My parents have wood burning stoves in their living room fireplaces, and both wood stoves have secondary burn functionalities built into them.
Secondary burn, also known as secondary combustion, helps to improve the heat output from burning wood, and helps wood burning stoves to achieve much higher efficiencies than traditional wood burning fireplaces.
It also provides other benefits such as helping to prevent blackening of the glass on stove doors and reduced emissions from burning wood.
Secondary burn plays a key role in making stoves as efficient as possible, so what is secondary burn on a wood stove?
Secondary burn or combustion on a wood stove is the process of burning off waste gases higher up in the stove in order to produce more heat and to reduce emissions. A second feed of air over the fire in a wood stove firebox helps secondary burn to occur.
The idea behind secondary burn in wood stoves is the same for all stove manufacturers, but my parent’s stoves offer secondary burn functionalities in slightly different ways.
I’ve explained in more detail below how secondary burn occurs in wood stoves.
What Is Secondary Burn On A Wood Stove?
Secondary burn in wood burning stoves helps to produce much higher heat outputs than stoves without a secondary burn process.
As a result, the majority of new wood burning stoves will typically come with secondary burn functionalities as standard.
Burning wood releases waste gases such as carbon dioxide, but the fire alone can’t burn all of these gases as it’s using much of its energy to burn off the excess moisture in the wood.
Between 50 & 60 percent of the heat generated by burning wood is through secondary burn of the waste gases. [MSU]
Secondary combustion of waste gases works by providing a source of fresh air to higher parts of a wood stove firebox, and most importantly above the flames.
You’ll typically see small holes at the top back of the firebox in a wood burning stove where the air is fed into the firebox for secondary combustion to occur.
Along with a fresh supply of air to the area above the flames, the high temperatures reached within a wood burning stove helps to reignite the gases, therefore providing more heat output than simply burning the wood alone.
Secondary burn typically starts to occur when the stove reaches higher temperatures. By closing down both the primary and secondary air vents on the stove you can learn how to control the airflow to maximize the amount of heat produced for the amount of wood used.
Efficiency of a wood burning stove means how well that stove converts the energy stored within the wood into heat. The higher the efficiency rating on a wood stove, the more heat it is typically able to produce per piece of wood burnt. Secondary burn helps stoves to reach very high efficiency ratings of 80% or higher, compared to traditional open wood burning fireplaces where as little as 10% or 20% efficiency can be seen.
This secondary combustion of waste products from a fire also helps to reduce the emissions caused by burning wood in your home, which is vital for use of stoves in smoke control zones.
A wood burning stove is also designed to keep the waste gases and smoke in the firebox for longer, thanks to a baffle plate located near the outlet to the flue pipe that also helps increase the pressure within the firebox. By increasing the pressure and the time spent in the firebox, there’s more chance for secondary combustion to occur.
The baffle also helps to reflect and radiate heat back into the firebox to aid with secondary burn.
My parent’s wood burning stoves both have secondary burn functionalities built in, but offer this feature in slightly different ways.
Wood Burning Stove Example 1
Below is a picture of my dad’s wood burning stove.
It’s manufactured by Clearview Stoves and their stoves have a patented ‘hot air wash system’, where air is sent around the stove before being fed into the firebox. This allows for the air to already be hot as possible before entering the firebox, and helps to improve the efficiency of combustion and to also reduce the potential for the glass stove door to darken.
This particular model of stove has two sets of controllable air vents; one located on the face of the stove and one located underneath.
The front air vent provides air to the fire through the ash tray compartment and up through the grate on which the fire is built. As this stove is a multi-fuel stove, this vent can be used to provide more oxygen to the fire when coal is also being burnt.
The vent underneath the stove provides the fresh air that helps initiate secondary combustion of waste gases and smoke from the fire.
The air warms up as it travels from the base of the stove to the top of the fire around the body of the stove, which according to Clearview Stoves helps to provide a cleaner combustion.
Wood Burning Stove Example 2
My mother’s wood burning stove also has secondary burn functionality, but works in a slightly different way.
Unlike my dad’s wood stove, there’s only one controllable air vent on the stove, but is also located on the base of the stove, with the handle located just under the stove door.
This air vent controls the primary air to feed the bed of the fire and also the secondary air that provides the ‘airwash’ system to keep the glass on the door looking cleaner.
There’s also a further air vent on the back of the stove that feeds air to a row of holes located at the top firebox surround. Fresh oxygen is therefore supplied to the firebox above the fire from this vent, helping secondary combustion of waste smoke and gases to occur.
Wood Stove Secondary Combustion Temperature
The temperature at which secondary combustion of gases occurs in a wood burning stove is upwards of 1100 degrees Fahrenheit (just under 600 degrees Celsius).
A wood burning stove burns wood in different stages.
Primary combustion is where the fire burns the wood and combustible gases are released from the wood, but the temperature in the stove isn’t yet hot enough for these gases to burn. Primary combustion starts at around 540 degrees (282 degrees Celsius) and continuous up to 900 degrees (482 degrees Celsius). [MSU]
Secondary combustion is the where the gases released from primary combustion of the wood start to burn from 1100 degrees onwards (593 degrees Celsius). This step releases up to 60% of the total heat output from burning wood. [MSU]
During secondary combustion, gases such as carbon dioxide and methane are burnt.
Each model of wood burning stoves is specifically designed to provide the right amount of fresh air to facilitate secondary combustion of gases, as too little air will hinder the secondary burn process, and too much air will cool the firebox down too much for secondary burn to even occur.
Benefits of Secondary Burn
There are a few great benefits with having secondary burn in wood stoves:
- Higher efficiencies meaning more heat output for the same amount of wood burnt.
- Reduced emissions, allowing some models to be used in smoke controlled areas.
- Potential reduction in the blackening of the glass on stove doors through use.
- Reducing creosote buildup within the chimney or flue, therefore helping to reduce the need to have it swept more than the recommended yearly clean.
Secondary Burn On A Wood Stove
Secondary burn on a wood stove helps to achieve much higher heat outputs than stoves without this feature.
A wood burn stove with secondary burn does not necessarily mean that a particular model of stove will make it onto an EPA Certified wood stove list or become at DEFRA Approved stove. An approved stove is most likely to have secondary burn however because it helps to greatly reduce the emissions released through burning wood.
If you’re looking to buy a wood burning stove then be sure to check its efficiency rating, as higher efficiency rating stoves will typically mean that the stove provides functionality for secondary burn of waste gases and smoke.
Great article, thank you. It brought home to me the importance of preheating the incoming air for secondary combustion and how getting the operating temps right is critical to complete burning.
Thanks for the information. I don’t think carbon dioxide is burnt in the secondary burn. The carbon is CO2 is already thoroughly oxidised so no further burning is possible.
From what I understand, you are correct. I believe the article meant to explain that carbon monoxide is burned… At higher temperatures, CO can be further combusted to CO2, releasing heat in the process, and I think this is what they meant to refer to
Very Good article, thanks
Great information and advice thanks
Very good article great advice and information I am a pensioner who struggles a bit with the settings on the Norvik 5 Multi – fuel stove thanks once again Ronnie
Very informative. I have to confirm what Dave said though, you can’t burn CO2. CO2 is produced by the secondary reburn, and that is exactly what you want.
CO2 is the final form of oxidised carbon (and wood is mostly carbon). Incomplete burning creates a lot of complex carbon-containing chemicals that are harmful and contribute to creosote formation. To avoid that, new stoves burn these harmful compounds away in a second burn process at high heat. Yes, CO2 is still released, but that’s inevitable if you burn something carbon based (wood, coal, oil, gasoline, kerosine …)
Thanks, I have a Cubic Mini Cub wood burning stove and I could never figure out what the bottom handle did (or didn’t) do. Their web-page mentioned ‘secondary-burn’ but never explained what it was or that the little handle at the bottom controlled it.