When Congress passed the Energy Policy Act of 1992, plumbing-fixture manufacturers were given less than two years to convert from the standard 3.5-gallon flush to 1.6 gallons. With little time available to completely re-engineer their products, most of them approached the problem in the simplest way possible: They rolled out water-saving toilets that were essentially modified versions of existing products, with smaller tanks and narrower trapways meant to give the reduced water volume enough velocity to provide an adequate flush (see Figure 1).

Figure 1.The pre-1992 fixture (top) had an open trapway design that resisted clogging but required 3.5 gallons of water per flush to develop the force needed to move waste through. The early low-flow toilet (center) relied on a much narrower trapway that added velocity to the flow but was far more likely to clog; the limited supply of water also...

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