I don't think it's necessary in the sense that you can successfully get water from a tank to a sump with only 1 drain. For this particular style, geared towards silence, minimal adjustments to the valves, and fail safe operation, I think it's necessary. I can't say that it's not possible to achieve all of those goals with only two drains.
What I've noticed so far is that all three drains are used at some point. When I shut down the return pump and start it back up, the closed channel takes about 10-15 seconds to establish a siphon. During that time, the open channel becomes overwhelmed and effluent comes out of the overflow channel (upside down elbow). Once the siphon is established in the closed channel, the open channel seems to always produce about half of the effluent of the open channel, and the overflow channel is dry.
I have both the open channel and the closed channel dialed back. If either are fully open, I get noise (gurgle from the open, sucking vortex from the closed). However, if one or both were fully open, the overflow channel may never be needed.
A couple of things that might reduce the need for the overflow channel:
-I haven't run my airline into the overflow box yet. When you do that, the open channel becomes a closed channel when the water reaches a high mark, and volume is increased.
-I use 1" 90 degree elbows. The building plan calls for 1.25" to 1" slip street sweeps with teeth cut into them. I haven't been able to find that type of elbow any where, but they are supposed to allow more flow through the open and closed channels without causing a noisy vortex.
