Originally Posted by jeffaquarius
I doubt it if you have ever used such overflow box. Your water will not just go up your tank and down to your sump. Your imagination will not work.
No, the water level in the overflow box will only drop to the level of the strainer grate on the in-tank portion.
As was pointed out, you would only lose siphon if air were allowed to accumulate in the overflow "tube".
When the power goes out, the overflow should only drain the last section, not the entire thing, and it should maintain siphon. If you raised the black "divider" to a point that it was almost the same level as the last divider, you could lose siphon.
I's the balance between the black adjustable divider and the final section that determines if siphon is lost when the power goes out at least on the CPR model it appears to be how it works.
Look at the pic, you'll see what I mean.