Removing hot air and other gases in equipment is just as important during operation as at start-up. For example, carbon dioxide or other non-condensable gases can be generated while equipment is operating normally due to insufficient treatment of the boiler feed water. Also, residual air inside equipment can be heated by the steam to high temperatures and interfere with heat transfer. If the air and/or non-condensable gases are not completely removed, the efficiency of the steam process can be severely impaired. Air venting should thus be ensured throughout start-up and during regular operation.

Air vents are particularly important when equipment configuration causes air build-up or when the capacity of the air venting function of a steam trap is insufficient (i.e. usually during start-up). When selecting an air vent for steam, the balanced pressure type of air vents are recommended over the bimetal type, and should be fitted in collection areas where air will be pushed on start-up.