If air can collect in a portion of the pipe then an air lock can form. I learned this the hard way in a small diameter gravity drain.
Only 4" of freeboard means the drain may never fully fill due to sucking air from the surface of the water. An anti-vortex device, like a hood, might help a little bit, but the pipe probably needs to be set lower.
Last edited by RAH; 12/08/19 10:05 AM.
