Power Flushing With Pressure-Assist Toilets

Many of today’s best-performing low-flow toilets enhance their flush with air pressure to force water into the bowl at high speed.

In a standard (non-pressurized) gravity-flush toilet, water fills a tank behind and above the toilet bowl. When the toilet is flushed, a valve at the bottom of the tank opens and water flows from the tank into the bowl, producing the siphon-action flush.

With a pressure-assist toilet, instead of an open tank behind the toilet bowl there’s a sealed plastic vessel hidden inside the porcelain tank. After the toilet is flushed, the pressure vessel fills from the bottom using standard water pressure. As water (1.6 gallons or less, depending on the model) flows into the vessel, air in the tank is compressed above the water, “charging” the tank. No special pump or compressor is involved; water pressure creates the compressed air.

When the user flushes the toilet, this compressed air forces the flush water into the bowl at a higher velocity—and shorter duration—than with a standard gravity-flush toilet. This burst of water does an excellent job at removing waste. According to Sloan, its Flushmate pressure-assist mechanism will deliver a peak flow of flush water at about 70 gallons per minute, which is twice the velocity of gravity-flush toilets.

Some pressure-assist toilets use as little as 1.0 gallon (4 liters) per flush to remove 1,000 grams of test media (based on the MaP testing protocol—see EBN Jan. 2004). Two manufacturers make pressure-assist flush systems: Sloan Valve Company and WDI International. These mechanisms are used in dozens of toilets made by leading manufacturers. The 1.0 gallon-per-flush (gpf) Sloan Flushmate IV mechanism (see image) is widely touted for its effectiveness and water savings.

An added benefit to pressure-assist toilets is that the porcelain tank will not sweat—a common problem with gravity-flush toilets in humid areas when cold water fills the tank. With pressure-assist toilets, the tank-within-a-tank construction provides an insulating air space.

A significant downside to pressure-assist toilets—and the likely reason they have not achieved a larger market share—is the flush noise. Pressure-assist toilets make a characteristic “whoosh” sound when flushed, and this can be fairly startling, particularly with older, 1.6 gpf pressure-flush mechanisms. The latest Flushmate IV model is much quieter than the 1.6 gpf Flushmate III, though still noisier than standard gravity-flush.

June 1, 2010

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All About Toilets

EBN: Feature - January 2004

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