You can do it from the "Chimney Effect" tables at engineeringtoolbox.com, too.
Natural Draft Ventilation Equations
You have to iterate the equations a bit, since the siphon has several channels doing different things up, down, and sideways. But it does seem to allow for pipe size and even lets you mess with friction.
Now I gotta check that boiler reference. Wish I wasn't so tired ... I'm curious if the equations are different for water vs. air. (they should be, to predict actual performance, but are they?)
-Erica