The images and information in this post are taken from Larry Gray, The Complete Book of Heating with Wood (Garden Way Publishing, 1974).
Franklin estimated contemporary fireplaces to be only 17 percent efficient, leading to the need for larger farms to employ a man full time to keep them supplied with cord wood. He designed his stove to be "luted" (that is, having its joints sealed) to reduce drafts, anticipating the "air-tight" stove not patented until 1836 by Isaac Orr of Washington, D.C. Franklin's design also included an "air-box", the forerunner of the heat-o-later, where air was drawn in from the cellar and forced to move through a long path before exiting at the top of both sides. Combustion gases were forced to draft downwards before leaving via the chimney. He commented, "People who have used these fireplaces differ much in their accounts of the wood saved by them. Some say five-sixths, others three-fourths, and others much less...My common room, I know, is made twice as warm as it used to be, with a quarter of the wood I formerly consumed there."
In 1771 he went even further with his "smoke consuming" stove which was designed for coal but could also be adapted for wood. This attempt to extract more fuel value from the volatile gases was even more impressive, given that Priestley had not yet discovered oxygen and Lavoisier's research into the chemistry of combustion was not to occur until 40 years into the future. Franklin's own experiences with this later stove, both in England and America, were that it demonstrated a bad tendency to back puff when opened, unless a good draft was operating in the chimney. Consequently, he recommended against its tending by "ignorant servants", preferring that it be used by men of letters who could better understand its tendencies.
Franklin never patented his inventions, including his bifocals, lightning rods and stoves, because he felt it unjust to do so when he benefited from the inventions of others. Consequently, manufacturers copied his work and changed it, ultimately resulting in today's "Franklin" stove, which Franklin himself would have despised.
For a detailed examination of the development of wood stove technology and patents, see J.H. Peirce, Fire on the Hearth (Springfield, Illinois: Pond-Ekberg, 1951). Commenting on this reference, Mr. Gray adds:
"Most schemes for extending the usefulness of wood stoves as found today in publications like The Mother Earth News are revivals of ideas that occurred to our great, great grandfathers. Lying fallow in the patent office are designs for the spiral stove, the nurse stove, the egg stove, and numerous improved stoves. American ingenuity also gave birth to a patented device for "leading the flame in stoves by gauze wire" and a "reverberating plate" to improve combustion."