Franklin's original design for the Franklin stove.
In colonial America, homes were warmed by a fireplace. The Franklin stove, invented in 1742, is a metal-lined fireplace that stands in the middle of a room. It has rear baffles for improved airflow. It provides more heat and less smoke than an open fireplace and uses less wood. This cast-iron furnace would radiate heat from the middle of the room in all directions, and the iron walls even absorbed heat, providing warmth to the room long after the fire went out.
Source: Franklin's Autobiography
In Order of Time I should have mentioned before, that having in 1742 invented an open Stove, for the better warming of Rooms and at the same time saving Fuel, as the fresh Air admitted was warmed in Entring, I made a Present of the Model to Mr. Robert Grace, one of my early Friends, who having an Iron Furnace, found the Casting of the Plates for these Stoves a profitable Thing, as they were growing in Demand. To promote that Demand I wrote and published a Pamphlet Intitled, An Account of the New-Invented pennsylvania fire places: Wherein their Construction and manner of Operation is particularly explained; their Advantages above every other Method of warming Rooms demonstrated; and all Objections that have been raised against the Use of them answered and obviated. &c. This Pamphlet had a good Effect, Govr. Thomas was so pleas'd with the Construction of this Stove, as describ'd in it that he offer'd to give me a Patent for the sole Vending of them for a Term of Years; but I declin'd it from a Principle which has ever weigh'd with me on such Occasions, viz. That as we enjoy great Advantages from the Inventions of others, we should be glad of an Opportunity to serve others by any Invention of ours, and this we should do freely and generously. An Ironmonger in London, however, after assuming a good deal of my Pamphlet, and working it up into his own, and making some small Changes in the Machine, which rather hurt its Operation, got a Patent for it there, and made as I was told a little Fortune by it. And this is not the only Instance of Patents taken out for my Inventions by others, tho' not always with the same Success: which I never contested, as having no Desire of profiting by Patents my self, and hating Disputes. The Use of these Fireplaces in very many Houses both of this and the neighbouring Colonies, has been and is a great Saving of Wood to the Inhabitants.