Quality parameters
Wood pellet parameters: what is measured and where the limits lie
Nine groups of parameters that determine how wood pellets burn in the boiler, how much ash they leave and how much energy we are actually buying. We take the limit values directly from the source documents.
01Guide to the parameters
Nine parameters, nine buyer’s questions
Each page explains what the parameter means in practice, how it is tested, what the requirements are and how to check it.
- 01A1 ≤ 0.70%
Ash content of wood pellets
Ash content tells you how much non-combustible residue every tonne of fuel will leave behind. It is one of the easiest parameters to understand — and one of the most frequently exceeded.
Read - 02A1–B ≤ 10.0%
Moisture content of wood pellets
Water in fuel provides no energy — it takes energy away, because it has to evaporate. That is why moisture content is the first parameter that affects how much heat you are actually buying.
Read - 03A1–B ≥ 4.6 kWh/kg
Calorific value of wood pellets
Net calorific value tells you how much heat a kilogram of fuel can deliver. In practice it is the figure you pay for — provided you know on what basis it is reported.
Read - 04A1 ≥ 98.0%
Mechanical durability of wood pellets
Wood pellets travel a long way: from the press into a bag or tanker, through warehouses and transport, to the boiler auger. Mechanical durability tells you how much of that journey the pellets can withstand without breaking down into dust.
Read - 05Bags A1 ≤ 0.5%
Fines in wood pellets
A little dust at the bottom of a bag surprises no one. But dust is not just a cosmetic issue — it changes fuel dosing, disrupts combustion and shows how well the pellets have survived the journey to the customer.
Read - 06A1 N ≤ 0.3 · S ≤ 0.04 · Cl ≤ 0.02%
Chlorine, sulphur and nitrogen in wood pellets
Clean wood contains very little of these elements. That is exactly why elevated levels are a good warning sign: most often they mean that something has got into the raw material that should not be there.
Read - 07A1 DT ≥ 1200 °C
Ash melting temperature and clinker
Clinker in the burner is one of the most troublesome problems with wood pellets. It is caused not so much by the amount of ash as by its composition — and that is described by the ash melting temperature.
Read - 08A1–B 600–750 kg/m³
Bulk density of wood pellets
Bulk density tells you how many kilograms of pellets fit into a cubic metre of hopper. It determines how large a store you need and how much energy the auger delivers with each dose.
Read - 09Ø 6 ± 1 or 8 ± 1 mm
Pellet dimensions: diameter and length
Pellet diameter and length may seem like details, but they determine whether the pellets flow evenly in the hopper, how the auger doses them and how quickly they ignite in the burner.
Read
- How much ash is left?Ash (dry basis)Result in the report; A1 ≤ 0.70%. Ash [kg/t] = result [%] × 10.Parameter details
- Will it form clinker?Ash deformation temperature (DT), raw materialDT ≥ 1200 °C for A1; ask about the cleanliness of the raw material (sand, soil).Parameter details
- Is it dusty?Fines < 3.15 mm, durabilityA1 bags ≤ 0.5% at bagging; check the bottom of the bag after transport.Parameter details
- Does it crumble in transit?Mechanical durabilityA1 ≥ 98.0%; the closer to 100%, the less dust reaches the customer.Parameter details
- Will the auger feed evenly?Length, share < 10 mm, bulk density, dust3.15–40 mm (max. 45 mm); L/M/S category; 600–750 kg/m³.Parameter details
- How much heat does a tonne give?Net calorific value as receivedA1 ≥ 16.5 MJ/kg (≥ 4.6 kWh/kg); not to be confused with the dry-basis value.Parameter details
- Will it harm the boiler and chimney?Chlorine, sulphur, nitrogenA1: Cl ≤ 0.02%, S ≤ 0.04%, N ≤ 0.3% (dry basis).Parameter details
- Is it dry?Moisture content as received≤ 10%; bags without tears, stored under cover.Parameter details
Threshold values according to ENplus ST 1001:2022 (2nd edition), Table 4. The result for a specific batch can only be checked in the test report.
02ENplus requirements
Full table of ENplus A1, A2 and B requirements
The ENplus scheme requirements are based on ISO 17225-2, but in several places they are stricter. Click a parameter name to go to its description.
| Parameter | Unit | A1 | A2 | B | Method | Notes |
|---|---|---|---|---|---|---|
| Diameteras received | mm | 6 ± 1 or 8 ± 1 | 6 ± 1 or 8 ± 1 | 6 ± 1 or 8 ± 1 | ISO 17829 | |
| Lengthas received | mm | 3.15 ≤ L ≤ 40 | 3.15 ≤ L ≤ 40 | 3.15 ≤ L ≤ 40 | ISO 17829 | No more than 1% by mass may be longer than 40 mm; no pellet may exceed 45 mm. |
| Share of pellets < 10 mmas received | % w/w | state value and category | state value and category | state value and category | ENplus guidelines | Categories: L < 20%, M 20–30%, S > 30%. |
| Moisture contentas received | % w/w | ≤ 10.0 | ≤ 10.0 | ≤ 10.0 | ISO 18134 | |
| Ashdry basis | % w/w | ≤ 0.70 | ≤ 1.20 | ≤ 2.00 | ISO 18122 | |
| Mechanical durabilityas received | % w/w | ≥ 98.0 | ≥ 97.5 | ≥ 97.5 | ISO 17831-1 | Determined at the point where the means of transport is loaded at the production plant. |
| Bulk densityas received | kg/m³ | 600–750 | 600–750 | 600–750 | ISO 17828 | |
| Fines < 3.15 mm (bulk)as received | % w/w | ≤ 1.0 | ≤ 1.0 | ≤ 1.0 | ISO 5370 | At the factory gate or when loading big bags and tankers for end customers. |
| Fines < 3.15 mm (bags)as received | % w/w | ≤ 0.5 | ≤ 0.5 | — | ISO 5370 | At the factory gate, when the bags are filled. |
| Net calorific valueas received | kWh/kg | ≥ 4.6 | ≥ 4.6 | ≥ 4.6 | ISO 18125 | Equivalent to ≥ 16.5 MJ/kg as received. |
| Additives (e.g. starch)as received | % w/w | ≤ 2.0 | ≤ 2.0 | ≤ 2.0 | — | Max. 1.8% during production; post-production additives (e.g. coating oils) max. 0.2%. |
| Nitrogen (N)dry basis | % w/w | ≤ 0.3 | ≤ 0.5 | ≤ 1.0 | ISO 16948 | |
| Sulphur (S)dry basis | % w/w | ≤ 0.04 | ≤ 0.04 | ≤ 0.04 | ISO 16994 | |
| Chlorine (Cl)dry basis | % w/w | ≤ 0.02 | ≤ 0.02 | ≤ 0.03 | ISO 16994 | |
| Ash deformation temperature (DT) | °C | ≥ 1200 | ≥ 1100 | ≥ 1100 | ISO 21404 | Ash prepared at 815 °C; the report should state all the characteristic temperatures. |
Source: ENplus ST 1001:2022, 2nd edition (approved on 18 June 2025, in force from 1 January 2026), Annex A, Table 4[1]. The limits are identical to those in edition 1; the method for determining fines has changed from ISO 18846 to ISO 5370. A result complies if the value reported by the laboratory falls within the limit. Outside the table, ENplus requires that bulk pellets leaving the plant be no warmer than 40 °C.
| Element | Symbol | A1 · A2 · B |
|---|---|---|
| Arsenic | As | ≤ 1 |
| Cadmium | Cd | ≤ 0.5 |
| Chromium | Cr | ≤ 10 |
| Copper | Cu | ≤ 10 |
| Lead | Pb | ≤ 10 |
| Mercury | Hg | ≤ 0.1 |
| Nickel | Ni | ≤ 10 |
| Zinc | Zn | ≤ 100 |
The same limits apply to classes A1, A2 and B; method ISO 16968[1],[25]. The Polish regulation sets no limits for heavy metals[36].
ENplus A1 · A2 · B — where the classes differ
- A1
- A2
- B
Ash (db)
maximum value
Mechanical durability
minimum value
Nitrogen (db)
maximum value
Chlorine (db)
maximum value
Ash deformation temp.
minimum value
03The law in Poland
ENplus and the Polish regulation Dz.U. 2025 poz. 618
Since 24 May 2025, wood pellets placed on the Polish market for households and installations below 1 MW must meet the requirements of a regulation issued by the Polish Minister of Climate and Environment.
The regulation Dz.U. 2025 poz. 618 (Polish Journal of Laws 2025, item 618) contains two tables for pellets. Table 3 (“A1”) covers pellets for heating appliances of class 3, 4 or 5 or those meeting ecodesign requirements; Table 2 (“A2”) covers pellets in general[36]. The ministry states that the values are consistent with the requirements of EN ISO 17225[37].
| Parameter | ENplus A1 | Dz.U. pellets A1 (Table 3) | Dz.U. pellets A2 (Table 2) |
|---|---|---|---|
| Diameter | 6 ± 1 or 8 ± 1 mm | 6 ± 1 / 8 ± 1 mm | 6 ± 1 / 8 ± 1 mm |
| Length | 3.15–40 mm (max. 45 mm) | 3.15–40 mm | 3.15–40 mm |
| Moisture (as received) | ≤ 10.0% | ≤ 10.00% | ≤ 10.00% |
| Ash (dry basis) | ≤ 0.70% | ≤ 0.70% | ≤ 1.20% |
| Mechanical durability | ≥ 98.0% | ≥ 98.00% | ≥ 97.50% |
| Fines | ≤ 1.0% (bulk), ≤ 0.5% (bags) | ≤ 1.00% | ≤ 1.00% |
| Calorific value (as received) | ≥ 16.5 MJ/kg | ≥ 16.50 MJ/kg | ≥ 16.50 MJ/kg |
| Bulk density | 600–750 kg/m³ | 600–750 kg/m³ | 600–750 kg/m³ |
| Additives | ≤ 2.0% | ≤ 2.00% | ≤ 2.00% |
| Nitrogen (dry basis) | ≤ 0.3% | ≤ 0.30% | ≤ 0.50% |
| Sulphur (dry basis) | ≤ 0.04% | ≤ 0.04% | ≤ 0.04% |
| Chlorine (dry basis) | ≤ 0.02% | ≤ 0.02% | ≤ 0.02% |
| Heavy metals | 8 limits (As, Cd, Cr, Cu, Pb, Hg, Ni, Zn) | none | none |
| Ash deformation temperature | ≥ 1200 °C | none | none |
Sources: ENplus ST 1001:2022 (ed. 2), Table 4[1]; Dz.U. 2025 poz. 618, Tables 2 and 3 of the annex[36]. “None” means that the regulation sets no such requirement.
04International standard
ENplus and ISO 17225-2
ISO 17225-2:2021 is the current standard for wood pellet classes. ENplus is based on it, but sets stricter requirements on several points.
The 2021 edition replaced the 2014 version. Among other things, it introduced mandatory ash deformation temperature thresholds, an upper limit for bulk density and informative parameters: particle density and coarse pellet fines. Alongside classes A1, A2 and B, the standard describes industrial classes I1–I3[11].
| Requirement | ENplus | ISO 17225-2 (FDIS) |
|---|---|---|
| Durability A1, 8 mm pellets | ≥ 98.0% (6 and 8 mm) | ≥ 97.5% for D08 (≥ 98.0% for D06) |
| Durability, class B | ≥ 97.5% | ≥ 96.5% |
| Sulphur, class B | ≤ 0.04% | ≤ 0.05% |
| Maximum length | 45 mm in all classes | 45 mm (A1), 50 mm (A2, B) |
| Fines in bags | ≤ 0.5% (A1, A2) | no separate limit (≤ 1.0%) |
| Class B raw material | no chemically treated or demolition wood | allows chemically treated by-products |
ENplus side: ST 1001:2022 (ed. 2)[1]. ISO side: values from the FDIS draft of 17225-2[12] and the scope of classes under ISO 17225-2:2021[11].
Sources and references
Full list of sources and editorial policy- [1]
European Pellet Council / Bioenergy Europe, DEPIapproved 18.06.2025, in force from 01.01.2026Certification schemeaccessed: 11/09/2026
Annex A, Table 4 (threshold values) and Table 5 (raw material). The limits are the same as in the first edition; the method for determining fines has changed to ISO 5370.
- [11]
ISO / TC 2382nd edition, 2021-05Standard paid documentaccessed: 11/09/2026
Edition and scope of changes confirmed in the official preview of the standard. Detailed ISO values were compared with the FDIS draft — see the separate source.
- [12]
ISO/FDIS 17225-2 — final draft of the standard (text published by AIEL) (opens in a new tab)
ISO / AIEL2020Standard requires verificationaccessed: 11/09/2026
Values taken from the FDIS draft; exact ISO figures must be confirmed in a purchased copy of the standard before they are published.
- [25]
ISO / TC 238Standard paid documentaccessed: 11/09/2026
- [36]
Minister Klimatu i Środowiskain force from 24.05.2025Legislationaccessed: 11/09/2026
Covers wood pellets and briquettes for households and installations below 1 MW.
- [37]
Ministerstwo Klimatu i Środowiska (gov.pl)Legislationaccessed: 11/09/2026
This content is an editorial summary based on the documents cited. Always confirm threshold values in the current edition of the standard or certification scheme document.
Read more
Next
Class limits are the minimum. What counts is the result for the specific batch.
See how to read a test report and how to compare wood pellets on the basis of measured values, not declarations.