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Biomass boilers and stoves

Biomass is rapidly gaining in significance as a source of renewable energy in the UK. People are increasingly heating their homes with wood-fuelled stoves or boilers, and the sophistication and versatility of such systems is constantly improving.

Wood is probably the oldest biofuel in the world, but the traditional sack of logs has been joined by other products, notably wood chips and pellets. Modern stoves can be over 90% efficient, and have thermostatic control systems and automated fuel supply. The technology has been developed in other European countries where wood is used much more widely. In Austria, for example, 80% of boilers installed burn wood pellets or chips.wood pellet boiler

Although the boilers are expensive, wood can be cheaper than conventional fuels, such as gas or oil, although this is not always the case, especially with pellets. Also, wood is essentially carbon neutral - the carbon dioxide emitted during combustion equals the amount absorbed during growth of the tree. However, inputs of energy for processing and transport must be considered, and a local supply of wood fuel is desirable to keep transport costs down.

Cutaway of automated feed for pellet-fuelled boiler

Types of biomass heating systems

Various systems use wood in different forms, but the two most popular are:

  • Stand-alone stoves for space heating - can use logs, wood pellets, or a variety of solid fuels ('multifuel' stoves); output is typically 5-20 kW and the most efficient models are rated at around 90%. Some can be fitted with a back boiler for water heating, although this is not recommended because it makes the stove less efficient and produces more smoke.
  • Boilers for central heating and hot water - can use wood pellets, logs or wood chips, although automated feed systems for domestic boilers generally require pellets. Output depends on the size of heating system. A typical three-bedroomed semi-detached house will need a 20-kW rated boiler.

What will it cost?

Stand-alone stoves typically cost £2000-4500 installed, but the price of fuel is highly variable; some can burn logs that are obtained for free, whereas pellet-fired stoves can be expensive to run. Also, according to the Energy Saving Trust, space heating using a stove is relatively inefficient compared with a central heating system. However, such stoves do cut the household's carbon emissions.wood pellet stove

Wood-fired boilers vary enormously in cost, depending on the type of wood fuel and the delivery system, but typically lie in the range £7000-£14,000 installed. For large houses the total cost may exceed £20,000. Again, running costs may depend on the price of pellets relative to gas or electricity. Systems using wood chips are cheaper to run, but tend to be more expensive to install. Given a cheap supply of fuel, biomass boilers can deliver annual savings of up to £800 on heating bills, compared with electric heating, and also substantially cut carbon emissions.

Wood pellets are typically around £220 per tonne when delivered in bulk, and £250/tonne in bags.

Pellet-burning stove for space heating

Other factors to consider

  • Safety and building regulations - check with your installer about compliance
  • Flue design -make sure the flue vent is of a suitable type and its location allows effective venting of flue gases. See the BFCMA guidance on flues and chimneys for wood-burning and multifuel stoves.
  • Fuel storage - space must be allocated for storing fuel and adequate access for deliveries
  • Smokeless zones - check that your appliance is exempted under the Clean Air Act, as a 'clean burn' model
  • Check with your planning authority whether or not planning permission is required

No fire without smoke?

  • Always burn properly seasoned wood with low moisture content. This burns most efficiently, with greatest heat output and the least smoke and other pollutants.
  • Burning green/wet wood produces acid that corrodes the equipment and tar that coats the interior of the stove and flue.
  • Ensure that the air supply is optimal. This creates a fierce burn and good secondary combustion of volatile gases to minimise harmful flue gases.
  • A lambda sensor on the appliance will regulate oxygen input to optimise efficiency.
  • Never burn treated timber (e.g. coated with paint or creosote), glued timber, or composite material such as MDF. These will emit toxic gases.
  • If you live in a Smoke Control Zone, check that your boiler/stove is on the list of exempt appliances.


Renewable Heat Incentive (RHI)

Heating systems fired by biomass boilers or biomass pellet stoves are eligible for incentive payments under the Renewable Heat Incentive (RHI) scheme, launched in spring 2014. Only systems installed after 15 July 2009 are eligible for the incentive payments. Click here to find out more, or visit the Ofgem website for full details.

Under the revised scheme, from September 2017 the rate is around 6.5 pence per kilowatt hour of heat, based on estimated annual heat use calculated from the property's Energy Performance Certificate. The tariff lifetime is 7 years. Any Renewable Heat Premium payments already received are clawed back through reduced RHI payments over the tariff lifetime.

You can find out how much money you are likely to receive by using the government's own Renewable Heat Incentive Calculator.

N.B. Note that stand-alone wood-burning stoves without a back boiler do not qualify for the RHI.

Biomass boilers and room heaters may qualify for grant assistance from the ECO programme (see Insulation and heating grants). See the Energy Saving Trust website for more information.