Nearly all closed-loop hydronic heating and cooling systems are supposed to be filled with water or a mixture of water and antifreeze. The only intentional air in the system is that contained in the expansion tank.
Transitioning a newly minted hydronic system, or an older system that has been drained, from air-filled to water-filled is known as purging. The effectiveness of this purging plays a major role in the reliable and efficient operation of the system.