I see one zone marked "basement". Is the basement zone an old radiant one, with copper in the concrete, or does any of the piping go into the concrete. If the answer is yes, then I will guarantee that there is a leak in the floor, which is where the air is coming from, either through the leak, or due to the constant introduction of water into the system, which brings oxygen/air into the system.
Also, that air eliminator would work better if it was straight up and down. Being crooked is going to prevent the float from moving up and down and releasing the air.
Hi, my name is Glenn, and I'm a Toolaholic!