How Dual Flush Toilets Work

By: Sara Elliott  | 
A dual flush toilet can help to make your bathroom more environmentally friendly. See more green living pictures.
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Key Takeaways
  • Dual flush toilets offer a choice between a full flush for solid waste and a half flush for liquid waste, helping to conserve significant amounts of water.
  • They utilize a larger trapway and a wash-down flushing design, which reduces the amount of water needed per flush and prevents clogs more effectively than traditional toilets.
  • Popular in regions with water scarcity, dual flush toilets are becoming more common in the U.S., due to environmental awareness and rising water costs.

Dual flush toilets handle solid and liquid waste differently from standard American style toilets, giving the user a choice of flushes. It's an interactive toilet design that helps conserve water that has caught on quickly in countries where water is in short supply, like Australia, and in areas where water supply and treatment facilities are older or overtaxed. The Environmental Protection Agency (EPA) reports that by the year 2013, an estimated 36 states will experience water shortages as a result of increased water usage and inefficient water management from aging regional infrastructures. Using less water to flush liquid waste makes sense, but in the United States there may be cultural biases that make accepting a more hands-on approach to personal waste harder to accept [source: Safe Plumbing].

For a time, toilets were called necessaries, one in a long line of euphemisms used to describe the business end of handling a simple biological process. One problem is that bodily waste is a delicate topic, so delicate that culture can be as much a factor in affecting change as necessity. Consider the words: defecate, poop and pee. They're not accepted in polite conversation, are they? We keep our bodily functions under wraps, so any changes in our approach to handling them can create culture shock and resistance.


Imagine being in charge of a household that relied on the safe and familiar use of the family outhouse. Now, consider the reaction you may have had when the outhouse moved indoors. Walking the privies, another euphemism for going to the outhouse, might have seemed a more sanitary option than moving personal waste management into your home, yet the bathroom still made its way inside.

Dual flush toilets may be another defining moment in the development of the American john: the introduction of environmental conservation to the process of elimination. Interest in low flow and dual flush toilets is on the rise in the United States, due in part to increased government regulation and the rising cost of water, and there are incentives for making changes in the way we use the commode. In the next sections, we'll see why change may be a good thing, learn more about the specifics of the dual flush and find out how government and business are coming together to help America flush responsibly.

Next up, let's take a closer look at the dual flush toilet's history and popularity around the world.


Dual Flush Toilet Technology

The Australians are credited with leading the way in the development of dual flush technology. In 1980, Bruce Thompson of Caroma Industries created the first two-button flushing system, a convenient method of manually selecting the water volume of each flush -- a half flush for liquid waste and a full flush for solid waste -- with the push of a button [source: Biotechnology Innovations]. Necessity was the driving force for the change. Traditional toilets used lots of water, a commodity that was in short supply on a continent that has erratic rainfall and experiences frequent and prolonged droughts.

The idea of evaluating waste to determine the most water-friendly way to getting rid of it caught on, and by 1993, a redesign reduced by half the amount of water used per flush. This led to international interest in the design. Most modern dual flush toilets use less than a gallon of water (3 liters, approximately) to flush liquid waste and around 1.6 gallons (6 liters) to flush solid waste [source: Nash]. This is a big savings over old toilet styles that used five gallons (19 liters) or more for each and every flush.


Today, dual flush toilets are used widely in Australia, Europe and Asia, and they're catching on in other areas as well. Increased environmental awareness, government regulation, the availability of monetary incentives and the rising cost of water are making the changeover to dual flush and low flow toilet designs more attractive to U.S. consumers.

So how do they work? In the next section, we'll take a look at how dual flush toilets handle waste.


How the Dual Flush Toilet Handles Waste

The way water is used to remove waste from the bowl has a lot to do with how much water is needed to get the job done. Standard toilets use siphoning action, a method that employs a siphoning tube, to evacuate waste. A high volume of water entering the toilet bowl when the toilet's flushed fills the siphon tube and pulls the waste and water down the drain. When air enters the tube, the siphoning action stops. Dual flush toilets employ a larger trapway (the hole at the bottom of the bowl) and a wash-down flushing design that pushes waste down the drain. Because there's no siphoning action involved, the system needs less water per flush, and the larger diameter trapway makes it easy for waste to exit the bowl. Combined with the savings from using only half-flushes for liquid waste, the dual flush toilet design can save up to 68 percent more water than a conventional low flow toilet [source: Green Building].

The dual flush toilet uses a larger diameter trapway that doesn't clog as often as a conventional toilet, needs less water to flush efficiently and saves more water than a low flow toilet when flushing liquid waste. But there are some disadvantages to consider, too. Dual flush units are a little more expensive than other low flow toilet designs. There is also the problem of aesthetics. If you like a tidy toilet bowl that's half full of sparkling clear water, the dual flush concept will be a bit of an adjustment. Typically, dual flush toilets only retain a little water in the bowl, and flushing won't always get rid of all the waste. Even in full flush mode, there's some occasional streaking. With a dual flush toilet, you'll probably use your toilet brush more often, but then you probably won't need to keep the plunger nearby.


In the next section, take a peek at what's involved in installing a dual flush toilet.



Installing a Dual Flush Toilet

The process of installing a dual flush toilet is similar to that of installing a low flow toilet. Although using a professional plumber is the easiest way to go, it's a job that a do-it-yourselfer can tackle in an afternoon.

If you're planning on doing the installation yourself, there are a few things to consider. The general suggestions below will give you an idea of what's involved, but dual flush toilet models differ, so pay close attention to the instructions that come with the dual flush toilet model you're considering:


  1. The standard rough-in measurement for a toilet is 12", but that can vary, so measure the distance from the wall behind the toilet to the center of the bolts affixing your toilet to the floor. You should also check the diameter and shape of the base of your toilet to make sure the replacement will cover the footprint your old toilet occupies. You'll need this information to select a replacement toilet.
  2. Turn off the water supply to the toilet.
  3. Remove water from the toilet by flushing it repeatedly. If there is any water left after flushing, try suctioning it up with a shop vacuum.
  4. To make it easier to remove the toilet, put plastic down along any carpeted hallways or rooms between you and the trash. A wheelbarrow or other aid may also help in getting the old toilet out of your home.
  5. Disconnect the toilet's supply hose.
  6. Disconnect the tank from the bowl. There are typically two bolts, one on either side of the toilet, that have to be removed.
  7. Unscrew the two bolts that attach the old toilet to the floor.
  8. Remove the old toilet.
  9. Place a rag in the floor drain temporarily to trap any gas that might escape from the drainpipe. Clean away any old wax.
  10. Install the wax seal or gasket of the new toilet according to the manufacturer's instructions. (Hardware and fittings are usually included with the toilet.)
  11. Install the offset collar/adapter of the new toilet to the closet flange, the fitting that connects to the drain line.
  12. Put the toilet in place. Install any necessary bolts.
  13. Install the rubber gasket on the outlet of the new water tank, and insert the screws and rubber washers. Attach the tank to the bowl by sliding the screws through the holes in the back of the bowl. Add the nuts and tighten them into place.
  14. Connect the supply hose. A new hose may be included in your toilet installation kit.
  15. Apply silicon seal around the base of the toilet. (A shim may be necessary if the toilet isn't level.)
  16. Reconnect the water line.
  17. Install the toilet seat.

If the prospect of installing an entirely new toilet doesn't appeal to you, there are retrofit kits available that will convert your existing toilet to a dual flush system. The bowl will still use a siphon system to evacuate waste, but you'll be able to conserve water by selecting which flush mode you want, partial flush or full flush.

Interested in the future of the U. S. toilet market? In the next section, we'll take a look where we're headed and what that might mean for dual flush technology.


The Future of the American Flush: High Efficiency Toilets

In 1994, the National Energy Policy Act was signed into law. It requires that toilets sold in the United States use no more than 1.6 gallons (6 liters) per flush [source: Green Living]. This mandate to conserve has given rise to a new generation of high efficiency toilets (HETs) that use technologies like pressure-assist, gravity flush and dual flush to whisk away waste using as little water as possible. Of the new technologies, the dual flush method has the advantage of intuitive flushing, where the operator can decide electively that less water is needed and use one gallon (3 liters) or less per flush instead of the 1.6 gallon maximum.

Although toilets purchased for new construction and retrofits must meet the new standards, millions of older water-guzzling toilets are still out there. As water and sewer costs keep rising, low flow toilets are becoming more attractive to the American consumer, and local and state governments are using rebates and tax incentives to encourage households to convert to these new technologies.


The advantages of low flow toilets in conserving water and reducing the demand on local water treatment facilities is pretty obvious. According to USA Today, the average person flushes the toilet five to eight times a day, and at a greedy five gallons a flush, the numbers start to add up quickly [source: Winter]. According to the Environmental Protection Agency, completely eliminating old style, water guzzling toilets would save about 2 billion gallons of water each day in the United States [source: Green Living]. With a growing population, an aging water treatment infrastructure and the looming threat of global warming contributing to uncertain weather, water conservation will continue to be a big issue.

How does the dual flush toilet fit into this picture? No one knows for sure. Most major toilet manufacturers are gearing up to provide these water saving toilets, but whether or not the American public will embrace a change in the way they approach their bathroom habits remains to be seen.


Frequently Asked Questions

How does the dual flush mechanism work?
Dual flush toilets feature two buttons or a handle mechanism that allows users to choose between a full flush for solids and a reduced flush for liquids.
Are dual flush toilets more expensive than standard toilets?
Yes, dual flush toilets can be more expensive initially, but they offer savings on water bills and environmental benefits.

Lots More Information

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