Only just noticed this post! Never said it was more efficient, I said it was just as hot. Franklin was not comparing his design to stoves but to badly designed fireplaces. I understood Franklin's argument to be that inefficient fireplaces have a low lintel and the grate set back too far so that most of the heat goes up the chimney. His design has a high lintel and a shallow grate which you would think would lead to a lot of smoke coming into the room
~The fact that it does not smoke is down to the clever design of the back wall, which slopes gently towards the full width throat. The. throat is only four inches deep across the width and only one brick back from the lintel. There is a metal flap across the whole width behind which id a large smoke shelf which catches and smoke coming down a cold chimney and takes it up with the heat from the throat. The metal flap also stops smoke coming back into the room. All I can say is that it works, and has worked here since 1983 when I built it!
