Wood Pellet vs Wood Chip vs Log: Choosing the Right Biomass Fuel

Choosing the Right Fuel for Your System

Biomass boilers are not interchangeable between fuel types. The boiler you install, the fuel store you build, and the handling system you specify are all tied to a fuel choice made early in the project. Getting this decision right at the outset saves significant time and money. Getting it wrong creates years of operational frustration.

This guide compares the three main solid biomass fuels used in the UK: wood pellets, wood chip, and firewood logs. Each has a specific set of applications where it performs well, and a different set of trade-offs.

Wood Pellets

Wood pellets are the most refined of the three fuels. They are produced by compressing dried, de-barked sawmill residues into a standardised cylindrical form — typically 6 mm or 8 mm in diameter, 10–40 mm in length — at a moisture content of around 8% or below.

Advantages

• Automated handling: Pellets flow reliably through augers, pneumatic delivery systems, and suction conveyors. Fully automated boilers require no manual intervention between deliveries.
• Consistent combustion: Because moisture content and energy density are tightly controlled, the boiler's combustion management system can operate precisely. Efficiency figures of 90–94% are achievable on well-maintained premium systems.
• ENplus certification: The ENplus quality standard (A1, A2, B grades) provides a verifiable benchmark for pellet quality. A1 is the specification required for most domestic and light commercial boilers. Buying certified pellets from a registered supplier protects both performance and equipment warranty.
• Compact storage: Pellets have a bulk density roughly double that of wood chip. A smaller store achieves the same energy content, which is valuable on constrained sites.
• Delivery flexibility: Pellets can be blown into a sealed store directly from a tanker, or delivered in bulk bags for smaller installations. This makes them practical for domestic properties without space for a large fuel building.

Disadvantages

• Higher cost per kWh: Pellets are typically 20–35% more expensive than chip from a reliable supplier, and significantly more than self-produced chip. For large consumers, this margin matters.
• Supply chain dependency: You are relying on certified suppliers and processing infrastructure. Rural locations may have limited options or longer lead times.

Best suited to

Domestic properties, smaller commercial buildings, any site where a large fuel store is impractical, and any application requiring a high degree of automation with minimal operator involvement.

Wood Chip

Wood chip is produced by chipping whole trees, branch wood, or round timber using a mobile or static chipper. Quality and consistency vary widely depending on the feedstock, the chipping process, and how the chip is stored.

Moisture content is everything

Unlike pellets, where moisture is standardised, wood chip can range from roughly 20% moisture (freshly chipped, air-dried material) to over 55% (freshly chipped green timber). Boilers designed for chip — including most agricultural and commercial units in the 50 kW to 1 MW range — are designed to operate within a stated moisture window, typically 25–35% for a chip boiler, or 20–30% for dryer-running premium units.

Running chip that is wetter than the boiler's specification does not simply reduce efficiency — it causes incomplete combustion, excessive clinker, rapid heat exchanger fouling, and accelerated wear. Moisture content should be tested using a calibrated probe meter before use, not estimated.

Advantages

• Low cost at scale: For farms and rural estates with access to their own timber — short rotation coppice, forestry thinnings, or hedgerow arisings — wood chip can deliver heat at very low cost per kWh. Even purchased chip is typically cheaper than pellets.
• Suited to large loads: Chip boilers scale well. Units from 150 kW to several megawatts are commonplace in agricultural and commercial district heating applications.
• Local supply chains: In many rural areas, local chip suppliers can deliver directly from nearby forests or processing yards, reducing transport costs and carbon footprint.

Disadvantages

• Storage requirements: Chip requires a substantial, ventilated fuel store — a dedicated building or bunker, not a compact silo. Rules of thumb vary by boiler size, but a full season's supply for a 100 kW boiler typically requires 50–100 m³ of covered, drained storage.
• Quality variability: Unlike ENplus pellets, chip quality is less standardised. Woodsure's Ready to Burn scheme provides some assurance for domestic firewood but the chip market is patchier. Know your supplier.
• Maintenance intensity: Chip boilers typically require more frequent ash removal and heat exchanger cleaning than pellet boilers running on A1 fuel, particularly when moisture content is at the wetter end of specification.

Best suited to

Agricultural operations, large commercial buildings, district heating schemes, any site with access to local or own-produced timber, and operators who can manage the storage and fuel quality requirements.

Firewood Logs

Log boilers occupy a different category to the automated pellet and chip systems. They require the operator to load fuel manually, typically in batches, and the combustion pattern is fundamentally different — a controlled, high-temperature single burn rather than a continuous trickle feed.

Gasification log boilers

The most efficient log boilers use downdraft gasification — the primary burn takes place slowly in the upper chamber, driving off wood gas which then burns at very high temperature in the lower chamber. This delivers efficiencies of 85–92% on dry logs, compared to 60–75% for a simple batch boiler.

These boilers require:

• Well-seasoned or kiln-dried logs, typically with moisture content below 20%. Wet logs in a gasification boiler produce tar, reduce efficiency, and damage the secondary chamber.
• A large thermal store (buffer tank), typically 1,000–2,000 litres, to absorb the full heat output of a single load and distribute it to the heating system over several hours.

Advantages

• Low equipment cost: Log boilers are typically cheaper to purchase than automated pellet or chip equivalents.
• Simple fuel supply: Logs are widely available in rural areas and can be self-produced with basic equipment.
• No fuel store infrastructure: Logs can be stored in an open woodshed or barn. There are no pneumatic delivery systems or augers to maintain.

Disadvantages

• Manual operation: Someone must load the boiler regularly — once or twice daily in cold weather. This rules out log boilers for unattended operation or holiday properties without a caretaker.
• Not eligible for automated RHI metering: The Domestic RHI metering and monitoring service requirements are met more straightforwardly by automated pellet or chip boilers.

Best suited to

Rural properties with access to their own timber, owners willing to manage the fuel themselves, and applications where capital cost is the primary constraint.

Summary Comparison

| Factor | Wood Pellet | Wood Chip | Log | |---|---|---|---| | Automation | High | Medium | Low (manual) | | Cost per kWh | Higher | Lower | Lowest (own timber) | | Storage footprint | Small | Large | Medium | | Quality standard | ENplus | Variable | Woodsure | | Best scale | Domestic/light commercial | Agricultural/commercial | Domestic/rural | | Maintenance demand | Low-medium | Medium-high | Medium |

The right choice depends on your site, your budget, your operational capacity, and the scale of your heat demand. For most domestic installations, pellets are the practical default. For farms and larger commercial sites, chip usually wins on economics once the storage and handling infrastructure is in place.

If you are unsure which fuel suits your project, the fuel choice should be part of the initial site survey — not an afterthought.