The smoke shelf is built just where it will prevent this action.
The second vital consideration is the introduction of what are known as a “smoke shelf” and a “smoke chamber.”
The depth of the smoke shelf should be the same for a 2-foot as for a 10-foot fireplace opening.
The throat will in this case be in the middle of the chimney with the smoke shelf on either side of it.
The smoke shelf should not be bricked up but should conform to the dotted lines.