Biomass - Useful information

The combustion of fuel, derived from wood, can be considered renewable, providing they are replanted as they are harvested. When burned, these fuels release only the amount of carbon dioxide that they absorbed whilst growing - unlike the carbon in coal, oil and gas, which was absorbed over millions of years but is being released in the space of a few decades.

There are two main types of wood-fuel systems. The first is a standalone stove, burning logs or pellets to heat a single room. Some can also be fitted with a back boiler to provide water heating.

The second type is a boiler which uses logs, pellets, or chips as a fuel and is connected to a central heating and hot water system. Larger wood fuel boilers can be used in district heating system.

Most pellet and chip boilers use automatic fuel feeders which are served by a fuel store which is periodically refilled by a fuel supplier. Many of these installations now include an auto de-ash and heat exchanger cleaning system.

There are 3 types of fuel:

  • Logs –They have good availability are cheap but bulky to handle and store.
  • Pellets –They are energy dense, requiring less storage space, but more expensive, approximately matching the p/kWh of mains gas (but cheaper than oil).
  • Wood chips–Considerably less expensive than

pellets but are bulkier. Moisture content and therefore energy density/quality can vary according to source. 

Fuel Type

Size of system

Cost per tonne

Moisture content













The biomass energy centre has useful data on cost and energy content of biomass fuel types.

Wood-fuel costs will also depend on the proximity of the supplier and the capacity to buy and store in bulk. Wood-fuel should be sourced locally to minimise the environmental impact of transportation emissions. Having an on-site resource of useable fuel, such as timber shavings or arboreal arisings can significantly reduce costs.

Feasibility and Development

There are a number of stages involved in assessing the feasibility of installing a wood-fuel system:

Siting and design considerations

  • Siting – a large dry area close to the boiler to store the wood-fuel.
  • Building regulations - For domestic installations the property insulation should be upgraded in line with Part L building regulations and a detailed calculation of the buildings heat losses or load should be undertaken to size the boiler. The lower the building’s energy demand the lower the running costs and the greater the CO2 emission savings. Installations under 50kW must adhere to Part J, Combustion Appliances and Fuel Storage Systems.
  • Fuel supply and quality - Fuel consistency in terms of size and moisture content is vital. Low quality chip is the largest reason for boiler breakdown to date, it is important to get written wood chip moisture content specifications from the boiler manufacturer/ installer and supplier of wood chips.
  • Maintenance -Regular serving and maintenance is required.  Although boilers are likely to have automatic removal of the low quantities of ash, the ash bin will need periodically emptying.


Advice should be sought from the local planning authority, some key points to consider are listed below:

Useful resources