I''m not familiar with that particular head (must have some kind of ''trash pump''?) but most are gravity feed. According to CG & Yachts standards, all below waterline thru hulls ending below WL (at tank) should have vented loop which is above WL at all angles of heel. This is to prevent flooding the boat and/or filling the tank should seacock fail or be left open. Manual for Jabsco Macerator pump states vented loop required for below waterline thru hulls also. Mostly affects line from macerator pump & water intake line for head. I know Catalina didn''t put loops in either place in my boat and it may be physically impossible to retrofit without extensive mods. They did put labels all over advising to only open thru hulls when in use which is what I do.
Just looked at that page posted above & looks good but a little confusing, maybe I''m just being dense today but the tanks higher than head is throwing me, flushing uphill?