As a plumbing contractor, I remember the response when the first generation of 1.6-gallon-per-flush (gpf) toilets hit the market in 1994: People hated them. They didn’t clear the bowl of solids and often left dirty porcelain. In the years since, plumbing manufacturers have redesigned their products so they are now less likely to clog.

The 1.6-gpf standard was a step forward, but in the face of drought and population growth, it has not been enough to eliminate water shortages, which are a serious problem in western states like California, Arizona, and Texas. Florida and other southeastern states are grappling with water problems as well. As a result, there has been an ongoing movement to enact even stricter conservation standards.

WaterSense Program

The EPA has established a program called WaterSense, which sets water conservation goals for a variety of plumbing products, including toilets, urinals, and faucets. Showerheads are expected to follow in the near future. The goal for toilets is to reduce usage by 20 percent, to a maximum of 1.28 gpf. A fixture that meets this standard is considered to be a high-efficiency toilet (HET) and is eligible to receive a WaterSense label.

Although the WaterSense program is voluntary, it’s just a matter of time before its provisions find their way into state laws and plumbing codes. This has already started happening in California and Texas, where it will be illegal as of January 1, 2014, to sell toilets that require more than 1.28 gpf. In anticipation of such standards, manufacturers have begun rolling out a new generation of HETs.

Comparing Performance

The poor performance of early water-saving toilets gave plumbing manufacturers a black eye, and they’ve made a concerted effort to avoid a repeat of that with newer models. A recent print ad from one major manufacturer bragged about the number of its toilets that belong to the “1,000 gram club.” This refers to the weight of solids that can be cleared from the bowl in a single flush as measured in a standardized Maximum Performance test, or MaP (for an explanation of MaP testing, see sidebar). To qualify for a WaterSense label, a toilet must be able to flush 350 grams of solids.

The best performers in the MaP ratings can clear 500 to 1,000 grams of solids. Comparing a toilet that flushes 500 grams with one that flushes 1,000 grams is like comparing an “A” product to an “A+” product: Few users will ever need a 1,000-gram flush, so the difference is more or less academic.

Other factors. Though important, raw flushing power is not the only consideration. The focus on MaP ratings has led some manufacturers to boost flushing power at the expense of other significant factors. In addition to clearing solids, an effective flush should scour the porcelain and leave perfectly clear water in the bottom of the bowl — and do so without making a lot of noise.

Flushing Technology

There are a variety of methods for achieving an effective flush and exchange of water, including siphoning, wash-down, and power flushing.

Siphoning. The toilets in most U.S. residences are siphoning models. When the toilet is flushed, water flows to the rim and to a jet near the bottom of the bowl. The jet, which is aimed toward the back of the toilet, pushes water up and over the high point of the trap, creating a powerful siphon that pulls water and solids into the drain. Water from the rim scours the porcelain and refills the bowl. You can identify a siphoning toilet by the sound it makes when it sucks the bowl empty.

The siphoning method allows for a large “water spot” — the area of the water in the bowl. A large water spot reduces the incidence of staining and “skid marks” by preventing solids from hitting the porcelain. Some manufacturers use other bowl-cleaning strategies as well, such as putting an ultra-smooth finish on the porcelain or incorporating rim jets designed to scour the bowl as it refills.

Wash-down. Although toilets based on the wash-down principle are the norm in Europe, they’re less common here. When the flush valve is opened, water floods the bowl from under the rim and pushes waste out through the trap. It has been my experience that backwash can occur when the wall of water hits the back of the trap, leaving discolored water in the bowl. To leave clear water in the bowl, the user may have to flush a second time.

Another shortcoming of the wash-down design is that it requires a bowl with steeply sloped sides so that the water falls with enough force to clear solids. This shape results in a small water spot, which greatly increases the incidence of staining. That’s why a cleaning brush stands next to most toilets in Europe.

The size of the water spot — which can usually be found in the manufacturer’s specifications — varies greatly among types of toilets. As an example, consider two HETs from Toto: The Eco Drake (siphoning) and Aquia II (dual-flush wash-down). At 10 1/4 inches by 8 1/2 inches, the Eco Drake’s water spot is more than three times the size of the Aquia II’s, which is 4 1/2 inches by 6 inches. This is typical of the difference between siphoning and wash-down fixtures.

Power-assisted flush. Inside the tank of a power-flush toilet is a vessel containing flush water and air compressed by incoming water. When the flush valve is opened, water is propelled into the bowl at high velocity. This type of toilet has a large water spot and may consume as little as 1.0 gpf in a single-flush model.

The biggest problem with power-flush toilets is that they’re noisy — anywhere from loud to explosively loud. As a result, we rarely recommend them to homeowners, although they typically do an effective job of clearing the bowl. Power-flush models are also more complicated than gravity models, making them more expensive to repair.

Dual-Flush Doubts

A dual-flush toilet has a full flush for solids and a partial flush for liquids. The full flush typically contains 1.6 gallons of water and the partial flush between 0.8 and 1.0 gallons. The performance goal for this design is to average less than 1.28 gpf over time, which qualifies the product as an HET.

Dual-flush models have been getting a lot of great press, but I’m not convinced they live up to the hype. Choosing between different flushes is confusing to people unaccustomed to this kind of toilet, so they may hit the wrong button. Also, the amount of water in the light flush may be insufficient to leave perfectly clear water in the bowl, leading users to double-flush or use the heavy flush for everything. When that happens, the toilet loses its water-saving advantage.

The most serious problem has to do with the nature of the flush. Most dual-flush models are wash-downs, because a partial flush is typically too small to create strong siphoning action. We have installed dual-flush wash-down toilets from Caroma and Toto and found they required regular brushing. Power-assisted dual-flush models are available, but they tend to be loud. American Standard recently introduced the H2Option, a dual-flush toilet that has a large water spot and is said by its maker to create strong siphonic action. We have yet to install one of these toilets, so we don’t know how well they actually work.

Conversion kits. The current emphasis on “green” has led to the introduction of products for converting single-flush toilets into dual-flush models. It’s a good idea on paper, but so far there’s been no independent testing to show that they actually work.

In any event, designing a truly effective conversion kit is a tall order. For a toilet to flush effectively, the size and shape of the bowl and trap must be matched to the size of the flush valve, and timing, location, and volume of the water flows all must be adjusted accordingly. Change any one aspect without accounting for the others, and performance is likely to suffer. The early 1.6-gpf models are a good example: Many performed badly because they were not designed from the ground up to work with that amount of water; in effect they were 3.5-gpf toilets with less water in the tank. Fixtures designed to work with a particular amount of water need that amount to flush properly.