Efficiency

Enclosed stoves hold out the possibility of greater efficiency, controllability and lower smoke emission than simple open fires. In free air solid fuels burn at a temperature of only about 270°C, too low a temperature for perfect combustion reactions to occur, heat produced through convection is largely lost, smoke particles are evolved without being fully burned and the supply of combustion air cannot be readily controlled.

By enclosing the fire a draft (draught) is generated pulling fresh air through the burning fuel or gas to raise the combustion temperate to a point where efficient combustion is achieved, the enclosure allows the ingress of air to be regulated and losses by convection are almost eliminated. It also becomes possible, with ingenious design, to direct the flow of burned gasses inside the stove such that smoke particles are heated and destroyed.

A first step in improvement was the fire chamber: the fire was enclosed on three sides by brick-and-mortar walls and covered by an iron plate. Only in 1735 did the first design that completely enclosed the fire appear: the Castrol stove of the French architect François Cuvilliés was a masonry construction with several fireholes covered by perforated iron plates. It is also known as a stew stove. Near the end of the 18th century, the design was refined by hanging the pots in holes through the top iron plate, thus improving heat efficiency even more.

In order to prevent air, and therefore smoke, from spilling back into the room a large updraft pulling air (and therefore heat) out of the chimney is needed. This both pulls heat away and pulls air from the rest of the house into the fire and then up the chimney. A fireplace consumes 200 to of air per minute, more for a very large fire. Even a mostly closed-off fireplace, for example a modern fireplace with glass doors closed, will use 50-150 cubic feet per minute. High airflow creates a draft which pulls heated air out of the house to be replaced with cold air leaking in from the outside. Second, in an open fire some of the combustible gas coming off the wood escapes and thus does not ignite and is lost. By controlling the inflow of air to allow only what a fire needs to burn, modern stoves can reduce the consumption of air to as little as 15-30 cubic feet per minute, though consumption varies.

Modern stoves also increase the completeness of combustion by capturing most of the heat from the combustion and exhaust. Some types use a catalytic converter which causes combustion of the gas and smoke particles not previously burned. Other models use a design that includes firebox insulation, a large baffle to produce a longer, hotter gas flow path. Modern enclosed stoves are often built with a window to let out some light and to enable the user to view progress of the fire.