How Low-flow Toilets Flush Your Waste Without Wasting Water

First let's talk about how low-flow toilets got to your bathroom. In the early 1990s, Congress mandated the first real law on toilet efficiency as part of the Energy Policy Act. Toilet manufacturers had to redesign their toilets to use 1.6 gallons (6 liters) or less per flush. Some were effective. Some weren't. Many manufacturers just reduced the size of the toilet tanks, and consumers complained the new toilets just didn't function effectively.

That led to a new testing protocol called Maximum Performance (MaP) testing. In 2002, a group of 22 Canadian and U.S. organizations developed the program to test toilets based on their flushing ability. The scores represent how much waste (soybean paste and toilet paper) that the toilet can totally remove in a single flush. It's now the de facto flushing performance test for toilets in North America.

Today's low-flow toilets are much more effective than those that first came onto the market. Low-flow toilets usually use one of two methods to clear waste: gravity or power flush. Both types use no more than 1.6 gallons (6 liters) of water or less per flush.

As the name implies, gravity toilets use gravity to remove the water and wastes from the toilet bowl. You just flush and water goes into the bowl, then into a trap that creates a natural syphoning effect that helps everything head into the sewer pipes. Gravity-assisted toilets are easy to repair; the technology has been the same for decades, it's just been improved.

Power flush toilets, by contrast, use pressurized air in the tank to force the water and waste as you flush. The air is compressed in a vessel inside the tank each time it refills. Pressure-assisted toilets are very efficient, but they're noisy and usually more expensive than gravity fed toilets.

As we've said, modern low-flow and high-efficiency toilets perform far better than the water-wasters of the past. But there are differences among toilets, and the following factors influence how well a new toilet performs.

What should you look for when buying a low-flow toilet to get the most out of your investment?

If you'd like to save water, but don't want to invest in a low-flow toilet, there are a few ways to retrofit a toilet so that it uses less water.

The most common approach is to use a household displacement device. These devices are placed inside the tank in a place where they won't affect flushing and do just what you'd think — displace water. Because they take up room inside the tank, the tank doesn't have to fill up with as much water each time you flush. Some typical displacement devices include plastic bottles, or bags filled with water or bricks.

But displacement devices and not recommended as long-term solutions. Bricks can deteriorate and cause damage to the flushing mechanism and plastic bottles or bags usually need to be weighted (with pebbles, for example) to make sure they're stable.

Another way to retrofit a toilet is to use a displacement dam. These plastic dams are wedged into the tank on both sides of the flush valve to decrease the amount of water per flush. One advantage they have over homemade displacement devices is that they're less likely to move around in the tank and disrupt the flushing mechanism.

One more retrofit device is the early-close flapper. It shuts off the water flow to the bowl before the toilet tank is empty. Early close flappers are adjustable so homeowners can adjust the water level so they can save water and still achieve a clean flush.

Still, all retrofit devices generally affect how well a toilet functions, and retrofitted toilets simply don't work as well as new low-flow and high-efficiency toilets in clearing waste. And since low-flow toilets with excellent performance are available at a range of price points, it's hard to find reasons to retrofit instead.