sc's right. KISS. The idea is to remove the air from the CLOSED LOOP system. Closed loop means once you purge the air (and there are no leaks) air is GONE. Air doesn't magically materialize once it is purged from the loop.
We also call this BURPING the engine's coolant
loop, bus heater or not.
All a header tank does is make a big box where one isn't really needed. All it is is a big box that can be replaced with a manual air vent, a small petcock, or any hose that is above the level of the coolant
so ALL the air is purged out when the system is filled.
Even with a bump in the hose from the engine
(engine to the right), air won't be trapped in that loop because once you fill the hoses from a high point the air will be gone throughout the system.
For those of us not born or trained as engineers, Just think of filling up a soda container with one of those tops with a hole in the middle and a straw through it. You can fill the container by filling it up through the straw, all the way to the top, and as liquid rises in the straw, you KNOW for sure that the container is full. (Ignore the fact that the analogy has an non-air tight plastic lid and connection between the straw and the lid.)