Primary air, Secondary air/airwash, Cleanburn and Tertiary air Explained

Primary air
Many years ago in the mid 19th century, long before the days of global warming discussions, stoves were being widely produced in Europe as cast-iron boxes with many chambers, designed with a firebox in the bottom chamber. Air entered the stove in many cases at the base of the fire bed to ignite and burn the fuel. This was known as primary air.Primary air is still used today in multifuel stoves as a controllable inlet at the base of the stove door. It is the primary or main combustion air inlet into the stove chamber under the fuel bed .The air ensures good ignition of fossil and non fossil fuels and if not controlled by the inlet control will cause the multifuel stove to burn fiercely. Multifuel stoves up to more recent years had only solid cast-iron doors, as pyroceramic glass was not available. All stoves with glass had those little strips, which I am sure many can remember, or mica, which yellowed over time.As our woodburning and multifuel stoves advanced, with panes of pyroceramic glass and better fuel combustion a requirement to reduce emissions, secondary air was developed.

Secondary air
Secondary air is an extra addition of air through an inlet or inlets above the stove door. The secondary air exit vent is close to the angled baffle plate at the top of the fire chamber, and angles downwards appearing as a narrow vent above the stove door and glass This introduces a stream of warm air just as the unburnt gases rise to pass the baffle plate, and enter the gas exit chamber. The warm air ignites them, thereby giving a secondary burn with more heat and less emissions. The secondary warm air from the vent flows downwards behind the glass to the top of the fire bed, also aiding in secondary combustion and presenting a warm air film over the glass. This hinders the smoke from blackening the glass. As the temperature in the fire chamber rises and the secondary air flowing downwards behind the glass gets hotter you can see the hot air visibly clean the glass. This action is called airwash. Woodburning stoves work best after initial ignition by only using this upper controllable inlet, as they burn differently than stoves that burn fossil fuels. The secondary air inlet should also be used to control multifuel stoves, once the fire is well established, with the opening up of the primary air inlet again when more fossil fuel is added or if the fuel is not combusting well.

Cleanburn or preheated secondary air
Cleanburn is really preheated secondary air. The secondary air as opposed to entering the woodburning or multifuel stove at an inlet or inlets above the door is very often brought up ducts from the lower back of the stove. The air is partially fed into the lower part of the fire chamber and rest of the hot air, is ducted above the stove door to a similar exit vent as described above, near the edge of the baffle plate. It again flows down over the door so give a hot airwash and into the fire chamber as hot secondary air. The next time you have a good look at a stove you will see the same channel just inside, above the door on both secondary air and cleanburn stoves. The difference being the clean burn stove will normally have its secondary inlet control at the base of the stove. This preheated secondary air ignites some unburnt gases in the lower and upper parts of the fire chamber, which burn off at higher temperatures than the direct secondary air can achieve. The glass usually stays super clean with preheated secondary air systems.The result of the cleanburn or preheated secondary air is a much better combustion and lower emissions from a woodburning stove.

Tertiary air
Tertiary air further compliments the cleanburn system as a third air intake, which I have seen working superbly in Norwegian woodburning stoves and modern/contemporary woodburning stoves with large burning chambers. The air is normally drawn up the back of the stove via a series of chambers and is then injected into the back of the upper fire chamber through small steel jets. The jets of superhot air ignite the remainder of gases that will only burn off at very high temperatures. . The emissions from these stoves as long as good seasoned wood is being burnt are very low with co emissions in some cases lower than .0.09 vol% at 13% CO2. Many of these contemporary/modern woodburning stoves carry the Din + standard for high combustion and low emissions.

I hope I have made this woodburning stove jargon a little more clear.
These triple burn stoves as I like to call them tend to be in the more expensive range. I suggest that as long as the woodburning stove that you purchase has the obligatory CE Approval or a Din Standard Approval you will be purchasing a good stove with acceptable emission standards. Always burn the recommended fuels to achieve these emissions with a chimney that is regularly serviced and a properly well fitted and maintained woodburning stove.
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17th June 2009

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