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.
So what is primary, secondary and tertiary air in a wood burning stove, and what does each one do?
Primary air is fed to the solid fuel located at the base of the firebox, typically through the ash pan, for the purpose of helping the fire to get going and the stove to get up to operational temperature.
Secondary air typically enters the stove through an air vent located above the door, or from underneath the stove, and can be used in the stove’s air wash system or for secondary burn of gases.
Tertiary air is fed into the firebox through the back of the stove to aid with secondary combustion and to further reduce emissions from burning wood.
All wood burning stoves are designed differently. Some stoves may only have primary and secondary airflows, while others can have all three forms of airflow.
As every brand of stove works in different ways, I’ve explained in further detail below how primary, secondary and tertiary air works on two of our very own wood burning stoves.
To help you understand the jargon used in this article:
Secondary burn is the process of combusting waste gases from the fire to produce more heat and reduce emissions. I’ve explained what secondary burn is in a wood burning stove in more detail here.
An air wash system in a wood stove helps to keep the glass on the door of the stove clean, and to provide a clear view of the fire. Click here to find out more.
Wood Burning Stove Primary Air Control
Primary air on a wood burning stove is the main source of oxygen for the fire as it starts and gets going.
The aim of the primary air is to get the fire up to sufficient temperature for secondary combustion of gases to occur, so that the stove can begin to provide its optimal heat output. By closing down the primary air vent when temperature in the stove is sufficiently high, secondary air can then take over as the main source of air for the fire.
Closing down the primary air vent helps prevent the wood from burning too quickly, which in turn would lead to literally burning through your supply of wood!
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.
Wood Burning Stove Secondary Air Control
Secondary air typically takes over as the main source of oxygen to the fire once the stove is up to operating temperature.
The Environmental Protection Agency defines secondary burn as:
‘The combustion of fuel materials that are not completely burned in the primary combustion zone, i.e., in the immediate vicinity of the wood.’ Secondary combustion can be achieved by mixing the gases from the wood and from the primary combustion with suitable oxygen at a temperature sufficient to ignite the mixture or sustain burning.’Environmental Protection Agency (EPA)
Secondary air therefore plays an important role in ensuring that the stove operates as efficiently as possible, and releases the maximum amount heat from burning wood.
Secondary air can be used within secondary combustion of gases that are release by the fire. This releases more overall heat into your home than simply burning the wood does, and also helps to reduce the 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 being burnt, while also ensuring that maximum heat is being generated from the stove, and that the gases are being burnt cleanly in order to reduce the total emissions.
Secondary air vents can usually be found above the door of the stove or under the base. Secondary air can be heated up on the way for use in secondary burn or air wash systems as it makes its way around the hot body of the stove.
If both the primary and secondary air flows are completed closed then the fire will eventually burn itself out. The secondary air vent should therefore typically never be closed unless you want to put the fire out.
While leaving the secondary air vent open too much can prevent secondary burn from working efficiently, too little secondary airflow can also cause the same effect.
Manufacturers should state how far open the secondary air vent should be to ensure that the wood is burning the most efficiently, and on one of our wood burning stoves it’s between a quarter and half open.
Wood Burning Stove Tertiary Air Control
In some cases, tertiary air will be used for secondary combustion, replacing or complimenting secondary air used for the secondary burn, which may instead be used for the air wash system.
Tertiary air typically can’t be manually controlled on wood burning stoves, and so is automatically fed to the firebox to facilitate secondary combustion (like on one our wood burning stoves shown below).
Tertiary air can also be heated up within a preheat chamber before entering the firebox, which aids in the ability for the stove to initiate secondary burn.
Tighter regulations on emissions from burning wood in homes means that the wood needs to be burnt very cleanly if the stove is to be used in smoke control areas. Tertiary air helps to provide the cleanest burn possible and so can be found on a number of DEFRA or EPA approved wood burning stoves.
Wood Burning Stove Air Flow (Example 1)
My dad has a traditional looking wood burning stove located in the fireplace in his living room.
The stove has two controllable vents: one located underneath the stove and one located on the front.
The stove uses both primary and secondary forms of air to feed the fire, and allows for both secondary combustion of gases and air wash of the door to occur. There is no form of tertiary air on this wood burning stove.
The front vent provides air to the base of the fire through the ash pan compartment, and up through the firebox grate.
This primary air inlet feeds air to the base of the firebox only and so is kept wide open when starting the fire to help it get going. The primary vent is then either closed or left partially open for the remainder of the fire.
The secondary vent controls are then used to control the rate at which the fire burns. The secondary air vent on this model of wood stove is located underneath the stove, and feeds secondary air to the top of the firebox to facilitate both the secondary burn and air wash systems.
This secondary air vent is also left wide open when getting the fire going, and is used to control the fire.
Again, there are no holes located on the back of this stove to provide a tertiary airflow.
Wood Burning Stove Air Flow (Example 2)
My mother’s wood burning stove is also located in the living room fireplace.
The stove utilizes primary, secondary and tertiary air to operate all of the wood stoves functions.
Unlike our other stove, there is only one controllable vent, and is located underneath the stove.
The vent under the stove controls both the primary air and secondary air to the firebox. Primary air is fed to the bed of the fire, while secondary air is directed to the top of the stove for the air wash system.
The stove manufacturer recommends that this air vent is left open between 25% and 50% to obtain the best results when burning wood.
There’s also a vent located on the back of the stove to provide tertiary air for secondary burn inside the firebox (I couldn’t reach around to take a photo unfortunately).
There are small holes located at the back of the firebox for the tertiary air to aid in secondary combustion of waste gases.
There is no manual operation required for this vent, and tertiary air is fed automatically to the fire. The air is heated up within the body of the stove before entering the chamber to further improve secondary combustion.
These holes are very small, and so force jets of hot air into the firebox to help reignite any left over gases at very temperatures to further increase heat output and reduce emissions.
Primary, Secondary & Tertiary Air On A Wood Burning Stove
It can be easy to misunderstand how a wood burning stove delivers air to the fire for primary or secondary combustion, or to help keep the glass on the stove door clean.
Be sure to read the manual for your wood burning stove to find out which vents control which air flow, and how best to use them to have the most efficient fire in your stove.
Misusing the air controls on your wood burning stove may be causing the fire to keep going out, but ensuring to burn low moisture content wood can be just as important as using the air vents correctly.