catchment tank overflowing the top

[Jay Bondesen]

As said just above, when too much water comes into the tank too quickly it can submerge the overflow inlet. Then like a sink or toilet the water will start swirling into the overflow pipe and create enough suction to draw air in as well as water and you loose the siphoning effect. Like household plumbing the system needs venting. After flipping my overflow set-up a few times to remove the air I added an open T fitting pointing up at the outlet, past the two U shapes that create the siphon. That also allowed me to plumb the water down to the ground and over 20' or so to drain in a better place.

Jay