal/3 litres in the reduced flush mode.) Generally speaking, dual-flush toilets installed in residences are flushed in the "reduced" mode once for every "full" flush. The average flush volume for a dual-flush toilet, then, is less than 1.28 gallons (or 4.8 litres)*, thereby meeting the requirements for an HET. Today, 57 manufacturers of dual-flush toilets offer 719 different models in the North American marketplace!
*-1 full flush at 1.6 gal (6L) and 1 'reduced flush' at 0.8 gal (3.0L); average of 2 flushes therefore is 1.2 gal (4.5L)
Gravity-fed toilets are the most common type found in homes. They depend upon the volume and weight of the water in the tank to create the flushing action. When the flush handle is depressed, a flush valve (with a flapper or other type of seal) opens and the water in the tank drops into the bowl, channeled either through rim openings, through the large jet opening at the bottom of the bowl, or through a combination of both.
In most toilets sold in North America, the force of the water coming from the tank acts to create a siphon in the exit from the bowl (known as the "trapway"). As a result, the siphon action pulls the waste through the trap and into the drain. Once this happens, an automatic valve begins to refill the emptied tank, getting the toilet ready for the next flush.
Less common in North America is the wash-down design of the toilet bowl. In these fixtures, no siphon is created in the bowl trapway. Rather, the weight of the water simply pushes the waste into the trapway. Wash-down toilets, therefore, usually feature a much larger diameter trapway (since no siphon is required) and may result in less clogging than a siphonic design. As with siphonic toilets, the tank begins to refill once the flush is completed.
The disadvantage of the wash-down bowl design is the small water surface area in the bowl (the 'water spot'). North Americans, however, are generally not accustomed to small water spots and find that with this smaller 'target area', bowl streaking results (AKA, 'skid marks'). On the other hand, wash-down bowls seldom clog (because the trapway is twice the cross-sectional area of a typical siphonic bowl).
Pressure-assist toilets use compressed air to create a more forceful flush action. These toilets take advantage of the pressure of the building water supply to get the job done, with the help of a pressure containment vessel. Water from the supply line is forced into the air-filled pressure tank at the building's water pressure of 40 to 80 psi. The air in the tank is compressed and when the flush lever is pushed, the water rushes into the bowl. (One benefit of a pressure-assist toilet in humid climates is that the water is contained inside the pressure vessel, which, in turn, is inside the vitreous china toilet tank. That design results in little or no tank sweating.)
Because of the design of the pressure-assist toilet, the flush action may be somewhat louder than a gravity-fed toilet. But that sound is of very short duration, unlike a gravity-fed toilet that can take 20 to 30 seconds to refill.
Round-Front vs. Elongated Bowl Design
Today's toilets come with either a smaller, round-front bowl or a longer, elongated-front bowl. The round-front bowl is ideal for compact bathroom spaces and is found extensively in older homes. Elongated bowls with a longer rim dimension are more comfortable for adult use and improved hygiene. Plumbing codes require elongated bowls in commercial applications.
Conventional vs. Elevated-Height Bowls (ADA Height)
Again, consumers and designers have choices when it comes to bowl dimensions. Generally speaking, two bowl heights are available: the conventional height bowl with a top rim generally in the range of 14 to 15 inches from the floor (excluding any toilet seat) and the chair-height bowl ranging from 16.5 to 18 inches. The Americans With Disabilities Act of 1990 sets the bowl height at a 17-inch (432mm) minimum. This chair-height design is becoming increasingly popular among all ages of the population and is frequently referred to as an "ADA height" bowl. Manufacturers have chosen their own terminology, including such descriptors as "comfort height®", "easy height", "right height", "smartheight", or some similar term.
Children's and Juvenile Toilets
A few manufacturers also market smaller toilets for very young children and suited to pre-school and day care installations. Go here for further information.