Many people seem to forget that a chimney is actually designed to “suck” warm air away from a room and the fire burning in the grate. This is the way that it is designed to work to ‘draw’ the smoke away with the hot air that rises above the fire, ensuring that it does not enter the room.
Cooler air replaces the air lost up the chimney and provides the oxygen required to keep the fire burning. The problems arises when the fire is not in use, as the hot air from the room will be drawn, unwanted, up the chimney. This is often referred to as the “Stack Effect”.