Resideo FV183 GoldTop Air Vent 3/4 Inch is a spring-loaded, float-type air vent for HVAC and hydronic heating/cooling systems. It automatically purges trapped air that accumulates when cold water is heated, using a float inside that rises to open the valve and release air. https://www.partshnc.com/fv183-resideo-34-goldtop-air-vent