As Manager of Engineering Training, I do a lot of Lunch 'n Learn sessions in the U.S. and Canada for specifying engineers. A question that comes up often is: how many automatic air vents should be installed in a hydronic system? My answer is at least one, probably more than one. It all depends on what is required to get rid of the air, based on system design. As a bare minimum, you need the automatic air vent on the air separator that is part of the boiler trim kit, for example the NA553 Series. For residential applications, with ¾" and 1" distribution piping, if the velocity is high enough (>3 ft/sec) to keep the air pockets and bubbles entrained (moving along in the water stream) then one automatic air vent on the air separator might be sufficient.
For lower velocities and/or larger piping you need to start looking at installing automatic air vents on high points in the system because velocity alone will not break up and entrain the pockets of air. Caleffi has 5 sizes (discharge rate) of automatic air vents to fit any application. The 5026, 5027 Series has 0.75 SCFM capacity, the 5020, 5021 Series MINICAL has 1.75 SCFM, the 5022, 5023 Series VALCAL has 2.5 SCFM, the 551 Series DISCALAIR has 4.5 SCFM, and the 501 Series extra high discharge vent has 9 SCFM capacity for those really big jobs.
Additional locations to consider are on the boiler, buffer tank, or anywhere else a pocket of air can develop. Another location for automatic air vents is on radiant manifolds. Due to low loop flow rates air can gather and become trapped when it gets into the manifold due to the increase in size from the PEX tubing to the body of a 1" or 1-¼" inch manifold.
Contact me if you have any questions about this (or anything Caleffi) or if you want to discuss an on-site training session for your hydronics staff.