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Flappers or Pistons

So what's wrong with flappers anyway. They've been in toilets for decades and decades. they do a decent job of letting the water out of the toilet tank and seal back up most of the time. They have to be replaced every couple of years because of cracking and warping or deterioration from caustic chemicals, but all in all they work pretty good. Better yet they're cheap to replace.

I never thought much about them until our company started getting involved with flushing mechanisms, i.e. Dual Flush Kits. They operate using a piston mechanism that is lifted quickly by a cable. A float assembly inside the dual flush mechanism determines when the piston will drop back down to it's normally closed position. Most high end dual flush toilets on the market today use this type of mechanism. I was confused at first as to what the big deal was until I started doing my own tests to determine how much water was necessary to accomplish the task.  Whats I found interesting was that if you flush a toilet using a conventional flapper valve, you can only lower the water level in the tank by about 1/3 before the bowl doesn't evacuate any more. This is especially true on 1.6 gallon toilets. On older 3.5 and 5 gallon toilets where there is quite a bit more water in the tank to start the siphon action, the level can be dropped farther.What I began to notice after installing Dual Flush Kits in different size toilets was that it took less water to do the same amount of work. I could get a 1.6 gallon toilet to function on sometimes as little as .7 gallons of water. This baffled me at first. Then I started to look closer. As it turns out, a toilet flapper is designed to restrict the flow of water on about 1/3 of its circumference. Flapper designers have tried to eliminate some of this restriction by elevating the hinge point to a point where it wouldn't interfere with the water flow but it still does. With a piston style dual flush mechanism, the entire circumference has an equal flow of water. This means that the water is going to leave the tank a little faster. Also, The Dual Flush Device is bored out a little bit to allow for increased flow as well. This makes the valve wall a little thinner but not so much as to effect the structural integrity of the valve. On an average, you can expect about a 20% increase in water volume leaving the tank when the valve is opened. This increased volume equates to increased siphon action in the bowl below. In other words, the water is entering the bowl at a higher velocity so it takes less to do the job.