Time to purge air in a system (baseboard water heat)

Gquinn1824

I'm a homeowner with water baseboard heat. There is air in the system that is very noisy despite my best efforts to get rid of it. I have three questions. 1. How long should it take a working system to purge the gases that are evolved from a new charge of city water? 2. Given that I cannot find a leak in any of the pipes, baseboards, expansion tank (brand new) or pump, where could air be entering such a system (pressure is 15psig)? 3. There is an occational drip from the back flow preventer attached to the city water line, on the city side of the pressure regulator. Can that be my problem spot?

Weezbo

Spiro vent clogged?

Gquinn1824

3 automatic vents. None clogged

I have 3 # 12 vents in the system. One for each of two zones and one on the main return. Only water comes out of the 1st zone valve. Air sometimes comes out of the 2nd zone valve, and no air comes out of the return valve.