Primary, secondary and tertiary air is delivered into the firebox of wood burning stoves through strategically placed vents and passageways. Some of these vents can be manually controlled, while others don’t require any intervention to work. Each form of air plays a different role in helping to create the most amount of heat from burning wood in a stove.
Wood Burning Stove Primary Air Control
Primary air in a wood-burning stove is the main source of oxygen for the fire as it starts and gets going.
The purpose of the primary air is to bring the fire to a temperature high enough for secondary combustion of the gases to occur so that the stove can begin to provide its optimal heat output. By closing the primary air vent when the temperature in the stove is high enough, the secondary air can then take over as the main air source for the fire.
Primary air is fed to the solid fuel located at the base of the stove firebox, and so primary air vents are typically found near the bottom of the stove. Air is usually fed through the ash tray to the bed of the fire. Primary air does not typically need to be heated prior to entering the firebox.
Closing down the primary air vent helps prevent the wood from burning too quickly.
Wood Burning Stove Secondary Air Control
Once the stove is up to operating temperature, secondary air typically becomes the main source of oxygen for the fire.
Secondary air therefore plays an important role in ensuring that the stove operates as efficiently as possible and releases the maximum amount of heat from the burning wood.
Secondary air can be used in the secondary combustion of gases released by the fire. This releases more overall heat into your home than simply burning the wood, and also helps to reduce emissions from burning wood.
Properly controlling the secondary air vent on your stove will ensure that you're regulating the rate at which the wood is burning, while also ensuring that maximum heat is being generated from the stove, and that the gases are being burned cleanly to reduce overall emissions.
Secondary air vents are usually located above the stove door or under the base. Secondary air can be heated en route for use in secondary combustion or air scrubbing systems as it makes its way around the hot body of the furnace.
If both the primary and secondary air streams are completely closed, the fire will eventually burn itself out.
While leaving the secondary vent open too much can prevent the secondary burn from working efficiently, too little secondary air flow can have the same effect.
Wood Burning Stove Tertiary Air Control
In some cases, tertiary air is used for secondary combustion to replace or supplement the secondary air used for secondary combustion, which may instead be used to keep the fire door glass clean with the air scrubbing system.
Tertiary air typically can't be manually controlled on wood stoves, so it is automatically supplied to the firebox to facilitate secondary combustion. Tertiary air can also be heated in a preheat chamber before entering the firebox, which aids the stove's ability to initiate secondary combustion.
Tighter regulations on emissions from burning wood in the home mean that wood must burn very cleanly. Tertiary air helps ensure the cleanest possible burn.
Conclusion
It can be easy to misunderstand how a wood-burning stove delivers air to the fire for primary or secondary combustion, or to keep the glass on the stove door clean.
Be sure to read the manual for your wood stove to find out which vents control which air flow and how best to use them to get the most efficient fire in your stove.