A beehive system includes a main hive body with opposed, insulated side walls having access ports formed respectively therein. A vent assembly fits into an access port, and includes a valve case with an outer housing, a coaxial core portion, and plural opposed panels between the outer housing and the core portion, with openings formed between the panels. A pivotally movable valve member includes a core portion and two or more blades attached to and extending outwardly from the core portion. The valve member cooperates with the valve case to regulate air flow through the vent assembly. A bimetallic coil interconnects the valve case and the valve member, and a perforated screen cover is attached to the valve case to allow air flow therethrough. A kit of components usable to retrofit an existing hive, and a method of retrofitting an existing beehive to add a vent thereto, are also disclosed.