Damper
A movable plate that controls airflow in a chimney or duct.
A damper is a plate that opens and closes to control airflow. In a fireplace, the chimney damper seals the flue when there's no fire so heated air doesn't escape up the chimney — leaving it open in winter is like leaving a window open. In HVAC ducts, dampers balance airflow between rooms. Check that your fireplace damper opens fully before a fire and seals when closed.