Gravity Tank Toilets:
Gravity two piece tank toilets are the most common toilet used in residential settings. They also are in some commercial/business settings. They depend on the volume of water (today generally about 1.6 gallons) in the tank to flush wastes and usually require water pressure of at least 15 psi (pounds per square inch) to operate properly. The tank and bowl are usually two separate pieces, although this is not obvious once they are in use.
A few one-piece toilets are also available and these don't always flush as well and are mostly chosen for their looks. Gravity tank toilets are relatively inexpensive, with retail prices for two-piece toilets ranging from $50 - $1,000 and one-piece models costing somewhat more.
Pressurized Tank Toilets:
This design uses water line pressure to achieve a higher flush velocity. Water is not stored inside the tank, but in a tank that compresses a pocket of air and releases pressurized water into the bowl and out the trapway. They generally require at least 25 psi of water pressure to operate well. Retail prices for these toilets are generally over $200 and theplumber.com does not recommend these types of toilets.
Flush Valve Operated Toilets:
This is the type of toilet usually found in many public and commercial restrooms. These toilets have no tank. Instead of a water storage tank, this toilet uses a valve directly connected to the water supply plumbing of a building. This valve controls the quantity of water released over time by each flush. Flushometer valves are typically made by one manufacturer and the china bowl by another. It is important that there is a proper match between the valve and the bowl when purchased. Unlike tank-type toilets, flushometer valve toilets must accommodate different water pressures at different points in a building. They are priced at a minimum of $275 for both the valve and the bowl.