A float type automatic air vent on the air scoop near the boiler and another at the highest baseboard in the sytem would be enough. They need to be used for a few weeks after the system is refilled to purge air left after a flushing purge of the loops, then kept closed to prevent air from reentering the system. It is a good idea to include a balancing valve boiler drain combination on the end of a loop to permit flushing air during a fill procedure. A boiler drain on the end of the zone valve loops will do. You can see some ideas for these at: http://www.hydronic.net at the free design lessons section.