Answer:
Air ends up in hydronic (hot water heating) systems in the following ways:
1. The system has just been installed or repaired, and it has been filled with fresh, air-laden water. Heating the water will purge the air, which then typically collects in high points. It will need to be purged out of the system.
2. A leak in the system causes fresh water to refill the system (through the feedwater valve).
3. Air migrates from the expansion tank (where it belongs) to one or more radiators (where it doesn't).
We don't want air in the system because:
a. the oxygen in it causes corrosion; and
b. it blocks the flow of heating water.
The way to repair the above problems is:
for 1. -- to purge the air from bleed valves daily until no air remains.
for 2. -- to locate and repair the leak, and then fill and purge.
for 3. --to purge the system, and also empty the expansion tank of water.
The HVAC Veteran