The way water is used to remove waste from the bowl has a lot to do with how much water is needed to get the job done. Standard toilets use siphoning action, a method that employs a siphoning tube, to evacuate waste. A high volume of water entering the toilet bowl when the toilet's flushed fills the siphon tube and pulls the waste and water down the drain. When air enters the tube, the siphoning action stops. Dual flush toilets employ a larger trapway (the hole at the bottom of the bowl) and a wash-down flushing design that pushes waste down the drain. Because there's no siphoning action involved, the system needs less water per flush, and the larger diameter trapway makes it easy for waste to exit the bowl. Combined with the savings from using only half-flushes for liquid waste, the dual flush toilet design can save up to 68 percent more water than a conventional low flow toilet [source: Green Building].
The dual flush toilet uses a larger diameter trapway that doesn't clog as often as a conventional toilet, needs less water to flush efficiently and saves more water than a low flow toilet when flushing liquid waste. But there are some disadvantages to consider, too. Dual flush units are a little more expensive than other low flow toilet designs. There is also the problem of aesthetics. If you like a tidy toilet bowl that's half full of sparkling clear water, the dual flush concept will be a bit of an adjustment. Typically, dual flush toilets only retain a little water in the bowl, and flushing won't always get rid of all the waste. Even in full flush mode, there's some occasional streaking. With a dual flush toilet, you'll probably use your toilet brush more often, but then you probably won't need to keep the plunger nearby.
In the next section, take a peek at what's involved in installing a dual flush toilet.