A dual flush toilet has two buttons that open the same flush valve by different amounts. The small button gives a reduced flush for liquid waste, and the large button gives a full flush for solids. Makers list about 0.8 to 0.9 gallons for the small flush and 1.28 to 1.6 for the full flush.
How does a dual flush toilet work?
A standard toilet has one flush volume. Pull the handle and the flapper lifts, and the same amount of water always drops into the bowl. A dual flush toilet replaces that with a flush valve that can release two different amounts.
American Standard describes its version this way: the toilet "uses a unique flush valve that achieves two distinct flushing functions." One is "the normal 1.6 gallons per flush for removal of solid waste," and the other is "0.8 gallons per flush (half the normal amount) for removal of liquid waste." The flush actuator is a top-mounted button "instead of the normal side-mounted trip lever." The button "consists of two separate valves." The smaller side is for a partial flush, and the larger side is for a full flush.
Inside the tank, the button works rods that engage the flush valve. American Standard's troubleshooting notes say the rods must be "fully extended and fully engaging with the flush valve when depressed." Kohler's guide groups dual flush models with standard floor-mounted toilets for troubleshooting, and adds a rod length check for dual flushing toilets.
Different brands use different valve designs, so the parts inside vary. The idea is the same: a short press lets less water out, and a full press lets the full amount out.
Which way the water swirls has nothing to do with how well a toilet flushes; why toilets don't flush the other way in the Southern Hemisphere explains what does.
How much water does a dual flush toilet use?
It depends on the brand and model, and makers list their own volumes:
- American Standard: 1.6 gallons for the full flush and 0.8 gallons for the reduced flush.
- TOTO Dual-Max: TOTO says the system "lets you choose between 0.9 or 1.28 gallons per flush."
- Other models list their own volumes, so check the label or the maker's page for yours.
EPA explains the saving. It says a WaterSense labeled dual-flush toilet can save even more water when you choose the reduced flush mode, which uses a smaller volume of water to remove liquid waste. Toilets are a big share of household water use. EPA says toilets account for nearly 30 percent of an average home's indoor water use, and that WaterSense labeled toilets use 1.28 gallons per flush or less. The 2024 International Plumbing Code's fixture table sets a maximum of 1.6 gallons per flushing cycle for a water closet.
How dual flush toilets are rated is changing. EPA says version 1.2 of its WaterSense toilet specification is in effect. In May 2024 it released version 2.0, which sets "the same maximum flush volume requirement - 1.28 gallons per flush - for both single-flush toilets and full-flush modes of dual-flush toilets." EPA has paused the effective date of version 2.0, and says version 1.2 remains in effect until further notice. Check the WaterSense label and the listed gallons per flush, not just the two buttons.
Your actual saving depends on how you use it. A toilet that needs a second flush to clear the bowl saves little.
What goes wrong, and how do you fix it?
Most dual flush problems come from a few causes.
A weak or partial flush. American Standard's troubleshooting steps start with the rods. If they are not fully extended and engaging the flush valve when you press the button, you can get a poor or partial flush, and it says to "adjust the rod length and test the flush." Its instructions for the push button rods on several of its models say to loosen the lock nut, turn the threaded rod one turn counterclockwise, retighten the lock nut and test. If that leaves the flush valve stuck open, turn it back half a turn clockwise. On other brands, follow the instructions for your model.
Low water in the bowl or the tank. Kohler's list for a poor flush includes confirming the waterline and water level match and adjusting the tank water level to the waterline mark if needed. American Standard says there should be approximately 9 inches of water coverage across the top of the bowl water, front to back.
A clog. American Standard suggests a bucket flush test: "taking a gallon of water and dumping it into the bowl. If the bowl does not clear, then there is a clog in the line." Kohler's guide also lists clearing obstructions from the trapway or jet and the waste line, and cleaning the rim holes. Our guide on why a toilet won't flush all the way covers those steps.
A loose button frame. On in-wall tank models, Kohler says to confirm the frame around the flush button is securely in place, because "if it is loose the buttons may not fully actuate the flush."
A toilet that refills by itself or runs. This is often a worn flush valve seal. Fluidmaster says that when the fill valve turns on and off periodically, called ghost flushing, "water is leaking from the tank into the bowl at one or more leak points." Clean the seal and the seat, and replace worn parts. Korky sells a dual flush seal kit with three sizes of seals designed to fit popular models, and says its seal fits a 3 inch flush valve opening. Seals are brand specific, so match yours to the model. Our guide on how to fix a running toilet walks through the checks.
A converted standard toilet. Fluidmaster makes a Duo Flush converter kit. Its instructions say the water level should be half way to all the way over the oval windows of the valve, and that "if your water level cannot be increased, the Duo Flush system should not be used." If you are thinking of a conversion, follow the instructions for that kit.
Is a dual flush toilet worth choosing?
It can be, if you like the idea of a reduced flush, and the toilet you choose flushes well on both settings. Look at the gallons per flush, the WaterSense label and how the maker supports parts. If you are weighing a new toilet against a repair, see how to know when to replace a toilet. When you shop, also compare comfort height and standard height and what a toilet replacement costs. New toilets also need a supply line that fits.
HQ Plumbing and Air can repair a dual flush toilet or install a new one. We are licensed and insured, we give free estimates, and you get the price up front.