Dual-flush toilets have two buttons or levers to flush: one for a large amount of water (most commonly 1.28 or 1.6 gallons) and the other for a smaller amount, mostly 0.8-1.28 gallons.
This system has been designed and introduced by Toto in 1960 in a successful effort to decrease the flush volumes. The original toilet even had an integrated toilet tank sink for further water saving. In retrospect, this had been revolutionary, but dual-flush toilets became more popular many decades later.
Water savings became more important after the Energy Policy Act of 1994, which allowed a maximum of 1.6 gallons of water per flush in an attempt to reduce water usage. Later, only 1.28 gallons could be used per flush in states with potential sparse water supplies such as California. This all stimulated further developments and popularity of the dual-flush toilet.
There is a large difference in the mechanism of dual-flush toilets. In the US dual-flush toilets are almost exclusively siphonic, while in other parts of the world such as Europe and Australia, the wash-down flush is standard. Flushing- and water saving characteristics are different and will be elaborated further down this article.
The general principle of dual-flush toilets is to use less water than the single flush toilets by having the option to use two buttons/levers: one larger flush for solid waste and a smaller one for liquid waste.
In the US popularity is dual-flush increasing as consumers are becoming more environmentally aware and motivated to save water. However, the water-saving capabilities vary a lot and some dual-flush toilets on the market use more water than efficient single flush toilets, which is remarkable and misleading.
Dual-flush in siphonic toilets
Before the following explanation can be properly understood, it is necessary to understand the general concept of the siphon. Another illustrated post on this website explains the siphonic flush.
What is important to realize is that in order for the siphon to work, two factors are important: 1) the speed of the drop of water into the bowl and 2) the volume of water. If the water flows to slowly, the water will just slowly flow in to the outlet without activating the siphon. If the volume of water is to low, the siphon will not completely fill and the mechanism will not be activated.
Among the reasons why dual-flush toilets became popular later in the US than in other parts of the world is the difficulty in activating a siphon with a as low possible volume of water. This is difficult as the water line in siphonic toilet is already a lot higher than in wash down toilets. This means that a flush always uses more water than is visible in the bowl before the flush is actuated. Therefore, a flush with a volume of less than a gallon is very difficult to attain and requires a lower water line and smaller siphon and smaller trapway diameter, which in effect makes clogging more likely.
- Large flush volumes:
- Varying between 1.28 and 1.6 gallons. Both are common, but in dual-flush toilet 1.6 gallons per flush is more common than in single flush toilets.
- Small flush volumes:
- Varying between 0.8 and 1.28 gallons. 1 gallon is common.
Dual-flush in wash down toilets
In wash down flushing toilets, dual flush is considerably less complicated. The mechanism does not require a certain volume to activate a siphon (there is none) and only the combination of speed and a volume large enough to clear the trapway is required. These toilets are popular in large parts of the world: Europe, Israel, Australia. This has been one of the reasons why these toilets were incorporated there (sometimes even mandated), because of the simple mechanism and water saving qualities. Generally these toilets are able to function with even less water than their siphonic counterparts.
- Large flush volumes:
- Varying between 1.2 and 2 gallons. 1.6 gallons per flush is most common. 2 gallons per flush is rare and is mostly an option in toilets that have adjustable flushing volumes (such as many Geberit toilets).
- Small flush volumes:
- Mostly 0.8 gallons. Notably smaller volumes than what is usual in siphonic toilets.
How much water will a dual flush toilet use?
A question difficult to answer, because it is so dependent on the size of your household, the number of smaller and larger flushes, the volumes of both flushes. The only sensible thing is to give you an option to calculate it yourself. Below you find the calculator. We used the characteristics of the American Standard H2Option as an example. Adjust as needed.
As you can see a huge amount of water is flushed every year, despite the 1.6 gallon per flush limitation in effect since 1994. Previously, toilets flushed volumes varying mostly between 3.5 and 6 gallons per flush.
How much water will a dual-flush toilet save?
It depends on the amount of water the particular model uses and the number of flushes. Water saving per toilet is related to the amount of people using it and the amount of toilets you have in the house. Let’s say you have one toilet and live with 3 people. Everybody uses the toilet 5-6 times on average (based on this information). In this example let’s assume 5 flushes a day is standard. From these 5 flushes everyone needs 1 big flush on average. So it’s 4 flushes per person which use 0.8 gallons (3 liter) instead of the 1.6 gallons (6 liter). This saves 3.2 gallons (12 liter) per person = 9.6 gallons (36 liter) a day.
This means dual-flush is going to save you 350 gallons (or more than 1300 liters) of water a year with just 3 people using the toilet! In the long run this is a very good investment and also good for the environment.
Calculate water savings by updating a single-flush toilet to a dual-flush toilet.
Fill in the table and calculate how much water you could potentially save by updating an existing single-flush toilet to a dual flush toilet.
Mind that there are also dual flush toilets flushing just 0.8/1.28 gallons, making them very-very water efficient.
Dual-flush toilets that will not save much water
Some dual-flush toilets are not as efficient as they might appear. Since the Energy Policy Act of 1994 toilet are prohibited of using more than 1.6 gallons per flush. Many toilets on the market, also single flush, use even less water, 1.28 gallons per flush, making them High-efficiency toilets and labeled WaterSense toilets. These are not rare, or very expensive toilets. They are even mandated in certain states that are often hit by drought, such as California.
However, some toilets on the market have a large flush of 1.6 gallons and a smaller flush of 1.28 gallons. The Horow HWMT-8733 is an example, (read the review here). The water saving features are in between a “regular” toilet and a high-efficiency toilet. It does not fulfill criteria for the WaterSense label. This is misleading, as one would expect that every dual-flush toilet is highly efficient and saves more water than other single-flush toilets.
The reduced flush in dual-flush toilets should use no more water than 1.1 gallons according to the ASME A112.19.2, a standard of 2013 for plumbing manufacturers. The aforementioned Horow toilet does not comply with its reduced flush of 1.28 gallons. However, on average, this toilet still uses less water than the federally mandated maximum of 1.6 gallons per flush and is in accordance with the law.