It is thought that a giant hearth would have been found in the very centre of the Great Hall, providing heat for the guests. Although there is no physical evidence left today, it would have sat in the entrance hall where you now enter the building. Its presence would be visually imposing as well as providing heat and a decidedly smokey atmosphere. There would be no chimney to let the smoke escape, and so it would have risen into the rafters and left a fog hanging in the room.
The main architectural clue to its existence is the roof itself. The presence of fire is almost certainly one of the main reasons why the roof timbers have taken on their rich dark colour.
Listen to a short interview with Richard Buckley, (lead archaeologist at the University of Leicester) describing the hearth.