Franklin stove

The Franklin stove is a metal-lined fireplace named after Benjamin Franklin, who invented it in 1742. It had a hollow baffle near the rear (to transfer more heat from the fire to a room's air) and relied on an "inverted siphon" to draw the fire's hot fumes around the baffle. It was intended to produce more heat and less smoke than an ordinary open fireplace, but it achieved few sales until it was improved by David Rittenhouse. It is also known as a "circulating stove" or the "Pennsylvania fireplace".

The two distinguishing features of Franklin's stove were a hollow baffle (a metal panel that directed the flow of the fire's fumes) and a flue that acted as an upside-down siphon.

Baffles were used to lengthen the path that either a room's air or a fire's fumes had to flow through ductwork, thereby allowing more heat to be transferred to the room's air temperatures or from the fire's fumes. Specifically, ducts could be installed within the brickwork around a hearth; cool room air would then enter the lower end of a duct, be heated by the hot walls of the duct, rise, and finally exit from the duct's upper end, and return to the room. The longer the path through which the air flowed, the more heat would be transferred from the fire to the air. Similarly, the longer the duct through which a fire's fumes had to flow before reaching the chimney, the more heat would be transferred from the fumes to the room's air.

The use of baffles to extract more heat from a fire and its fumes was not new. In 1618, Franz Kessler (c. 1580–1650) of Frankfurt-am-Main, Germany published Holzsparkunst (The Art of Saving Wood), featuring a stove in which the fumes from a fire were forced to snake through five chambers, one above the other, before entering the chimney. Kessler also documented an enclosed heating stove that, like Franklin's stove, had a baffle directly behind the fire, thereby lengthening the path that the fire's fumes had to travel before reaching the chimney.

In 1624, a French physician, Louis Savot (1579–1640), described a fireplace that he had built in the Louvre. Ducts passed under, behind, and above the fire in the hearth. Cool air in the room entered the lower opening of a duct, was warmed, rose, and returned to the room through the duct's upper opening. In 1713, Frenchman Nicolas Gauger (c. 1680–1730) published a book, La Mécanique du Feu (The Mechanics of Fire), in which he presented novel designs for fireplaces. Gauger surrounded the hearth with hollow spaces. Inside these spaces were baffles. Cool room air entered the spaces through lower openings, was warmed as it snaked around the baffles in the spaces, and returned to the room through upper openings.

In Franklin's stove, a hollow baffle was positioned inside and near the rear of the stove. The baffle was a wide but thin cast-iron box, which was open to the room's air at its bottom and two holes on its sides, near its top. Air entered the bottom of the box and was heated both by the fire and by the fumes flowing over the front and back of the box. The warmed air then rose inside the baffle and exited through the holes in the baffle's sides. Franklin's baffle thus performed at least two functions: like Kessler's heating stove, it lengthened the path that the fire's fumes had to follow before reaching the chimney, allowing more heat to be extracted from the fumes; and like Gauger's fireplace, it placed a duct near the fire, which heated the room's air via convection.

Some early experimenters reasoned that if a fire in a fireplace were connected by a U-shaped duct to the chimney, the hot gases ascending through the chimney would draw the fire's smoke and fumes first downwards through one leg of the U and then upwards through the other leg and the chimney. This was what Franklin called an "aerial syphon" or "syphon revers'd". This inverted siphon was used to draw the fire's hot fumes up the front and down the back of the Franklin stove's hollow baffle, in order to extract as much heat as possible from the fumes.

The earliest known example of such an inverted siphon was the 1618 fireplace of Franz Kessler. The fire burned in a ceramic box. Inside the box and behind the fire was a baffle. The baffle forced the fire's fumes to descend behind the baffle before exiting to the chimney. The intention was to extract as much heat as possible from the fumes by extending the path that the fumes had to follow before they reached the chimney.