Damper (chimney)
A movable plate in a chimney that opens for fires and closes to block drafts.
A chimney damper is the metal flap inside the flue above the firebox. Open it before lighting a fire so smoke escapes; close it when the fireplace is cold so heated or cooled indoor air doesn't pour up the chimney. A damper stuck open is a big, silent energy leak — and one stuck closed is a smoke hazard. (Not to be confused with HVAC duct dampers.)