Which pellets for flame, retort and gravity burners
"A flame burner is too fussy about pellets", "a retort burns anything", "a gravity burner has to be shaken every 4-8 hours": owners of different burners describe different experiences with the same bags. That is because the burner type determines which pellet class and how much dust it will forgive.
Three burner types in 2 minutes
Flame burner: pellets drop from the auger onto an inclined or horizontal grate in the burner tube, a fan blows along it, and the flame comes out as a torch into the boiler's combustion chamber. This is the most common type: almost all burners fitted into a standard solid fuel boiler and most inexpensive household pellet boilers. Pro: simple and cheap, works in any boiler. Con: the ash has to fall off the grate on its own; whatever does not fall off fuses.
Retort burner (cup, volumetric combustion): fuel is fed by an auger from below into a cup, burns on its surface, and fresh fuel pushes the ash over the edge. More expensive models have a moving grate or self-cleaning. Pro: tolerates ash and different raw materials. Con: price, the cup needs cleaning, more electronics.
Gravity burner (no electricity): pellets fall from the hopper by gravity onto a grate and burn on natural draught, like firewood. Pro: works during power cuts. Con: air is not regulated by a fan, the bed has to be shaken by hand every 4-8 hours, and it is sensitive to everything.
Flame burner: A1 only and clean only
In a flame burner the combustion zone is small and very hot, air blows through a thin layer, and everything that has not burned and has not fallen off the grate stays on it. That is why the three requirements for pellets are strict. Ash content up to 0.7% (ENplus A1): at 1.2% (A2) the grate will need cleaning twice as often, and at 2-3% (unclassified, with bark) every day. Minimal dust: fines flare up in the tube, overheat the zone and produce soot; the norm is up to 1%, and that is exactly why flame burner owners buy pellets in 15 kg bags rather than repacked from big bags. Uniform length and 6 mm diameter: long pellets (over 30-40 mm) jam the auger and pile up on the grate.
Agro pellets do not work in a flame burner: husk ash melts at 650-750 °C, and that is exactly the temperature in a flame burner tube, so you get a cake within a few hours. A mix with wood pellets slags too. More on this in the article "Agro pellets: which boilers they suit".
Settings: a flame burner has to be re-tuned for every new batch, because a different pellet density and length change the feed. The target is a whitish-yellow, clear flame without smoke; a sooty yellow flame means too little air, and half-burnt pellets flying off the grate mean too much. Clean the grate every 2-7 days depending on the pellet class.
Retort burner: A1, A2 and, if the passport allows, agro
In a retort, ash does not interfere with combustion as much: fresh fuel lifts it up and it spills over the edge of the cup. That is why a retort burner runs happily on A2 (ash content up to 1.2%), and models with a moving grate also run on agro pellets or a mix. Other things matter more for it: consistent pellet length (the auger feeds from below, so long pellets jam it even more than in a flame burner) and dryness (swollen pellets get stuck in the auger). It tolerates dust better, but not endlessly: once a month the hopper should be cleared of fines.
Settings: less sensitive to the batch, but after switching from A1 to A2 or agro you need to increase the air and reduce the feed per cycle, otherwise the bed in the cup overheats and slags. Clean the cup once a week on A1, every 2-3 days on A2 and agro.
Gravity burner: dry, durable, dust-free and wood only
Without a fan there is no way to "blow through" the bed, so anything that restricts airflow is critical: dust, brittle pellets, fusing ash. The requirements are similar to a flame burner (A1, clean, in bags), plus one more: pellet durability, because the bed is shaken periodically and a brittle pellet turns into dust right in the bed. Agro pellets: never. Pellet length should be medium, 15-30 mm: very long ones get stuck in the hopper, very short ones pack tightly and do not let air through.
Operation: shake the grate every 4-8 hours, empty the ash pan daily, keep the damper open wider than for firewood. And remember that this is a "for power cuts" fuel, not a way to save money: without air control the efficiency is lower, and you will use 10-15% more pellets.
Summary table
| Burner | Class | Dust | Agro pellets | Packaging | Cleaning |
|---|---|---|---|---|---|
| Flame | A1 only | up to 1%, critical | no | 15 kg bags | grate every 2-7 days |
| Retort without moving grate | A1, A2 | tolerates | with caution | bags, big bags | cup every 3-7 days |
| Retort with moving grate | A1, A2, industrial | tolerates | yes, if the passport allows | big bags, bulk | cup once a week |
| Gravity, no electricity | A1 only | critical | no | 15 kg bags | shake every 4-8 hours |
What to check in a batch before filling the hopper
For flame and gravity burners: all six tests from the article "How to check pellets at home in 5 minutes", especially the bottom of the bag and the burn-on-a-plate test. For a retort burner, the water test and pellet length are enough. And in any case: a batch passport with the ash content. Without that figure, all the advice above only half works.
Our wood pellets in 15 kg bags are screened, packed in a moisture-proof bag and come with a batch passport; for a flame burner they are the default choice. Not sure which burner you have? Call us or order a trial bag: one evening is enough to see how it performs.
Questions and answers
Can I burn A2 pellets in a flame burner?
Technically yes, but a flame burner is designed for A1 with ash content up to 0.7%. On A2 (ash content up to 1.2%) the grate will need cleaning roughly twice as often, and on unclassified pellets with bark (2-3% ash) every day. So for a flame burner the default choice is A1 wood pellets in bags.
Why do agro pellets fuse in a flame burner?
Sunflower husk ash melts at 650-750 °C, and that is exactly the temperature in a flame burner tube. The ash does not have time to fall off the grate and forms a cake within a few hours. A mix of agro and wood pellets slags too; agro pellets suit retort burners with a moving grate. More in the article "Agro pellets: which boilers they suit".
How often should a pellet burner be cleaned?
It depends on the burner type and pellet class. A flame burner's grate is cleaned every 2-7 days; a retort burner's cup once a week on A1 and every 2-3 days on A2 or agro. A gravity burner without electricity needs the grate shaken every 4-8 hours and the ash pan emptied daily.
Does a pellet boiler work without electricity?
Flame and retort burners do not work without electricity: they have an auger and a fan. Only a gravity burner runs during a power cut: pellets fall from the hopper by gravity and burn on natural draught. But the air there is not regulated, so efficiency is lower and you use 10-15% more pellets, and only durable, dust-free A1 wood pellets will do.
What pellet length and diameter does a burner need?
For a flame burner: 6 mm diameter and uniform length; pellets longer than 30-40 mm jam the auger and pile up on the grate. A retort burner is even more sensitive to long pellets, because the auger feeds from below. For a gravity burner the optimum is a medium length of 15-30 mm: long pellets get stuck in the hopper, and very short ones pack tightly and let little air through.
How do I set up the burner for a new batch of pellets?
A flame burner is re-tuned for every batch: a different pellet density and length change the feed. The right flame is whitish-yellow, clear, without smoke; a sooty yellow flame means too little air, and half-burnt pellets flying off the grate mean too much. In a retort burner, when switching from A1 to A2 or agro, increase the air and reduce the feed per cycle, otherwise the bed in the cup overheats and slags.
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