Why I'm ditching my herbie style overflow

[asylumdown]

I think the return chamber/overflow water height relationship thing is a function of the herbie system in general, regardless of how you have it plumbed. I'm not a physicist at all, but in a 'normal' overflow, the overflow pipes always have more capacity to drain than the return pump can pump, do any added water to the system falls to the lowest point in the system (the return chamber). This adding more water to a normal system just increases the return chamber water level.

In a herbie, the draining capacity of the overflow pipes is throttled, so the height of the water in the overflow and return chamber become highly related variables. Basically, I think that the height of the water in the overflows (and thus the return chamber) becomes a function of the restricted rate of flow, and the total amount of water moving through the system. If you add more water, rather than "falling" to the lowest level like it would with a normal overflow, it gets "held" above the point where the flow is restricted. Essentially it means that you need an ATO that adds exactly the amount of water you need and no more, otherwise you risk slowly overfilling your tank.