Top-sealing chimney dampers help eliminate a major source of heat loss by allowing you to close off your chimney when your fireplace is not in use. They basically function as a “lid” for your chimney, which you can open or close at will.
Without a damper, your chimney is an open path for heat to escape—you should always have it closed when you’re not actively burning in your fireplace. Yet if you are burning, make sure you keep it open. In any case, a top-sealing damper should never be used with vented or vent free gas logs.