“I bought new pellets and suddenly there’s a hard stone in the burner. This has never happened to me before. Are the pellets bad or is the stove broken?”
This can really surprise the owner of a pellet stove. Instead of the usual fine ash, they will find a hard, sometimes glassy substance in the burner, resembling a piece of slag.
Sometimes it can be easily removed, but sometimes it sticks firmly to the rust and blocks the intake openings for combustion air.
The good news is that the formation of deposits does not automatically mean that the oven is defective. However, it is necessary to find out why they occur.
What are clumps and how do they form?
When wood pellets are burned, a small amount of minerals remains, forming ash.
Under normal conditions, the ash remains mostly loose. However, some mineral components can soften and solidify at high temperatures.
After cooling, hard lumps form, which we refer to as clumping or slag.
What is crucial is not only the amount of ash, but also its chemical composition and the temperature at which it begins to soften.
Two pellet brands with the same ash content can therefore behave differently during combustion.
1. Are low-quality pellets responsible for the clumping?
Yes, they can. But it’s not the only possible cause.
High-quality wood pellets must meet requirements not only regarding ash content, but also regarding their behavior at high temperatures.
The ENplus certification specifies the minimum ash deformation temperature:
| Pellet class | Minimum ash deformation temperature |
|---|---|
| ENplus A1 | 1 200 °C |
| ENplus A2 | 1 100 °C |
| ENplus B | 1 100 °C |
This is not the temperature at which all the ash melts at once. It is a value determined in the laboratory at which its deformation becomes noticeable.
The less prone the ash is to clumping at operating temperatures, the lower the risk of hard deposits forming.
Therefore, ENplus A1 pellets are recommended for most household pellet stoves. However, the manufacturer’s instructions for the specific appliance always apply.
2. Why does clumping only occur after changing the pellet brand?
“I’ve been heating with the same pellets for three years and never had any problems. Now I’ve bought cheaper ones, and after two days I have slag in the burner.”
Such an experience can truly highlight the difference between the fuels.
Pellets from different manufacturers can have different mineral compositions, different ash contents, and different deformation temperatures.
Sometimes, simply changing the fuel is enough to lead to the formation of hard deposits, even if the operation is otherwise identical.
However, this does not automatically mean that new pellets are defective. They may have properties that are unsuitable for your appliance, or adjustments to the combustion process may be necessary according to the manufacturer’s instructions.
If the problem occurs immediately after changing the fuel, it is advisable to check the fuel first.
3. Could it be due to insufficient air supply?
Yes.
Pellet stoves require sufficient air for proper combustion. The air supply is usually regulated by a fan and the combustion process settings.
If the combustion is not properly adjusted or some of the air passages are blocked, the conditions in the burner can change.
This can contribute to the formation of deposits and impair combustion.
Attention: It is not correct to automatically assume that adding or removing air will eliminate clumping.
An incorrect setting can lead to unstable combustion, increased emissions, or other operational problems.
Therefore, do not adjust the fan settings or fuel metering by trial and error. Follow the instructions or contact a service technician.
4. What happens when the deposits clog the holes in the burner?
This could pose a more serious problem.
The air supplied through the holes in the burner is important for proper combustion.
If these holes gradually become clogged with hard deposits, the supply of combustion air can deteriorate.
The result can be an irregular flame, incompletely burned pellets, increased environmental pollution, or an error message from the oven.
Some devices have an automatic burner cleaning function, but that does not mean they can remove all stubborn deposits.
If the burner clogs repeatedly, do not ignore the problem.
5. Do the deposits mean that the oven is heating too hot?
Not necessarily.
The formation of corrosive substances depends on the temperature at the combustion point, but also on the properties of the ash itself.
Some types of ash begin to soften at lower temperatures than others.
Therefore, the presence of deposits does not automatically mean that the oven is being operated at too high a power level.
Likewise, automatically reducing the stove’s power to a minimum is not the right solution.
Pellet stoves have an operating range and combustion parameters specified by the manufacturer. It is important that they are operated under suitable conditions.
6. How can I determine if the problem lies with the fuel or the stove?
A simple test at home can help to better assess the problem.
If the deposits only formed after changing the pellets, try reusing the originally proven fuel after cleaning the stove, provided it meets the manufacturer’s requirements.
Observe whether deposits form again during comparable operating procedures.
If the problem disappears when using the original pellets, the cause is probably in the properties of the fuel.
If clumping occurs regardless of the brand used, it is advisable to also check the condition and settings of the device.
However, changing the fuel alone is not conclusive proof of the cause.
7. How do you properly remove deposits from the burner?
Above all, safe.
First, switch off the stove as usual according to the operating instructions and allow it to cool down completely.
Only then should the burner be removed or cleaned according to the manufacturer’s recommendations.
Stubborn deposits may need to be removed mechanically. However, only use the tools and procedures specified in the operating instructions.
Do not use force, sharp tools or unsuitable chemicals that could damage the burner or its surface.
Pay particular attention to the air intake openings. These must remain unobstructed, but must not be damaged or enlarged during cleaning.
If the dried material cannot be safely removed, please contact customer service.
8. What can be done to prevent clumping?
Their occurrence cannot be completely ruled out, but the risk can be significantly reduced through proper operation.
- Use pellets that meet the requirements of the stove manufacturer.
- Prefer fuel with tested parameters and valid certification.
- Clean the burner regularly according to the operating instructions.
- Check that the combustion air holes are not blocked.
- Observe the prescribed maintenance intervals.
- If the problems persist, have the combustion settings checked.
If the deposits only form after changing the fuel, you don’t necessarily have to assume an expensive defect in the stove.
First, you need to find out what has changed.
Can encrusted deposits damage the stove?
A small knot alone may not pose an immediate danger.
The problem occurs when hard deposits obstruct the air supply, prevent proper burner cleaning, or cause repeated combustion problems.
If these problems are ignored for an extended period of time, this can lead to a malfunction of the device and, in some cases, to damage to its components.
If the stove repeatedly reports an error, produces an unusual amount of unburned fuel, or the burner clogs quickly, further operation without eliminating the cause is not advisable.
What can you take away from this?
Hard sinter is not ordinary fine ash. It is formed by the sintering of its mineral components at high temperature.
They can be related to the quality and properties of the pellets, but also to the combustion conditions or the technical condition of the oven.
If the problem occurred after changing the pellet type, first check the fuel. If it persists regardless of the pellet type used, you should also check the stove itself.
And above all: Do not try to solve the problem by arbitrarily changing the operating parameters. A properly functioning pellet stove requires not only the right fuel, but also the correct combustion conditions.