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.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
translated into a smaller water spot in the bowl, increasing
customer complaints about "skid marks." While the modern
fixture, at bottom, also has a narrow trapway, better
engineering provides a more efficient flush and a water spot
almost as large as that of the old 3.5-gallon
The results left a lot to be desired. The early low-flows
quickly earned a reputation for clogging and failing to clear
the bowl, often requiring the user to double flush. "It was
like Las Vegas," says Virginia plumber Rex Cauldwell. "You'd
pull the lever and take bets on whether it was going to go down
That reputation persists because toilets last a long time and
many of those early versions are still in service. In reality,
though, fixture manufacturers have come a long way in the past
decade. Most of the current crop of 1.6-gallon toilets perform
as well as or better than the 3.5-gallon models they
Like their predecessors, most of today's low-flow fixtures are
powered purely by gravity. Depressing the flush lever lifts a
flapper valve in the base of the tank and allows water to flow
through the hollow interior of the rim and through
strategically placed holes in the rim. At the same time, part
of the flow -- the so-called siphon jet -- is directed into the
outlet, where it pours up and over the trap to start the
siphoning action that drains the bowl of waste.
Traps and tanks. What makes
the new toilets perform so much better than the old ones? For
one thing, the science of trapway design has come a long way in
the past few years. Toilet manufacturers now use sophisticated
computer-modeling techniques to develop traps that provide
optimum flow and reduce the frequency of clogs.
Another improvement has been the use of larger tanks to
provide increased head. Victorian-era toilet designers
accomplished this by mounting the tank on the wall, several
feet above the bowl, but modern low-flows use a simpler
approach. While the toilets use the required 1.6 gallons per
flush, their tanks may contain three gallons or more, which
means that the tank never empties completely. As the water
flows from the bottom of the tank during the flush cycle, the
added weight of the unused water above provides a more forceful
This suggests that, all other things being equal, a one-piece
"lowboy" toilet could be expected to deliver a less forceful
flush that its two-piece counterpart. American Standard
engineer Peter DeMarco confirms that one-piece fixtures do pose
some design challenges. "You don't have the gravity head, so
it's more difficult to get them to perform," he says. "You have
to change the internal configuration in other ways to
Another advantage of increased-head tanks is reduced sweating
in hot, humid weather. Because the tank isn't completely
refilled with cold water after each use, the water left in the
tank has time to warm up to room temperature. That tempers the
incoming water and is often enough to keep the outer surface of
the tank above the dew point.
Valve size. Most toilet
manufacturers use the 2-inch flush valve that has been the
industry standard for about a century. The sole exception is
Toto, which introduced a 3-inch valve in 1997 (Figure 2).
According to Toto engineer Fernando Fernandez, the resulting
high rate of flow from tank to bowl results in an especially
rapid, forceful flush. Fernandez claims that other
manufacturers haven't adopted the 3-inch valve because Toto
holds the patent to the specific trap configuration needed to
make its system work.
Figure 2.The 2-inch flapper valve at right has
long been the industry standard. A new 3-inch valve recently
introduced by Toto (left) empties the tank faster and provides
an aggressive flush with little swirling in the
Pressure- and Vacuum-Assisted
Consumers who want or need a more powerful flush than gravity
alone provides can choose from a wide range of
pressure-assisted models. In place of the unpressurized water
tank, flapper, and fill valve found in the back of a standard
toilet, pressure-assisted fixtures contain a manufactured
energizing unit that works something like the pressure tank in
a private well system.
After each flush, household water pressure fills a sealed
vessel, creating a cushion of compressed air at the top. When
the lever is depressed again, opening the flush valve, the
pressurized air rapidly forces the trapped air into the bowl to
provide what's best described as a commercial-type flush. In
fact, pressure-assisted units were largely confined to
commercial applications until a decade or so ago, when the
relatively poor performance of the early low-flow fixtures
brought them into the residential market. Two Michigan-based
companies -- Sloan Flushmate and W/C Technology Corp. --
manufacture all of the pressure units installed as original
equipment by U.S. plumbing-fixture manufacturers (Figure 3). In
addition to the standard 1.6-gallon unit, Sloan recently
introduced a new model, the Flushmate IV, which uses just 1.1
gallons per flush.
Figure 3.Pressure-assisted toilets use air trapped
in a sealed vessel to deliver a forceful -- and noisy -- flush.
Although pressure-assisted fixtures are outwardly similar to
conventional gravity toilets, bowl and trapway designs differ.
As a result, existing gravity fixtures can't be adapted to work
with a pressure-assisted unit.
Bang for the buck.
Pressure-assisted toilets typically cost about $150 more than
comparable gravity fixtures. Some plumbers find that they're a
good choice in problem applications, such as in an old drain
system with marginal venting.
Aside from the added cost, the biggest drawback to
pressure-assisted units is noise. A pressure-assisted flush is
both louder and more abrupt than that of a gravity fixture,
which gives them a high "startle factor."
"Women hate them," says Bellevue, Wash., plumber Terry Love.
"When you're trying to be quiet when you get up in the middle
of the night, you're not going to want to flush it at all."
Another drawback, Love observes, is that the rapid flow through
the trap tends to shred toilet paper, allowing small particles
of paper to drift back into the just-emptied bowl.
The "tank-within-a-tank" approach of pressure-assisted toilet
units prevents condensation from forming on the external
porcelain tank. On the other hand, condensation can form on the
surface of the pressure vessel inside the tank, where it may
provide a breeding ground for mildew.
Fixtures with pull. A
quieter but less aggressive method of adding force to a flush
is the vacuum assist incorporated in several models offered by
Crane and Briggs. Like pressure-assisted fixtures,
vacuum-assisted models contain sealed vessels that refill with
each flush. But instead of becoming pressurized during the fill
cycle, the vacuum chamber is designed to create negative
pressure as water drains from the tank during the flush.
Integral tubing connects the vacuum chamber to the upper bend
in the trapway, providing added suction to help clear the bowl
Figure 4.The outwardly conventional-looking Briggs
Vacuity (top) develops increased flushing force with an
internal tank-mounted vacuum-assist unit (above). As water
drains from the closed unit during the flush cycle, an internal
connecting tube induces a partial vacuum in the upper portion
of the trapway.