Here in NE USA, with the concern about heating costs, fireplaces usually have a built in flap, locally called the damper, which closes off the chimney much like the stone mason's pane of glass, to keep the warm air in the room from blowing up the chimney when the fire as out, and outside beasties like squirrels and birds from falling into the room at other times. This also regulates the draft/intensity of the fire much like the adjustable grill deliberately placed in the stove pipe of most heating stoves which serves the same purpose.

These dampers are opened to get a fire going, and then closed partially to control the flow of air up the chimney or stove pipe, or completely when the fire is out to keep warmth in and outside critters out.

Dave