Not going to back siphon. Its a overflow like any other. Just splits into 2 lines where it drains into sump. And I have 2 shutoffs, one on each drain into sump. So, the theory is, I'll throttle the "sump" return to what I need, then leave the "pump" line wide open. Might need to throttle them both a bit though, as I have dual 3/4" durso overflows feeding the 1" drain manifold. heres a pic
Ohh, and for the pump feed line to the tank, I have a swing check valve in there, AND the outlets are just below the water surface, around overflow level. Soo, any drinback will be the same as the overflow anyways. Ohh, and the pump is running on a UPS