Here’s Todd in his own words:
WOW!!!!

I can not believe the efficiency of this thing. Try as we might we have not been able to get it to clog or not fully flush even with the .9 gallon flush. Read more: DIY Installation and performance of Low Flush Toilet.

There are several aftermarket retrofit kits available today that promise to transform a standard toilet into a dual flush model that can “save more water than a high priced high-efficiency toilet.”

Unfortunately, these gadgets simply regulate the amount of water entering the bowl and do nothing to alter the water flow in or out of the bowl.  Since bowl design is the most important factor in a toilet’s performance – and even more critical in low-flow toilets – these kits promise far more than they deliver.

As the Professor has previously explained, standard and dual flush toilets have different flushing mechanics.  While standard toilets depend on siphonic action to “pull” waste out of the bowl, dual flush toilets rely on the “push” of water to clear the bowl.  More advanced technology, such as the WaterSense-certified H2Option Dual Flush Toilet, combines the traditional siphonic “pull” force with the newer “push” action associated with the washdown flush.

Because standard toilet bowls are not specifically engineered for less water, homeowners will have as much luck using these retrofit kits as they would adding a brick to the toilet tank.  Both strategies try to “trick” toilet science and will likely result in incomplete flushes.  Worse, users will likely overcome this problem by – you guessed it – flushing again.  Multiple flushes eliminate any possible water savings.

In addition to voiding the American Standard warranty on toilets, installing these types of gadgets will frustrate homeowners and discourage any future use of proven water saving technologies such as HETs and dual flush toilets.

Physics, as it turns out, is it right up there with “can’t fool Mother Nature.”

Water-saving toilets in general and dual flush toilets in particular are seen as increasing in popularity, according to the AIA survey published on KBBonline.com.

Also trending in bathrooms:

• Bath sizes are not increasing
• Energy and water efficiency are important, but so are designs to accommodate aging in place.
• Aging in place features on the rise include doorless and/or no-threshold showers, and handshowers.
• In the current economy, other high-end bath features such as towel warmer are not being installed as often.

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.

Glenn Hasek of Green Lodging News, notes the downside to some models regarding the small area of water in the bowl also known as the water spot:

“…some dual flush toilets do not clean the bowl as efficiently as traditional single-flush versions. Be sure to ask vendors about this. Without getting into too much detail here, you obviously do not want to leave your guests with a bad impression and you do not want to add to the work load of your housekeepers.”

Hasek echoes what the Professor has said before about dual flush toilets.  “Dual flush toilets that use a siphonic design have a water spot the size found in standard toilets.“  Learn more from Toilet Scientist Jim McHale and follow Hasek’s blog at Green Lodging News:

How can just 0.9 gallons of water flush 5 large hot dogs?   Because H2Option is the first  truly siphonic dual flush toilet, with strong push and pull action created by forceful but quiet jetted action under the rim.

H2Option

H2Option is the first truly siphonic dual flush toilet, with strong push and pull action created by forceful but quiet jetted action under the rim.

Why does that matter?   As we learned earlier in Flushology, dual flush  toilets do a great job of saving water, and many work great at the toilet’s most important function–getting everything out of the bowl without clogging.

We also learned that dual flush toilets cleanse the bowl with what is known as a washdown flush.  Washdowns can create a little unpleasant splash action.  You know what I’m talking about.

H2Option adds more push to the water from under the rim of the bowl. We engineered the water pathways so that when the water is released from the tank,  some of it is 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, pressurized, but not mechanized, so it’s a quiet action.

There is no sound associated with the added force; and a lot less splashing.  The marketing types call it “Staying Clean While Going Green,” which actually is one of their better headlines.  This is the kind of thing we get to invent at the American Standard New Product Design Center.

Dual flush toilets are a considerable advancement over that old 1970s wheeze:  if  it’s yellow, let it mellow.   Today’s motto is go greener, but do it cleaner.

As noted here, on How Stuff Works, most dual flush toilets do not use siphonic action to clean the bowl.  Until now, most dual flush toilets rely on what is known as a washdown flush.  With a washdown flush, the push of water rushing from the tank to the bowl cleanses the bowl and removes waste.  It is hopefully forceful enough to do a thorough job without as much of the pull that siphonic toilets create.

Push action that is forceful enough to cleanly remove waste is forceful enough to create splashing.   Even the most enthusiastic dual flush owners will admit to keeping a brush and towel nearby for washdown models: the former to thoroughly clean the bowl, and the latter to dry off after unintended splashing.

The better bet is to look for new dual flush models that have perfected the true siphonic pull action of other toilets.