Ah, air! It’s grand for living creatures but often a problem for hydronic heating systems.
Air is basically 80% nitrogen and 20% oxygen. Oxygen likes to corrode iron and steel, and there’s plenty of that in our hydronic systems. The oxygen enters with the cold fill water. It’s dissolved in solution and it will react with the metal to create rust. What’s left after that takes place is nitrogen, which will also dissolve in the water and come out of solution, and only when the water gets hot or the system pressure gets low.