Dual Flush Toilet Review: Washdown vs. Siphonic Action


Professor Toilet is pleased to see enthusiasm for dual flush toilets finally catching on in the United States.

Top view of a dual flush toilet Designed more than 30 years ago by an Australian Toilet Scientist, dual flush technology can reduce water usage by up to 67%. To put this into perspective, that’s like saving an average of 9,600 gallons a day over the old 3.5-gallon behemoths, or enough water to fill three hot tubs each day when compared to a 1.6-gallon toilet.

Dual flush toilets are actually mandated in Australia and Singapore, and catching on fast in North America because of the simple premise: “push 1 for 1 and push 2 for 2,”  A number 1 flush uses less water, a far more pleasant alternative to “if it’s yellow, let it mellow.” A number 2 flush is designed to take down solids without clogging, typically using the same 1.6 gallons per flush (gpf) as standard toilets like the Champion 4 and Cadet 3.

Dual Flush Washdown vs. Dual Flush Siphonic

One issue slowing more widespread adoption of dual flush toilets in the US is a technical term that toilet scientists call skid marks. The Professor assumes that you don’t need a visual on that; in short, it’s a bowl that won’t stay clean after pushing 2 for 2.  You may use other words for it when it happens.

Dual flush toilets rely on the “push” of water to cleanse the bowl in what is known as a washdown flush.  In contrast, standard toilets depend on the more popular siphonic action to “pull” waste out.

To strengthen the push action, the bowls of dual flush models are sloped more sharply than standard toilets to give the water greater momentum, which can also increase splashing.  The steeped bowls mean a smaller “water spot” or water surface area in the bowl, which makes it more likely that toilet paper or other waste will cling to the sides of the bowl, staining and generating odors.

The toilet engineers at American Standard have overcome these two hurdles with a dual flush toilet that removes every last trace of paper and waste, as well as one that offers a large traditional water surface area.

The WaterSense-certified H2Option Dual Flush Toilet is the first truly siphonic dual flush toilet, which generates strong push and pull action through forceful but quiet jetted action under the rim.  When the user flushes H2Option, some of the water is instantly diverted to the rim where there are a series of chambers.  The air in the chambers pushes the ongoing coming water forcefully out into the bowl. The resulting downhill rush of water creates an all-but-irresistible pull on the water behind it, so that every last trace of waste is removed from the bowl.

Share and Enjoy:
  • Digg
  • StumbleUpon
  • Facebook
  • Yahoo! Buzz
  • Twitter
  • Google Bookmarks
  • BlinkList
  • blogmarks
  • Blogosphere
  • connotea
  • Diigo
  • eKudos
  • Faves
  • FriendFeed
  • Google Buzz
  • HelloTxt
  • LinkaGoGo
  • LinkArena
  • LinkedIn
  • Live
  • MisterWong
  • MyShare
  • MySpace
  • Netvibes
  • NewsVine
  • Orkut
  • Plurk
  • Propeller
  • Reddit
  • Simpy
  • Slashdot
  • SphereIt
  • Sphinn
  • Technorati
  • Tumblr
  • Upnews



i noticed that dual flush toilet have a score of 600 in one store, in another store it is rated differently 1000. both american standard.


I installed a Cadet 3 Dual Flush yesterday. #1 and #2 buttons both seem to send the same amount of water. Instructions say “Pressing both buttons at once will flush the low flush amount.” I doubt that is correct. If I flush with lid off the see what is happening water splashes up and out. Instruction booklet picture of tank “guts” does not match my tank works. No instruction on how to set flush valve adjustments. One part was missing from box and I had to go back to Home Depot to get them to open another box and get me the part. The box opened was packed very differently (less packing material) than my box. I would not buy this toilet again.


Next time I go to Lowe’s or Home Depot I’ll take the short white hose (refill tube) with me and buy one twice as long. A test by attaching a plastic straw to the existing white hose seems to show that water splashing up on the top cover (then dripping down the back of the tank onto my tile and grout) is stopped from doing that with a lengthened hose. Let the new longer hose hang down into the water, almost to the bottom of the tank. Previous post should read, “If I flush with lid off TO see what is happening…..”


Hello Tom:
The two flushes can appear to be the same but they are definitely not. The flush volumes can only be confirmed by measuring the amount of water that leaves the outlet of the bowl. Yes, flushing both buttons at once will only flush the lower volume. The splashing water can be rectified by calling 800-442-1902 and asking for a splash cap for the refill tube. The difference between the instructions and the toilet itself are being addressed. We apologize for any inconvenience.

Paul, on behalf of ASB


Hello Tom:

Please call 800-442-1902 and ask for a splash cap for the refill tube.

Best regards,

Paul, on behalf of ASB


I have a 1947 “Standard” toilet at my house, which is the original model installed when the house was built. This is a forward-trap design, which was discontinued in America in the late 1960s. I LOVE this bowl! A superior flusher, it has easily handled big messes that would leave any of these newer “low-flow” toilets stopped up, big time! Yes, this toilet has a big tank and can pass 5 gallons with a single flush, but I know how to manipulate the handle so as to control the amount of water that comes out. For a “number one,” I can get a thorough flush with only approx. 1.8 gallons of water, or nearly the same as a “low-flow” model.


i have a duel flush system (b&q)and i am getting a leak through the push button i have dropped the water level in the cistern below cable/pipe entry but it still persists does button have to be above the cistern or is there another answer ?? has this happened to anyone else ?

Professor Toilet
August 6th, 2015 at 9:27 am

Hello Darryl,

Great question! The water level in the tank should not reach any push button or trip lever, each tank should be marked where the water line should be in relation to the top of the overflow cap on the flush valve. Hope this helps!

The Professor

Leave a Comment