How Tankless Toilets Work

Tankless toilets can conserve water and take up less space than standard models. See more pictures of green living.

Have you ever wondered why there is a tank of water attached to some toilets? Sinks and bathtubs aren't attached to tanks, so why does the toilet need one? The reason is that most residential flush toilets operate using a siphon, which is a tube at the bottom of the bowl fixture. Water coming into the toilet must do so fast enough to fill the siphon tube, allowing the water and whatever else is in the bowl to be sucked through and pulled down the drain.

Most residential water supply lines don't allow water to enter a toilet fast enough to trigger the siphon effect, so the tank provides a solution. When a toilet is flushed, the water held in the tank cascades down with enough force to activate the siphon. Without that gallon or so of water pouring into the bowl all at once, the water would simply spill over into the siphon tube and remain more or less level without creating an actual flush.

As the name suggests, a tankless toilet is any toilet that does not rely on a tank of water to clear its bowl. Instead, tankless toilets receive water directly from a supply line at a high enough pressure that a single flush can carry human waste through the drainage system. For the most part, these toilets are powered using only the force of water entering from the supply line. In buildings where water pressure is lacking, such as most private homes, tankless toilets can be helped along with pumps or other technologies that power the flush.

All urinals and the vast majority of toilets in public restrooms are tankless. There are also tankless toilets in some homes. They range from low-tech units made for urban apartments to high-tech futuristic models created for the dedicated toilet enthusiast. Traditionally, a variety of factors have prohibited the widespread use of tankless toilets in U.S. homes; however, that may be changing. Tankless toilets are steadily growing in popularity and could one day surpass the tank toilet as America's commode a la mode.

History of Tankless Toilets

The original flush toilet is believed by many to be a tank-type model invented by a 16th century English poet named John Harington [source: Lienhard]. This device followed the same basic principles of the modern flush toilet in that there was a tank of water used to flush waste out of the bowl and, with any luck, through the drain lines. The main problem with this toilet was that few people had indoor plumbing during this time. Still, the invention was considered a smashing success. According to some experts, Harington even designed a version for Queen Elizabeth [source: Lienhard].

Tankless toilets debuted on the plumbing scene with the invention of the first flush valve toilet by Sloan Valve Company in 1906 [source: Sloan]. In this model, a valve controlled the flow of water with each flush, allowing a specific amount to pass from the supply line into the toilet fixture at a pressure sufficient enough to cleanse the bowl. Sloan's basic design was improved upon over the years, rising in popularity as indoor plumbing became widespread. Most tankless toilets still work using this same basic design.

The first tankless toilets didn't immediately transform the world of plumbing -- Sloan sold only three units in the first two years of production. But the popularity of these flush valve toilets continued to grow throughout the 20th century, particularly in industrialized areas. Sloan's tankless toilet became the model for commercial flushometers, a type of tankless toilet that is now by far the most common fixture in public restrooms throughout the Western world.

The tank-style toilet also underwent a redesign during 20th century. The massive migration that occurred during America's Industrial Age led to the construction of high-rise apartments in which space was a precious commodity. During this time, water tanks were moved to a perch on the wall several feet above the porcelain throne. The release of water from such a height made for an extremely powerful and cleansing flush. As it turns out, this was overkill. In the next section, we'll learn more about tank-style toilets, including the amount of water it takes to power their flush and how some early models paved the way for the use of tankless toilets in urban apartments.

Tank-style versus Tankless Toilets

Standard tank toilets need water to enter the bowl at a high enough speed to activate the siphon and drain the bowl's contents.
Standard tank toilets need water to enter the bowl at a high enough speed to activate the siphon and drain the bowl's contents.
Peter Dazeley/Getty Images

Understanding how tankless toilets work requires a quick review of how tank-style toilets work. You can read about this in greater detail in How Toilets Work, but as we mentioned in the previous section, tank toilets are gravity-powered. The basic idea is that water is dumped into the bowl fast enough to activate a siphon, which pulls the water and waste out of the bowl and into the drain line. Because of this gravity-powered flush assistance, tank toilets can function on a water pressure as low as 10 pounds per square inch (psi).

Tankless toilets are a different story. They use approximately the same amount of water as a tank-type toilet, but the water enters the fixture at a greater pressure. This is generally achieved by sending the water through the line at a higher rate of speed, though the size of the feed pipe is also a consideration. For the most part, commercial buildings and some residential structures have enough water pressure to power the flush of a tankless toilet without any sort of mechanical assistance. This type of tankless toilet is usually called a flushometer.

Flushometers vary in their requirements, but generally need at least 15 to 20 psi of water pressure to function properly, sometimes more. Urinals are an exception. They generally operate using the same basic principles as regular flush valve toilets, but they require less water pressure because of the nature of the material being flushed (liquid versus solid waste). For this reason, urinals can run on much smaller water supply lines. They also require much less water to complete a flush.

Whether tankless toilets are poised to replace traditional models is a matter of some debate, but many experts say that tankless toilets produce a cleaner, more powerful flush than their tank-style counterparts. And because tankless toilets are connected directly to water supply lines, they can refill faster and be flushed again more quickly than a tank toilet. This is an important consideration for toilets in commercial operations, which are often heavily used. In the next section, we talk more about the use of tankless toilets in commercial buildings.

Tankless Toilets in Commercial Buildings

Tankless toilets, which include urinals, are by far the most common type of toilet found in public restrooms. The vast majority of these units operate using a valve, which is metered with either a piston or diaphragm. The valve is designed to shut automatically after completing a flush cycle, so there is no computer or other technology that regulates the operation.

In the world of plumbing, traditional flush valve tankless toilets are considered low-tech yet reliable workhorses. However, there can be some degree of user control with these devices. For most models, building owners can make minor adjustments to flush volumes, though in the United States and many other countries they must comply with national standards for water usage. A number of features can also enhance the performance of a flush valve toilet in commercial settings, including hands-free flush technologies and water conservation devices.

Within commercial settings, all tankless toilet models pretty much work in a similar way. When a toilet is flushed, a valve opens a supply line to allow a predetermined amount of water to pass through into the bowl. According to the U.S. Department of Energy, the current standard for water usage in the U.S. is 1 gallon (3.8 liters) for urinals and 1.6 gallons (6 liters) for regular toilets [source: Department of Energy]. The standards are similar or even stricter in other parts of the developed world, such as Australia, where average flush volumes must not exceed 0.6 gallons (2.2 liters) for urinals and 1.5 gallons (5.5 liters) for toilets [source: Australian Government].

Recently, one of the biggest trends for tankless toilets in commercial settings has been hands-free flush technology [source: Koeller]. This feature can be powered by battery or hard wiring, but there is typically a manual flush mechanism that still operates in the case of power failure. Hands-free flushing is beneficial in that it helps prevent the spread of germs. However, there is a common problem with this technology: Automatic flushers can misfire, leading to either wasted water or leftover waste.

For the most part, water pressure in commercial plumbing systems is high enough to power the flush of tankless toilets. Because water pressure in residential buildings and private homes is not as strong, it has limited the use of tankless toilets in many U.S. homes over the past century. There are some exceptions to this rule, which we will discuss in the next section.

Tankless Toilets in Urban Apartments

Tankless toilets in residential settings that rely on flush valve technology are essentially the same as those in public restrooms. However, these toilets are much less common in homes because of their high water pressure requirement. One exception is New York City, where many residential toilets are essentially identical to the flush valve style of toilets found in public settings.

In New York and other big cities, the main reason for having a tankless toilet in a residential setting is space-savings. In the case of New York, apartments built before the 1930s were fitted with high-tank toilets, in which a water tank was mounted on the wall several feet above the bowl. Because the tank didn't sit behind the bowl fixture, architects didn't allow for the extra space to accommodate them. Based on this layout, bathrooms in most pre-1930s apartments do not have room for both a bowl and a tank.

One of the best things about flush valve tankless toilets is that they are generally very reliable, though they are often a bit more expensive than tank toilets. One of the biggest downsides to this type of tankless toilet is noise, an important consideration in small apartments with shared walls.

Whether they are used in commercial or residential settings, these traditional tankless toilets are typically called flushometers. The modern version of the tankless toilet is a sleeker, high-tech model that usually comes with special features such as heated seating and personal cleansing systems. They are commonly referred to as performance toilets and we will review these in the next section.

Performance Toilets

Performance toilets aim to combine the most basic of human functions with extreme luxury and elegance. As a result, these days, homeowners can enjoy the benefits of a tankless system without having to settle for an industrial-looking loo. The catch is that they have to invest in a toilet equipped with some sort of flush assistance.

Flush-assisted toilets come in a variety of shapes, colors and sizes. A range of technologies power the flush of tankless systems, each with its own clever yet stylish way of whisking away waste. One example is Kohler's line of toilets with a technology called Power Lite [source: Kohler: Power Lite]. With this system, a tankless toilet is flushed using a 0.2 horsepower pump. Kohler also makes the Pressure Lite toilet [source: Kohler: Pressure Lite]. In this system, the flush is powered by a pressurized vessel held inside a tank that's connected to the bowl. So, technically, there is a tank involved, but it is not used to hold water in the way that traditional toilets do.

Another example of a flush-assisted tankless toilet is TOTO's Neorest. These high-tech models are powered by a flush engine (aptly named the Cyclone). As with traditional tankless toilets, Neorest's flush is controlled by a valve. The difference is that the valve is computer-controlled to release water in stages, though it uses only 1.2 gallons per flush [source: TOTO]. As with the Power Lite, Neorest toilets are run on electricity, so they will not work in the event of a power outage.

Another type of seemingly tankless toilet is the concealed tank toilet. As the name implies, concealed tank toilets are not tankless, but run on a gravity-powered tank system that is hidden from view, usually behind a wall. They are becoming increasingly popular because of space issues in many homes -- concealed tank toilets can save an average of 6 to 8 inches of space in a bathroom [source: Duravit].

These high-tech tankless toilets do not come cheap. Starting at about $1,000, they are significantly pricier than the standard $100 tank models. On the other hand, these high-performance devices do more than just carry away your waste. Most can be outfitted with a wide array of cool gadgets and fancy features, which we'll take a look at in the next section.

Special Features of Tankless Toilets

Many tankless toilets come with a sleek, modern style.
Many tankless toilets come with a sleek, modern style.
© Bondarchuk

If you're looking for a tankless toilet for your home, you'll soon find that there are countless add-ons that can be applied to them. For example, some models allow users to adjust the volume of water used in a flush, depending on the nature of the waste. In other words, you can choose a normal flush for solids or a partial flush for liquid waste. Also available on some standard tank toilets, this is an excellent tool for water conservation and a feature that will likely be available on more toilet models in the coming years.

Another plus with high-performance tankless toilets is aesthetics. They typically come in a sleek, modern design that takes up less space -- and is less noticeable -- than tank-style toilets. They can also be much quieter than standard toilets, assuming they are fitted noise-reduction technology.

They are also are marketed as having improved performance when it comes to bowl cleanliness. In other words, the powerful flushes are supposed to prevent any left over waste in the bowl. In addition, they often come with a variety of comfort features, including:

  • Personal cleansing
  • Heated seats
  • Spray massage
  • Warm air dryers
  • Air purification systems
  • Hands-free automatic flushing

Traditionally, looks and the luxury add-ons were the main motivation for buying a performance toilet. These days, space-saving is a major motivation, with water conservation coming in at a close second. In the next section, we review the water-saving potential of tankless and other toilet models.

Water Conservation with Tankless Toilets

Toilets are by far the biggest water guzzlers in most U.S. homes. According to the U.S. Geological Survey, toilets account for approximately 30 percent of indoor water usage at homes, sucking down about 25 to 30 gallons (95 to 114 liters) per day per person [source: U.S. Geological Survey]. That's the equivalent of about 10,000 gallons (37,800 liters) of water per person per year. But it's not all bad news: These days, homeowners are taking steps to conserve water, and tankless toilets can help with those efforts.

During the first half of the 20th century, toilets used at least 5 gallons (19 liters) of water per flush (gpf). Today, the U.S. Department of Energy requires that residential toilets use no more than 1.6 gpf (6 liters) [source: Department of Energy]. Toilet manufacturers are looking for ways to reduce flush volumes even further. The latest technology available on tankless systems allows for light flush modes of about 1 gallon per flush or less.

In the United States, federal law also addresses commercial tankless toilets (i.e., flushometers). The current standard specifies that flushometers use no more than 1.6 gpf, though many manufacturers are shooting for 0.8 gpf (3 liters) [source: Koeller]. Urinals are also improving: The U.S. Department of Energy's standards for urinals are currently at 1 gpf (3.8 liters) [source: Department of Energy].

An important water-saving feature of commercial flushometers is that they can be designed to let users push a handle in one direction for liquid waste and another direction for solid waste. The latter would trigger a full flush while the former would trigger a partial flush. This dual-flush technology is becoming common other parts of the developed world, including the U.K. and Australia, though it has yet to become as popular in the United States. With wider use of these features, water efficiency is expected to improve for all toilet types.

Learn lots more about toilets and water use by visiting the links on the next page.

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More Great Links


  • Australian Government. "Water Efficiency Guide: Office and Public Buildings." Department of Environment and Heritage. October 2006. (accessed December 22, 2009)
  • Australian Government. "Water Efficiency Labeling and Standards (WELS) Scheme." September 2009.
  • Duravit. "Wall Hung Toilets." (accessed December 11, 2009)
  • Koeller, John, spokesperson for Green Plumbers USA and Technical Advisor to the U.S.-Canadian Alliance for Water Efficiency (AWE). Personal interview. (December 9, 2009.)
  • Kohler. "Power Lite Technology." (accessed December 11, 2009)
  • Kohler. "Pressure Lite Technology." (accessed December 11, 2009)
  • Lienhard, John H. "Harington's John, Engines of Our Ingenuity." University of Houston, 1988-1997. (accessed December 17, 2009)
  • Roman, Angel, Deputy Press Secretary, NYC Department of Environmental Protection. Personal correspondence. (December 17, 2009)
  • Schroeder, Tim C., president, Duravit USA. Personal interview. (December 11, 2009)
  • Sloan Valve Company, "Our Heritage" (accessed December 17, 2009)
  • Smith, Matt, P.E. Personal interview. December 11, 2009
  • TOTO USA, Tankless Toilets. Personal correspondence. (December 11, 2009)
  • TOTO USA. "Neorest." (accessed December 11, 2009)
  • UK Environment Agency. "Toilets." December 2009 (accessed December 22, 2009)
  • U.S. Department of Energy. "Best Management Practice: Toilets and Urinals." Federal Energy Management Program. July 2009. (accessed December 16, 2009)
  • U.S. Department of Energy. "Energy Policy Act of 1992." (accessed December 17, 2009)
  • U.S. Environmental Protection Agency. "WaterSense Labeled Toilets." February 2008. (accessed December 16, 2009)
  • U.S. Environmental Protection Agency. "WaterSense Labeled Flushing Urinals." September 2009 (accessed December 16, 2009)
  • U.S. Geological Survey. "Water Q&A: Water Use at Home." May 13, 2009. (accessed December 17, 2009)
  • Waterwise. "Reducing Water Wastage in the UK: Toilet Flushing." (accessed December 22, 2009)