Air Vent is a valve that discharges air and other gases that were accumulated in pipelines or vessels. For normal operation, when liquid is pumped through the system at startup, the initial air inside the piping is pushed into the air vent by the pressure of the flow. The float remains in the lower part of the air vent, allowing the valve to remain open and continuously discharge air.

In steam applications, a thermostatic type of air vent is used which contains a welded stainless steel thermal element that expands when heated and contracts when cooled. When air and non-condensable gases are present, the valve is in the open discharge position. When steam reaches the air vent, the element expands and closes the valve off tightly.