A properly plumbed hydronic heating system makes it easy to add water and remove air, during the initial fill / purge and in later servicing.
We remove all of the air to make the hydronic heating system work mechanically correct. Small amounts of air can cause gurgling noises and large amounts of air can cavitate the circulators pumps causing the heating loop to stop flowing, resulting in a lack of heat.
When filling or purging a hydronic heating system, it is easiest when the system is broken down into smaller sections or loops. Each loop / zone is connected to the hydronic boiler fill valve and has the ability to be isolated from the other loops. Ball valves and drains isolate the system.
Water is introduced into the hydronic system in one way, flowing down the piping and out the boiler drain, carrying the air with it.
The process of isolating and filling is repeated until the hydronic system is full of water and the air removed.
Flushing the hydronic system works in reverse, replacing the old water with new. Flushing the hydronic system is important. The hydronic heating system should be flushed to remove and flux or solder that may be mixed in the water.
This is only an example on flushing a system - actual piping would be different as a Primary Loop would be required and a Secondary Loop with Pump and maybe mixing valve.