Many boilers have an air eliminator on top of the boiler. Years ago, I saw a
demo of a "Spirovent" which I thought was totally fascinating. Perhaps the
OP air eliminator isn't working properly.

Christopher A. Young
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"Limp Arbor" wrote in message
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Vic has it right. Any air in the system will be picked up by passing
water. The water will then release this air at the point of least
pressure. This is usually at the highest/farthest radiator. Rich
Trethewey gave a great explanation of this on one episode of TOH. A
simple explanation is that water will boil more vigorously in a pot
with the lid removed than a pot with a lid on.

You need to bleed every radiator in the system until all air is gone.
I usually do this first with the system off then run the pump(s) for a
while then shut down and bleed again. In a big system like you have
it can take a long time but you will eventually get all of the air
out. Then you just have to keep the pressure high enough to prevent
the new water from releasing air again.