Biomass Boilers


The three main types of fuel most widely available are Logs, Wood Chips and Wood Pellets. Of these, the most commonly used are wood chips and pellets due to their quality and ease of use.

Wood pellets

Energy Content: Wood pellets have a higher energy content and are made from compressed sawdust, wood shavings and other biomass products.

Storage: Wood pellets require less storage space compared to logs or wood chips.

Heat Output: Wood pellets have an average heat output of 4.8kWh per kilogram in which two kilograms would equate to one litre of oil. A higher standard of pellet will consist of better quality virgin wood and therefore release more energy per surface area.

Moisture: The moisture content of a pellet is important as the lower the level of moisture in the pellets, the higher the heat output and the more efficiently your boiler will operate. A good quality pellet typically has a moisture content of in between 5-10%.

Size: Each wood pellet boiler is designed and commissioned to accept specific size of pellet and are uniform in size and shape. The most common size is between 6mm and 8mm. Using the wrong size of wood pellets can alter combustion conditions and reduce efficiency.

Wood Chip

Energy Content: Wood Chip is a suitable biomass fuel choice if there is access to (non-contaminated) waste wood (e.g. farm/woodlands by products) and a wood chipping machine. Burning waste wood can be more cost effective than purchased wood pellets.

Storage: Wood chip storage requires a large volume of space, on average three times that of a pellet store. Chips must also be constantly kept flowing into the boiler with the help of an agitator mechanism in which the chips are continuously stirred to keep them in a fluid like motion.

Heat Output: Wood Chip has an average heat output of 3.5kWh per kilogram.

Moisture: Wood must be treated properly by being dried for 12 months so that it reaches a moisture content of 25% or less. Wood chips can also be purchased from local suppliers.

Size: Depending on the equipment they are to be used with, wood chips are typically 20-50 mm in length.


Energy Content: Log wood can be low cost option if there is access to (non-contaminated) waste wood. Logs must be seasoned to reduce their moisture content and can be of hardwood or softwood. The heat output of hardwood by volume is roughly twice that of softwood.

Storage: Log wood must be stored in a large, covered, well-ventilated area. A log-fueled boiler typically needs filling up at least once a day.

Heat Output: Log wood has an average heat output of 4.1kWh per kilogram.

Moisture: Logs should be sustainably sourced, seasoned to ensure they have a *moisture content of under 20%. *Unseasoned or wet logs burn inefficiently, reducing the heat produced. Hard woods will take longer to season (2 years) than soft woods (1 year).

Size: For a batch-burning boiler, the ideal log size is 50cm in length and 12-15cm in thickness.


The table below shows the approximate figures for how much energy is contained in each different fuels per unit weight, or the ‘energy density’ of a fuel.

FuelEnergy Density by Mass (GJ/tonne)Energy Density by Mass (kwh/kg)Bulk Density Kg/m3Average Price (p per kWh)
Wood Chips (30% MC)12.63.52502.6
Log Wood (20% MC)14.74.1350 – 500
Wood (Oven Dried)18 – 205 – 5.6400 – 6003.5
Wood Pellets17 – 184.7 – 5.0600 – 700
Miscanthus (25% MC)133.6140 – 1802.6
House Coal27 – 317.5 – 8.68508.57
Natural Gas (NTP)54150.74.1
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