Pellet classes A1, A2 and B — what A1 means, how A2 differs and which to choose

A1, A2 and B are quality classes of wood pellets, not marketing labels invented by shops. Behind them stand concrete, measurable thresholds from the ISO 17225-2 standard, which the ENplus and DINplus certification schemes apply. In short: A1 means the least ash and the strictest requirements on raw material, and the further down the alphabet, the looser the permitted values. The calorific value is the same in every class.

What "A1 pellets" means

"A1" on the bag means the producer declares the strictest of the three classes: ash up to 0.70%, moisture up to 10%, calorific value at least 4.6 kWh/kg, mechanical durability at least 98% (i.e. little dust and few broken pellets), nitrogen up to 0.3% and an ash deformation temperature of at least 1200°C. The raw material may only be stem wood and chemically untreated residues from the wood industry — no bark from logging residues, no recovered wood.

Two things "A1" does not mean: that the pellets heat more strongly (the energy minimum is shared by all classes), and that anyone has checked. A class declaration is a declaration. The check is the certificate number (ENplus or DINplus) — more in the last section. In practice almost all pellets sold in Poland for domestic boilers are labelled A1; in our table that is the vast majority of offers with a declared class.

A2 pellets — what they are and when they make sense

A2 is the class with a looser ash limit (up to 1.20%) and looser raw-material rules (whole trees without roots and logging residues are allowed, i.e. more bark). It heats the same — the 4.6 kWh/kg minimum is identical — but leaves up to 70% more ash and has a lower required ash deformation temperature (1100°C), i.e. a greater tendency to clinker in the burner. It makes sense in boilers whose manufacturer permits A2, with a clear price difference and easy access to the ash pan. There is little of it in Polish online shops: our table currently tracks no offer with a declared class A2.

Class B

B allows ash up to 2.00% and chemically untreated used wood. It is a fuel for larger, industrial installations, not a domestic boiler — and it practically does not appear in shops for households.

The concrete differences between classes

The values below come from Table 4 of the ENplus® ST 1001:2022 standard ("Threshold values of essential pellet parameters"):

Parameter	ENplus A1	ENplus A2	ENplus B
Ash (dry basis)	≤ 0.70%	≤ 1.20%	≤ 2.00%
Moisture	≤ 10.0%	≤ 10.0%	≤ 10.0%
Calorific value	≥ 4.6 kWh/kg	≥ 4.6 kWh/kg	≥ 4.6 kWh/kg
Mechanical durability	≥ 98.0%	≥ 97.5%	≥ 97.5%
Nitrogen (dry basis)	≤ 0.3%	≤ 0.5%	≤ 1.0%
Ash deformation temperature	≥ 1200°C	≥ 1100°C	≥ 1100°C
Diameter	6 ± 1 or 8 ± 1 mm	6 ± 1 or 8 ± 1 mm	6 ± 1 or 8 ± 1 mm
Bulk density	600–750 kg/m³	600–750 kg/m³	600–750 kg/m³

Note two things that usually get lost in discussions.

The calorific value is the same in all classes — a minimum of 4.6 kWh/kg (equivalent to ≥ 16.5 MJ/kg). That means A2 does not heat worse than A1. Buying A1, you're not paying for more heat per kilogram.

The real difference is ash: a permitted 0.70% in A1 versus 1.20% in A2, i.e. about 70% more ash at the upper limit of the class. That is the number that translates into your day-to-day boiler maintenance.

Where the difference comes from — raw material

The standard also specifies what pellets may be made from in each class:

• A1 — stem wood and chemically untreated residues and by-products from the wood industry.
• A2 — additionally whole trees without roots and logging residues. More bark and forest residues mean more mineral matter, and so more ash.
• B — also permits chemically untreated used wood.

Worth knowing: ENplus certification permits no demolition wood or chemically treated wood in any class — on this point the scheme is stricter than the ISO 17225-2 standard itself.

What it means in practice, at the boiler

A higher ash content translates into three things:

1. Emptying the ash pan more often. With A2 there's simply more ash — how much is left from a tonne and what can be done with it is worked out in a separate article.
2. Cleaning the heat exchanger more often. Deposits lower efficiency, so a neglected boiler burns more fuel — and part of the saving on the cheaper class comes back as higher consumption.
3. Risk of clinker (slagging). Here the last row of the table matters: the ash deformation temperature is ≥ 1200°C for A1 and ≥ 1100°C for A2. A lower temperature means a greater tendency for ash to fuse into lumps in the burner.

Which class to choose

Check your boiler manual — that's the deciding argument. Many pellet-boiler manufacturers require class A1 fuel, and using a lower class can be grounds for refusing a warranty claim. That one piece of information matters more than the whole table above.

If the boiler permits A2, the choice becomes a sum: does the price difference compensate for the more frequent maintenance and cleaning? At a large price gap A2 can be sensible, especially in simpler designs with easy access to the ash pan. At a gap of a few dozen złoty per tonne it usually isn't worth it.

Class B is in practice fuel for larger installations, not a domestic boiler.

Note: "A1" on the bag is not yet a certificate

The caption "A1" is not in itself confirmation of anything. Confirmation is an ENplus or DINplus certificate number that can be checked in the certifying body's database. We write about it separately: ENplus and DINplus certificates.

That's why in our pellet price listing we assign a class only when the shop itself declares it on the product page. When it doesn't, we mark the offer as unknown class — and such an offer will never trigger a price alert, even if it's the cheapest in the list. We'd rather not notify you of a bargain whose quality we can't confirm.

Sources

• ENplus® ST 1001:2022 — ENplus® wood pellets: Requirements for companies (Bioenergy Europe / DEPI), Annex A, Table 4 "Threshold values of essential pellet parameters" and Table 5 "Wood types that are permitted to be used for wood pellet production". All numerical thresholds in this article come from this document.