How long do wood pellets last? The UK winter storage guide

Buying wood pellets in bulk pallets is the most cost-effective way to run a biomass boiler. However, it often means storing hundreds of kilograms of fuel for several months. A common concern for homeowners is whether the fuel has a shelf life. Will pellets left over from the end of spring still be usable when the freezing weather returns in November?

The short answer is that high-quality wood pellets do not have an expiration date. However, their longevity depends entirely on how they are stored. In the notoriously damp UK climate, poor storage will ruin an entire pallet in a matter of weeks. Here is how to protect your fuel supply.

Wood pellets are manufactured by compressing ultra-dry sawdust under extreme pressure until it forms a shiny, solid cylinder.

Because they contain less than 10% moisture, they are highly hygroscopic. This means they act like magnets for water. If wood pellets come into contact with direct rain, or even high levels of humidity in the air, they will absorb the moisture. When this happens, the natural bonds break down, the pellet swells to three times its size, and it instantly crumbles back into a pile of useless sawdust.

The absolute best place to store wood pellets is indoors, in a dry, well-ventilated garage, utility room, or sealed shed.

• Keep them off the floor: Even inside a garage, concrete floors absorb moisture from the earth and experience temperature fluctuations that cause condensation. Always keep the bags elevated on the wooden shipping pallet they arrived on.

• Keep them away from walls: Never stack the bags tightly against a cold external brick wall. Leave a gap of a few inches to allow air to circulate around the entire stack.

Most premium pellets are delivered in thick, UV-resistant 15kg plastic bags. As long as these bags remain completely sealed, the pellets inside are perfectly protected from atmospheric humidity.

The danger arises once a bag is opened. If you only pour half a bag into the boiler hopper, do not leave the open bag sitting on the floor. Roll the top of the plastic down tightly and secure it with heavy-duty tape or a large clip to seal the remaining pellets away from the damp air.

Storing wood pellets outside is highly discouraged in the UK. While the plastic bags are waterproof, they are easily punctured by sharp stones, garden tools, or rodents looking for nesting materials. A single tiny hole in the plastic will allow damp winter air to enter, ruining the entire bag. If you absolutely must store them outside temporarily, keep them fully elevated on a pallet, wrapped securely in a heavy-duty, completely waterproof tarpaulin, and move them indoors at the earliest opportunity.