[untamed]

Originally Posted by GreenSpottedPuffer

So you were saying to have the primary drain as just a bulkhead right?

That's right. The primary drain (with the gate valve on it) is just a bulkhead on the bottom of the overflow. The emergency drain has a standpipe.

When you first turn it on, water should fall right to the bottom of the overflow box and go out the primary drain. As you close off the valve, you'll hit a point where the water level in the overflow box starts to rise. You want to dial up the water level as high as possible, short of the emergency drain.

My overflow boxes are only 12" deep. I found that I need to keep the water at least 6+" deep to prevent air bubbles being drawn down into the primary when the water falls into the overflow.