Pellet dimensions: diameter and length

Diameter: 6 or 8 mm

ENplus provides for two diameters for all classes: 6 ± 1 mm and 8 ± 1 mm, measured as received according to ISO 17829^[1]. The Polish regulation Dz.U. 2025 poz. 618 (Polish Journal of Laws 2025, item 618) permits the same diameters for A1 and A2 pellets^[36]. In the standard, the diameter is written as a dimension class symbol, e.g. D06 (Diameter class designation in ISO 17225-2: D06 means pellets with a diameter of 6 ± 1 mm, D08 — 8 ± 1 mm.)Diameter class designation in ISO 17225-2: D06 means pellets with a diameter of 6 ± 1 mm, D08 — 8 ± 1 mm. or D08^[12].

In residential boilers and stoves, 6 mm pellets are the standard. The diameter has to match the appliance — it is the boiler manufacturer's instructions that decide, not price or availability. On an ENplus bag the diameter must be stated explicitly (“6 mm” or “8 mm”), and each diameter requires a separate, approved bag design^[2].

Pellet length

Under ENplus, pellet length must fall within 3.15–40 mm. In addition, at most 1% of the pellets by mass may be longer than 40 mm, and no pellet may exceed 45 mm^[1]. The Polish regulation gives a range of 3.15–40 mm^[36].

ENplus is stricter than the standard here: the ISO 17225-2 draft allows a maximum length of 45 mm in class A1, but 50 mm in classes A2 and B, also with the proviso that at most 1% may be longer than 40 mm^[12],[1]Data require verification: The ISO values were read from the FDIS 17225-2 draft; they need to be confirmed in the published ISO 17225-2:2021..

Share of pellets shorter than 10 mm

ENplus requires the producer to state the share of pellets shorter than 10 mm — both the exact value and the category^[1]:

• L — less than 20% of pellets shorter than 10 mm (“long” pellets);
• M — from 20 to 30%;
• S — more than 30% (“short” pellets).

This is not a limit but mandatory information. It helps match the fuel to the appliance and compare successive deliveries: if pellets from the same producer change category, the way the auger doses them changes too.

Why length matters

• Auger and hopper. Very long pellets flow less well, can jam in the auger and at the transitions between hopper and feed system, and in the hopper they are more likely to form bridges (An arch of wedged pellets above the hopper outlet with an empty space underneath — fuel stops flowing into the auger.)An arch of wedged pellets above the hopper outlet with an empty space underneath — fuel stops flowing into the auger.. Hence the hard 45 mm limit in ENplus.
• Dosing. Short pellets pack more tightly than long ones, so there are more of them in the same volume. A change in the proportion of short and long pellets changes the bulk density and the amount of energy in each dose delivered by the auger.
• Ignition and combustion. Shorter pellets have a larger surface area relative to their mass, so they heat up and ignite faster. Very long pellets can lie unevenly in the burner and burn less uniformly.
• A quality signal. A large share of very short pellets and crumbs can result from the pellets breaking in transport — in that case it is worth looking at mechanical durability and fines.

How dimensions are measured (ISO 17829)

The method for determining pellet length and diameter is described in ISO 17829, the current edition of which dates from 2025^[21]. Pellets are measured individually, and the result describes the length distribution in the sample.

To determine the share of pellets shorter than 10 mm, ENplus requires 100 pellets to be measured after the sample has been screened on a 5.6 mm sieve — ISO 17829 recommends 50 pellets^[1]. Screening separates the pellets from the crumbs, which are assessed separately as fines.

Dimensions on the bag and in the report

An ENplus pellet bag must state the diameter — “6 mm” or “8 mm” — and for each diameter the producer must have a separate bag design approved by the scheme management and published on the ENplus website^[2]. Other fuel properties, including length, are voluntary information on the bag. If the producer states them, they must correspond to the ENplus threshold values or be stricter, and be backed by test results^[2].

A test report gives fuller information on dimensions. It is worth checking it for:

• the diameter and length distribution of the pellets, determined according to ISO 17829^[21];
• the share of pellets shorter than 10 mm, together with the category L, M or S^[1];
• the share of pellets longer than 40 mm, and confirmation that no pellet exceeds 45 mm^[1].

If the boiler ran well on one fuel but started to feed or ignite less reliably after a change of supplier, compare the length category of the two products. This is often a simpler explanation than “poorer quality” — and easier to correct with the settings.