Overflow box not siphoning

[TheReefGeek]

Is this overflow a u-tube design? Is your SEIO 820 for sucking out air in the top of the u-tube?

If it is a u-tube, are you priming it? Are both ends of the u-tube submerged in water when power goes out? My guess is your u-tube has broken siphon, you need to prime it, and make sure both ends stay submerged when you power goes out, this is easy to test.

Johnny, can you explain why your overflow almost flooded? I just dont want to scare people away from them without knowing why. External overflow boxes can be setup completely flood-proof IME, but most people do not take the necessary precautions, the devices are as safe as they are setup and maintained.