Cleanburn combustion or Secondary Air/Tertiary Air
Introducing pre-heated, secondary/tertiary air into the firebox allows there to be combustion of unburnt hydrocarbons in the smoke. This 'cleanburn' process can improve the heating efficiency you obtain from your fuel and give you an improved flame visual.
Usually, the secondary air enters through a control on the front (top)of the stoves .Use for log burning.
All wood and multi-fuel stoves with a heat output above 5kW require the provision of additional air for combustion.
An External Air facility allows this air to come directly from outside your building rather than through a vent into the room, thus eliminating draughts and adding to the overall heating efficiency.
If a strong development of smoke is produced when wood is added to a small bed of embers and the primary air supply is opened, it can produce an explosive air/gas mixture and result in deflagration.
For safety reasons it is recommended you initiate a new heating process.
This is the air that is drawn into the stove, typically at a low level to maintain the combustion of the solid fuel being burnt. Usually, the primary air enters through a control on the front (below)of the stoves which can be adjusted to control the amount of air entering the firebox and, thus, giving you the opportunity to regulate the intensity of the fire. This, in turn, will alter the heat output.
Primary air is the best way of controlling a stove burning solid mineral fuels and may also be used to start a wood fire however primary air is not normally used in a log fire once the logs are burning well.