6 mm or 8 mm pellets? Your boiler decides

The short answer is unglamorous but the only correct one: your boiler chooses the pellet diameter, not you. Before comparing any prices, open the boiler manual and check which diameter the manufacturer permits. If it permits only one — discussion over. The rest of this article is for people whose boiler accepts both.

What the numbers mean at all

6 mm and 8 mm is the diameter of a single pellet. The standard on which ENplus certification rests permits exactly these two diameter classes: 6 ± 1 mm or 8 ± 1 mm, with the same pellet length (3.15–40 mm) and the same quality requirements. In other words: diameter is not a quality attribute. 6 mm and 8 mm pellets of the same class A1 are the same fuel in a different size — they differ in how they behave in the boiler, not in how much heat they carry per kilogram.

Practical differences

A consistent picture emerges from trade guides (sources below):

6 mm burns faster and more easily, so the boiler controller can dose fuel more precisely — that matters especially in lower-output boilers with smaller burners, typical for detached houses. Finer pellets pass more smoothly through an auger feeder, reducing the risk of jams, and finer particles clear more easily from the grate.

8 mm burns slower and longer, and tends to be recommended for higher-output boilers — i.e. more often for larger installations than a domestic boiler room. It may also require appropriate boiler settings.

If after that list you get the impression that for a typical detached house the choice comes down to 6 mm — that's the right impression. And the market confirms it.

What our data shows

In the price table we track offers from a dozen or so Polish shops. Every offer that declares a diameter on its product page declares 6 mm. We haven't yet come across a retail 8 mm offer in the tracked shops. That doesn't prove 8 mm isn't on sale — but it shows that in Polish retail bagged pellets 6 mm is the de facto standard, so with 6 mm you also have the widest choice of offers to compare and the easiest path to a good price.

A side note: some shops don't state the diameter in the product description at all. Since almost the whole retail market is 6 mm, that's usually what's meant — but if your boiler is fussy, ask before ordering a pallet rather than assuming.

How to check your own

1. The boiler manual or rating plate — look for "fuel" or "pellet Ø". Manufacturers state it plainly.
2. When both diameters are permitted — for a domestic boiler with an auger feeder, 6 mm is the safer choice for the reasons above, and incidentally what you'll actually find in the shops.
3. Don't experiment outside the specification. Pellets larger than the burner is designed for can block the feeder or burn incompletely — and using fuel contrary to the manual is a short road to the warranty discussion we wrote about under classes A1 and A2.

Summary

Diameter is a parameter of fit to the boiler, not of fuel quality. Quality is told by class and certificate — look at those when buying, as described in ENplus and DINplus certificates. And once you know what you're looking for, compare prices in zł/t — the differences between shops for the same pellets are often larger than you'd expect.

Sources

• ENplus® ST 1001:2022 — ENplus® wood pellets: Requirements for companies (Bioenergy Europe / DEPI), Annex A, Table 4 — diameter classes 6 ± 1 / 8 ± 1 mm, length 3.15–40 mm, identical quality thresholds for both diameters.
• europol1.pl — "Pellet 6 i 8 mm: jakie są różnice i jak wpływają na wydajność ogrzewania" — combustion characteristics of both diameters, matching to boiler output, feeder-jam risk (Polish).

The observation about 6 mm dominating retail comes from our own catalogue of tracked offers — a description of what we see in monitored shops, not a study of the whole market.