I have not tried this method (many different methods to accomplish different ends) though I am pretty sure I will use it on my new build. From what BeanAnimal writes the second pipe is used to silence the system and the third is a fail safe (like I said I have not tried it but according to a person who has)
Yes different methods accomplish different ends, or put better--"results." You would be hard pressed to come up with a system that will accomplish the same results as this (Bean's) system does, or that would provide better performance....
Though the quote below, is in context, this conclusion (to silence the system) is actually out of context.
From BeanAnimal
The solution was simple. Incorporate a siphon into the overflow setup and back it up with an open channel standpipe. If the siphon is adjusted to a flow rate that is slightly lower than what the return pump provides, then water will backup in the overflow box and be handled by the second (open channel) standpipe. If the system is adjusted properly then the volume of water flowing through the second standpipe will gently cling to the walls of the pipe and be silent. A benefit of such a setup is that the siphon standpipe can handle a significantly higher flow than an identical standpipe that has water mixed with air. I added a third standpipe to the system that stays dry during normal operation. This third standpipe provides some of the fail-safe features that allow for either of the two primary standpipes to become fully clogged without causing a flood or system malfunction!
A siphon by nature is silent. There are exceptions to this rule, however, at the flow rates common with our systems, for all practical purposes the siphon is silent as no air enters the line--and no other adjunctive aid is required to silence the siphon.
Open channels on the other hand, operate on a completely different set of physics, and with the addition of air in the line, if the pipe is more than ~ 1/4 full of water, the air and water mix. This causes noise and other problems common to "durso" type drains--which is what the open channel is. This is pretty much common knowledge now, though some still debate the physics.
Bean's statement concerning silence in the above quote is a comment concerning the open channel--not the system as a whole. Because the volume of water flowing in the "open channel" is very low, the OPEN CHANNEL aka DURSO will operate silently, (additionally--bubble free, and free of the problems common to "durso" style drain systems.) If the flow is higher, the open channel will create problems such as noise. Lacking the open channel the system would still be silent.
The "open channel" is present to provide a self adjustment feature for the system, and as a 2nd fail safe for the system, things that "herbie" lacks.
The system that has been dubbed "The Herbie" is also a silent system, though it does not utilize an "open channel" aka durso stand pipe. It is silent because it utilizes a siphon.
thanks
