Air in Hot Water System

scottbenent

I have a situation.  I have a hot water boiler that feeds a baseboard system.  There is a tankless coil in the boiler that supplies domestic hot water.  The outer plate of the tankless is rotted and was leaking (since stopped).  My plan is to replace it in the spring (live in MA, don't want to take system off-line in case boiler replacement is needed due to corroded/damaged boiler flange).  Even though it is no longer leaking, I still have air (sometimes lots of air).  I have been purging the system periodically. 

The arrangement of the system in the direction of the flow:  pump is located a few feet upstream of boiler, then boiler, then expansion tank mounted to air scoop w/ air vent on top of scoop, then the piping splits into the 2 zones, then a check value, the the 2 zones (baseboard, 1st fl is a zone, 2nd floor is another), then drain/purge spigots, then valve to close piping to purge air, then pump.

I've read about the "pumping away" arrangement and it appears my system was designed differently.  My question is this:  Is it possible all the air I'm getting is from bubbles in the city water expanding ?  Over the past few weeks I've put a lot of water into the boiler (due to leak), but I'm getting alot of air.  Could air be entering the system some other way.  I wouldnt think so because for air to go in, water must have to leak out (kinda like pouring liquid from a bottle).  Thoughts ?  Thanks in advance for your help.