Herbie works by using the gate valve to match the overflow gph to the return pump gph exactly. This then does not let air into the the plumbing and therefor is very quiet. To do this, the main drain needs to be a few inches underwater in the overflow. Or you can just use a strainer in the bottom, doesnt matter.
As for a power outage, I dont see how its much different than any overflow...It will drain until the water goes below the teeth of the overflow box and then water will stop flowing into the overflow and then can't possibly go into the sump anymore. So basically maybe an inch of water. Where the pipe is, doesnt make a difference.
Your idea for the skimmer is good. I may have to just put it in the sump though depending if I make my stand higher or not. If its not made higher, I would not be able to have the skimmer raised up so that the return feeds back down inot the refugium.
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