In addition to having an air purger near the expansion tank, your system should have a manually operated bleed on each manifold.
Better manifolds allow you to isolate each loop on that manifold. Open the bleed nipple while the water is circulating. As each loop is purged, close the loop.
When all the loops on that manifold are purged move on to the next manifold.
Do you have a water make-up valve in your system? If not, consider addiing one that admits water whenever the system pressure drops below 12psi.
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