Phase 3 not firing

I think you are correct (HVACNUT), that I haven't a clue how to purge this system. To clarify, I did not replace the relay, just mentioning that it reads red correctly when a zone calls for heat. After your post, I attempted to fire a separate baseboard zone just to see what happens, and the boiler would not fire for that either. Although, the relay continues to recognize each of the zones (including the Phase III zone) with a corresponding red light when called, nothing fires the boiler. So I shut the boiler down last night. Which is where it stands now; entire system is now completely cold. Flipping the power switch on and pushing the restart did not fire it up, stone cold….Here are pics of the system. Is it possible that the entire system (not just the Phase III zone) needs to be purged of air? Assuming, that is, that I can figure out how to fire up the boiler to get adequate PSI going…Re the purge procedure I did last night (that didn't work), I shut the vale directly above, exiting the circle pump, then connected a hose to the valve at the end of that pipe where it enters the Phase III. Cold water came out; no air.