The water that is used initially to fill a hot water heating system contains dissolved air
Make-up water subsequent added will similarly have a high air content
Heating this water releases the air and permits it to be circulated in the system, for which it must be vented
The purpose of the air purges is to continuously separate and collect any air from the water as it circulates so that it may be vented automatically by afloat air vent without the necessity for frequent manual venting
System connection sets the point of no pressure change