From the preceding section you can see that modification can help provide control. A free standing fireplace often has a conical hood. In recent years these have been converted from fireplaces to stoves with glass or metal sides and doors. The Franklin fireplace and similar models are really stoves with doors which can be opened for viewing the fire. These are less efficient because entering air is not completely controlled.
Air control is important since the rate of combustion and thus the rate wood is used depend on it.
Without proper air control, gases from the wood escape up the chimney unburned. Many older model stoves were designed to burn coal, but used wood occasionally as a substitute. These are equipped with grates with an air intake below the grate. In these updraft stoves, gases and other products of combustion were drawn rapidly away from the flames and up the chimney. This method is seldom used in airtight stoves.