If you are hoping for a thermosyphon system, than the system needs a consistent gradient with the heat source at the low point, and the heat storage at the peak
without high spots or air traps. One air bleed at the top to evacuate trapped air, no motor required.
If you have a positive displacement pump (gear, piston) than any combination of gradients can be forced to work with enormous grief and excessive delay while it forces the air through the system if there is a place for the air to escape the piping system into a storage reservoir, once a hydraulic lock has been achieved (no air in the circulating system) than system efficiency has been achieved, and a remarkably small motor is adequate.
If you have a slip type pump (centrifugal, vane, screw,) than it is a balancing act consisting of horsepower vs air compression rebound , and sometimes air rebound wins over a fairly large motor,, multiple air bleeds are you friend in this case.
For a real world example, at my house where it drops to - 10 Fahrenheit I have 2, 150 foot runs, leaving a boiler dropping 5' below ground looping through 2 furnaces, 2 heat exchangers for domestic
hot water and 2 clothes dryers, with a 700 gallon non pressurized tank at the high end.
A (1) Bell and Gosset 1/25th HP pump 3/4" is adequate for both runs because efforts were made to ensure there were no high spots to entrap air and once the water is pushed through all entrapped air escapes via the tank vent. I pump 180 (F) degree water through and choke off the return line to equalize temperature (around 150 (F) once the system equalized) between the two runs.
Conversely I've seen industrial jobs that required a two HP centrifugal pump for less than 1000 linear feet of piping and were a constant headache with hours of time devoted to purging air every single time the system was opened.
