Wood burns from the top down, and burns best sitting on a bed of ash, with it’s combustion air coming from the top. A wood burning stove needs two things to work efficiently.
Firstly, it needs heat and so it will often be lined with firebricks, but also it is helped by sitting on a bed of ash, as the ash acts as an insulator, directing the heat upwards, into the fire.
Secondly, it needs combustion air from the top, in the form of a good supply of oxygen. There are two types of air intake. Primary Air is the air usually taken in through a control at a low level at the front of the stove to maintain combustion. It is the best way to control a stove burning solid fuels, and is only needed to start a wood burning stove, as once the fire is burning well, it will not need primary air. This control can be adjusted to regulate the air coming into the firebox. Secondary air is a stream of warm air that ignites the unburnt gases, which is known as secondary combustion. This control is typically above the door or to the top of the front of the stove. Secondary air flows downwards along the glass giving a warm air film which helps to keep the glass from blackening. This is known as ‘airwash’. Once the fire is lit a wood burning stove works best by controlling air using the secondary air control.