Got under the house this evening and rerouted the siphon to travel downhill the entire time. Unfortunately, the siphon still will not start WITH the gate valve in the correct position, which is almost closed. Matter of fact, it is difficult to see that it is open in the picture.
If I take the gate valve off, no problem starting the siphon.
I think I over-estimated my flow rate. I have about 11 feet of pipe between the return pump and the tank. About 6 feet of that is vertical. Head loss charts say the Mag 9.5 at 10' will push 400gph. At 6 feet it will push ~700.
Some testing shows the siphon starts just fine with the gate valve open 1/2 way. It takes about 2-2.5 minutes for the siphon to full start. My logic ----> if it works with the gate valve half open, I need a bigger pump to supply enough water to run the overflow with the gate valve 1/2 open.
Thoughts?