Combustion is a chemical oxidation reaction that occurs when any substance burns – in this case, pellets – typically releasing a large amount of energy in the form of heat and light, which is visually manifested as fire in the pellet stove.
Every combustion process requires:
- Fuel: the material that burns. In pellet stoves, these are the pellets.
- Oxidizer: the material that enables burning. Oxygen.
- Temperature: the point at which pellets and oxygen begin the combustion reaction. Electric resistance.
How should combustion work in a pellet stove?
In a pellet stove, you need to achieve an adequate and proportional amount between fuel and oxidizer (pellets and air) to achieve proper combustion.
Let’s look at the different combustion scenarios you might encounter in your pellet stove:
How to know if my pellet stove is burning correctly
Regarding combustion in your pellet stove, you may encounter the following types of combustion, and you should always aim to achieve complete combustion.
Combustion with excess air:
Incomplete combustion in a pellet stove happens when excess air is supplied to ensure complete fuel oxidation, beyond what is theoretically necessary.
Signs of excess air:
- The flame causes incandescent pellets to exit the burn pot
- The flame has a torch-like pointed shape, progressively reducing in size with an intense yellow color
- The flame will typically self-extinguish without interaction with the control panel, triggering a safety alarm (flame outage) after a certain time
- The burner will be completely empty of pellets once the device is off
- You’ll need to adjust the air by reducing the amount supplied. If this isn’t sufficient, increase the amount of pellets fed
Combustion with insufficient air:
Combustions with insufficient air occur when there’s less air than the stoichiometric requirement, resulting in unburned gases or smoke and efficiency losses.
Signs of insufficient air:
- A flame with insufficient air causes excess unburned pellets in the burn pot
- The flame resembles that of a wood stove, with a large undefined flame and orange/black color
- Generally, the flame will take longer to extinguish, depending on the amount of pellets in the burn pot
- It’s recommended to turn off the stove immediately
- Once turned off, you’ll notice the burner completely full of unburned pellets
- Produces higher amounts of CO
You’ll need to adjust by increasing air supply. If this isn’t sufficient, reduce the amount of pellets fed
Complete combustion:
This is the correct combustion for a pellet stove:
- Live flame with minimal pellets in the burn pot
- The flame has a uniform shape and a yellow/white color
- Optimal combustion requires no adjustment
In practice, it’s been shown that working with a slight excess of air is ideal, minimizing unburned fuel and chimney losses while optimizing process efficiency.
Other factors affecting combustion in a pellet stove
Stove maintenance:
A well-maintained stove will help ensure proper combustion. For example, a malfunctioning air supply fan will cause poor combustion.
Pellets:
Different pellets can change the flame appearance and thus combustion. Moisture levels and pellet composition can cause variations in the flame.
Conclusions
As you’ve seen, you can encounter these three types of combustion in your pellet stove, and you should always adjust it to achieve complete combustion.
If you have excess air or incomplete combustion, adjust by reducing air supply or increasing pellet quantity
If you have insufficient air combustion, do the opposite: increase air supply to the combustion chamber or reduce pellet quantity
Check your pellet stove’s user manual as you should be able to adjust it yourself. If you can’t, call the manufacturer’s Technical Service for a stove inspection, especially regarding combustion
Remember that poor combustion can be dangerous and will always make the stove dirtier, resulting in more maintenance tasks and lower stove efficiency.
Your stove’s performance and combustion will also be affected by the type and quality of wood pellets used, which can impact the heat output of the stove to the environment.
In this website I write about installation, maintenance and other news about pellet stoves, trying to help and clear doubts about the subject.
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- How to Achieve Perfect Combustion in Your Pellet Stove - January 18, 2025
