How Flushometer (Flush Valve) Toilets and Urinals Work

Walk into almost any office, school, stadium, or restaurant restroom and the toilets and urinals look stripped down compared to the ones at home: no tank on the back, just a chrome valve bolted to a pipe coming out of the wall. That valve is a flushometer, and it is the reason commercial fixtures can flush again two seconds after the last person walked away. Instead of waiting for a tank to refill, a flushometer takes one metered slug of water straight from the building’s pressurized supply line and sends it through the bowl on demand.

This guide explains the mechanism itself: why commercial buildings skip the tank, how a diaphragm valve and a piston valve each meter a single timed flush, what supply pressure and pipe size the valve actually needs to fire, the manual, sensor, and dual-flush ways it gets triggered, and how to read the flush volume stamped on the valve. It stops at how the mechanism works. For why these valves run or leak and how to diagnose them, see our guide on why commercial faucets and toilets run or leak (238). For how building supply pressure is generated and distributed, see our guide on how commercial water pressure and booster systems work (209).

Tank Toilets vs. Flushometer Valves: Why Commercial Buildings Skip the Tank

A flushometer flushes from line pressure, while a residential tank toilet flushes from water it has already stored. That single difference drives everything else about how the two behave.

A home toilet keeps a few gallons sitting in the tank above the bowl. When you push the handle, a flapper lifts and gravity dumps that stored water into the bowl. The tank then refills slowly through a fill valve, which is why you wait roughly a minute before the toilet is ready to flush hard again. The setup is quiet and works on modest household pressure precisely because the storage tank does the heavy lifting, not the pipe.

A flushometer has no tank and no stored water. The valve sits directly on the supply line, and when it opens it lets the building’s own pressure push a fixed amount of water through in a few seconds, then closes. Because there is nothing to refill, the valve is ready almost immediately, which is what makes flushometers the standard in high-traffic restrooms where fixtures get used back to back. The tradeoff is that the building has to supply enough pressure and flow on demand, since the pipe, not a tank, is delivering the entire flush.

That tradeoff has a practical edge worth remembering: a flushometer can fail to flush in a low-pressure situation where a tank toilet would still work fine, because the tank toilet already has its water in hand and only needs gravity. A flushometer needs the line to deliver pressure and volume at the instant of the flush. This is covered in detail in the pressure section below.

Federal efficiency rules apply to both kinds of fixture. Under the Energy Policy Act of 1992, toilets manufactured for the U.S. market are limited to 1.6 gallons per flush and urinals to 1.0 gallon per flush, whether the fixture uses a tank or a flushometer. Those limits took effect for commercial buildings in 1997.

Diaphragm vs. Piston Flushometers: How Each Meters a Flush

Both flushometer types meter a flush the same basic way: a small amount of water held above a flexible part keeps the valve shut, and releasing that water lets line pressure open it for one measured burst. The difference is the part that moves.

Inside the valve, a chamber is split into an upper and a lower section. The lower section connects to the pressurized supply. A tiny bypass opening lets water bleed into the upper chamber until the pressure on both sides equalizes, and that balanced pressure, helped by the larger surface area on top, holds the valve firmly closed. According to manufacturer technical literature from Sloan, that pressure balance is what keeps a flushometer sealed between flushes.

When the valve is triggered, a relief valve tips open and dumps the water from the upper chamber faster than the bypass can refill it. The balance breaks, line pressure shoves the moving part up, and the supply rushes through to the fixture. As water keeps bleeding back into the upper chamber through the bypass, pressure rebuilds, the part settles back down, and the valve reseals. The size of that bypass and the volume of the upper chamber are what set how long the flush runs, which is how the valve delivers a consistent metered amount rather than running until you let go.

The moving part is the whole distinction:

• A diaphragm flushometer uses a flexible rubber membrane stretched across the chamber. It is the older and more common design, and its flexible seal tends to tolerate a wider range of water pressure.
• A piston flushometer uses a cup-shaped piston that slides inside the valve instead of a flexing membrane.

According to Sloan, the choice between the two is driven by the building’s conditions, including water pressure, water quality, pH, chlorine levels, and traffic volume, and each design is better suited to different environments. The practical reason this matters to a facility is repair: a diaphragm valve and a piston valve take different rebuild kits, so identifying which one you have determines which parts you order. The diagnosis and parts side of that is covered in our guide on why commercial faucets and toilets run or leak (238).

The Supply Pressure and Pipe Size a Flushometer Needs to Fire

A flushometer needs both adequate pressure and a large enough supply pipe at the moment it fires, because it is pulling the entire flush from the line in seconds rather than from a tank. This is the dependency that most overviews skip, and it is where flushometers fail when a tank toilet would not.

On the pressure side, the International Plumbing Code sets minimum flowing pressures for flushometer valves. The code requires at least 15 psi flowing for the highest group of fixtures when that group includes flushometer valves, and it specifies a minimum of 20 psi flowing for flushometer valves on siphon-jet water closets and 25 psi flowing for blowout water closets and blowout urinals. “Flowing” pressure is the pressure measured while water is actually moving during the flush, which is lower than the static pressure you would read with everything shut off. The exact figures and which code is in force vary by jurisdiction, so confirm your locally adopted code. Manufacturers commonly specify their own minimum operating pressure for a given valve, which is why a valve that worked in one building can run poorly after being moved to a lower-pressure zone.

On the pipe side, the supply has to be sized to deliver that water quickly. The International Plumbing Code lists minimum fixture supply sizes, and a water closet served by a flushometer valve calls for a 1-inch supply, while a flushometer-valve urinal calls for a 3/4-inch supply. Those are minimums for the connection to the fixture; the branch and riser feeding a bank of flushometers have to be sized for the combined demand, which is engineering work for a licensed plumbing designer. A supply line that is fine for a tank toilet, which sips water slowly to refill, can be too small to deliver a flushometer’s fast, high-volume pull.

There is also a ceiling, not just a floor. Where building pressure exceeds 80 psi static, the International Plumbing Code requires a pressure-reducing valve to bring it down, because excessive pressure stresses fixtures and valves. So a flushometer lives inside a working window: enough flowing pressure and pipe size to fire a full flush, but not so much static pressure that the system has to be throttled. How that pressure gets produced and boosted in a tall or large building is its own topic, covered in our guide on how commercial water pressure and booster systems work (209).

Manual, Sensor, and Dual-Flush Activation Options

How a flushometer is triggered is separate from how it meters water: the same valve body can be opened by a hand lever, an electronic sensor, or a dual-flush control, and all three end up tipping the same relief valve to start the flush.

A manual flushometer is the familiar chrome handle. Pressing or pushing it mechanically tilts the relief valve, dumps the upper chamber, and starts the metered flush. It needs no power and has the fewest parts, which is part of why manual valves remain common in heavy-use restrooms.

A sensor (automatic, touchless) flushometer replaces the handle with an infrared sensor and a small solenoid that opens the valve electronically when a user steps away. Touchless operation reduces hand contact and prevents the unflushed fixture left by someone who did not press the handle. These valves run on a battery, a hardwired transformer, or a small turbine that generates power from the flush itself. The sensing and electronics layer is its own subject, covered in our guide on how sensor (touchless) faucets and flush valves work (239).

A dual-flush flushometer offers two flush volumes, a full flush for solids and a reduced flush for liquids, so the fixture uses only as much water as the situation needs. On manual dual-flush valves the user chooses by lifting or pushing the handle in different directions; on sensor dual-flush valves the control decides the volume automatically. Dual-flush is a water-saving feature layered onto the same diaphragm or piston mechanism, not a different way of metering.

Whatever the trigger, the metering physics underneath stay identical. The activation method only decides what tips the relief valve open.

Reading the Stamped Flush Volume (gpf/gpm) on the Valve

The flush volume a flushometer delivers is set by its internal parts and is marked on the valve and fixture, so you read it rather than adjust it. A flushometer is not freely tunable the way a tank toilet’s water level is; the metered amount is built into the diaphragm or piston assembly.

Look for the rated flush volume stamped or labeled on the valve body, usually expressed in gallons per flush (gpf). According to manufacturer repair guidance from Sloan, the flush volume of a valve is identified by markings on the valve and fixture and by the specific internal parts, where the color and shape of components such as the relief valve and flow ring correspond to a particular gpf rating. That is why ordering a rebuild kit means matching not just the valve model but the flush volume; dropping a 1.6 gpf kit into a valve meant to deliver a different rating changes how much water it sends.

The number matters for water use and for code. Under the federal limits noted earlier, a flushometer water closet is capped at 1.6 gpf and a urinal at 1.0 gpf. The U.S. Environmental Protection Agency’s WaterSense program labels more efficient fixtures that beat those limits: a WaterSense labeled flushometer-valve toilet uses no more than 1.28 gpf, and a WaterSense labeled flushing urinal uses no more than 0.5 gpf. WaterSense also sets a minimum flush volume for those toilets, 1.0 gpf, so the fixture still moves waste reliably rather than saving water it cannot afford to lose.

Because the volume is fixed by the internals, the way to change a flushometer’s flush volume is to change the parts to a different rated assembly, not to “turn it down.” If a valve is delivering too little or too much water, that points to a parts or pressure issue to diagnose, not a setting to dial in.

Frequently Asked Questions

What is a flushometer?
A flushometer is a valve that flushes a toilet or urinal using water directly from the building’s pressurized supply line instead of from a storage tank. When triggered, it releases one metered burst of water in a few seconds, then reseals and is ready to flush again almost immediately. The tankless design is why flushometers are standard in commercial and high-traffic restrooms.

Why do commercial toilets not have a tank?
Commercial restrooms get used back to back, and a tank toilet has to refill for about a minute between flushes. A flushometer draws each flush straight from the supply line and resets almost instantly, so it keeps up with heavy traffic. The tradeoff is that the building must provide enough pressure and a large enough supply pipe to deliver the whole flush on demand.

What is the difference between a diaphragm and a piston flushometer?
Both meter a flush by holding water above a moving part to keep the valve shut, then releasing it so line pressure opens the valve. A diaphragm valve uses a flexible rubber membrane, and a piston valve uses a sliding cup-shaped piston. According to Sloan, the better choice depends on conditions like water pressure and water quality. They take different rebuild kits, so identifying which one you have determines which parts you order.

How much water does a commercial flushometer use?
Federal standards limit flushometer toilets to 1.6 gallons per flush and urinals to 1.0 gallon per flush. More efficient WaterSense labeled models use no more than 1.28 gpf for toilets and 0.5 gpf for flushing urinals. The rated volume is built into the valve’s internal parts and marked on the valve, so you read it rather than adjust it.

Why does a flushometer need higher water pressure than a regular toilet?
A tank toilet flushes from water it has already stored and only needs modest pressure to refill slowly afterward. A flushometer delivers the entire flush straight from the line in seconds, so it needs adequate flowing pressure and a large enough supply pipe at the moment of the flush. That is why a flushometer can fail to flush in a low-pressure zone where a tank toilet still works.

This article is general information, not professional advice. Sizing, pressure adjustments, and any work on the building supply that feeds a flushometer should be handled by a licensed commercial plumber, and code requirements vary by jurisdiction.

Sources

• U.S. Environmental Protection Agency, WaterSense, Flushometer-Valve Toilets Specification and Certification (1.28 gpf maximum, 1.0 gpf minimum): https://www.epa.gov/watersense/commercial-toilets
• U.S. Environmental Protection Agency, WaterSense, Urinals (0.5 gpf labeled maximum): https://www.epa.gov/watersense/urinals
• U.S. Department of Energy, Energy Efficiency and Renewable Energy, Urinals (federal 1.0 gpf urinal standard, EPAct 1992): https://www.energy.gov/cmei/buildings/urinals
• International Code Council, 2021 International Plumbing Code, Section 604.8 (Water pressure-reducing valve, 80 psi static; code varies by jurisdiction, confirm your locally adopted code): https://codes.iccsafe.org/s/IPC2021P1/chapter-6-water-supply-and-distribution/IPC2021P1-Ch06-Sec604.8
• International Code Council, 2018 International Plumbing Code, Table 604.5 (Minimum sizes of fixture water supply pipes): https://codes.iccsafe.org/s/IPC2018/chapter-6-water-supply-and-distribution/IPC2018-Ch06-Sec604.5
• International Code Council, 2018 International Plumbing Code, Appendix E, Table E103.3(2) (Minimum flowing pressure for flushometer valves): https://codes.iccsafe.org/content/IPC2018/appendix-e-sizing-of-water-piping-system
• Sloan, Diaphragm or Piston Flushometers? It Depends (white paper, diaphragm vs. piston operation and selection): https://www.sloan.com/resources/education/white-papers/diaphragm-or-piston-flushometers-it-depends