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Biomass heating

Heating a single room using a wood burning stove – or a whole property using a wood-fuelled boiler – is known as biomass heating.


Biomass can usually be integrated into a home’s existing heating and hot water system and can be an attractive option for households with a reliable wood supply, reasonable storage space and delivery access.

Because the CO2 absorbed by the trees when they are growing is approximately the same as the CO2 released when the fuel burns they are considered to be ‘low carbon’. They are not ‘carbon neutral’ or ‘zero carbon’ because there are still carbon emissions associated with the fuel’s extraction, processing and transportation.

Some forms of biomass (e.g. wood pellets, below) use material grown especially for the purpose, such as field-scale willow.

Two main types of biomass heating appliance

1) Stoves:  These burn logs or pellets to provide space heating for individual rooms. Stoves can also be fitted with a back boiler to provide hot water for the kitchen and bathroom. Stoves are usually around 7kW and have an efficiency of between 60-80%.

2) Boilers:  Larger and more industrial in design, these provide whole house heating and hot water. Domestic log boilers range from 20-50kW and are stoked by hand. Pellet boilers range from 8-30kW and often incorporate a pellet hopper, which automatically feeds the boiler with fuel.

Chip boilers are most efficient for larger (50kW+) systems so are generally not suitable for individual domestic properties. Some biomass boilers are designed to take a variety of wood fuels, including logs, chips and sawdust, and might be viable if you have access to a range of different wood sources.

Heating a single room using a wood burning stove – or a whole property using a wood-fuelled boiler – is known as biomass heating.


Biomass can usually be integrated into a home’s existing heating and hot water system and can be good for households with a reliable wood supply, reasonable storage space and delivery access. 

Broadly, there are three diffent types of biomass heating fuel: logs, wood chips and wood pellets

  1. Logs require little processing except for 'seasoning', the drying- out process which can take up to 3 years but means the logs burn hotter and produce less smoke. Logs are usually cheaper than other types of wood fuel but need more storage space and can only be used in manual-feed boilers or stoves. 
  2. Wood chips are made from forestry ‘thinnings’ or untreated waste wood, and are typically used in larger heating systems in e.g. schools or blocks of flats. In smaller (e.g. domestic) systems the chips must be produced to a standard size and with low moisture content. 
  3. Wood pellets are are more energy-dense than logs or chips and require about a third of the storage space. Their dryness and uniform shape makes pellets ideal for automated systems. They are often made from by-products such as sawdust or form material grown especially for the purpose, such as field-scale willow. However, some pellets come from unsustainable sources (e.g. forests in the US) and should be avoided.

And there are two main types of biomass heating appliance:

  1. Stoves: These burn logs or pellets to provide space heating for individual rooms. Stoves can also be fitted with a back boiler to provide hot water for the kitchen and bathroom. Stoves are usually around 7kW and have an efficiency of between 60-80%. 
  2. Boilers:These provide heating and hot water for the whole house. Domestic log boilers range from 20-50kW and are stoked by hand. Pellet boilers range from 8-30kW and often incorporate a pellet hopper which automatically feeds the boiler with fuel. 

Chip boilers are most efficient for larger (50kW+) systems so are generally not suitable for individual domestic properties. Some biomass boilers are designed to take a variety of wood fuels, including logs, chips and sawdust, and might be viable if you have access to a range of different wood sources.

Cost & savings

The initial cost of installation depends on the size and type of system: a log burning stove might be in the region of £2,000; a stand-alone pellet stove around £4-4,500; and an auto-feed pellet boiler for an typical family house around £11,500. You should allow for a further £100 per year for annual maintenance for a boiler.

The financial and CO2 savings will vary depending on what fuel you are currently using for heating your home. If you’re using electricity or oil biomass will save more than if you’re using mains gas. And ensuring your property is fully insulated will reduce your heating demand and mean you can fit a smaller and cheaper system.

The following table shows potential financial and CO2 savings for a 3 bedroom semi-detached property with loft and cavity wall insulation:

Fuel replacedSavings £/yearSavings CO2/year
Mains gas £100 3000kg
Oil £280 4000kg
Coal £300 7500kg
Electricity £580 7500kg
LPG £720 3500kg

An additional financial benefit is the Renewable Heat Incentive (RHI) though which you will be paid for each unit of heat that your renewable heat technology produces. But, in order to be eligible, both the system and installer must be registered with the Microgeneration Certification Scheme.

Other things to consider:

Biomass heating systems require an adequate air supply and ventilation to work efficiently and safely. This may require a new flue and additional vents, or the strengthening or lining of an existing flue or chimney. Biomass systems and flues must meet building regulations, and in designated smoke control areas (e.g. many towns and cities) only certain boilers and stoves can be used. Check with your local authority for details.

Biomass boilers and stoves don’t generally need planning permission as long as the flue sticks up less than 1m above the roof and is not on the home’s ‘principal elevation’. But if you live in a listed building, a conservation area or a World Heritage Site you should contact your local planning department before installing any renewable energy technology.

Don't forget that you’ll need a reliable supply of wood fuel; and please check that the pellets, logs or chips come from a sustainable source.  


Biomass is considered to be a ‘low carbon’ technology, rather than ‘carbon neutral’ or ‘zero carbon’. This is because even though  the CO2 released by the wood pellets or chips is is approximately the same as the CO2 absorbed by the trees when they are growing, there are still carbon emissions associated with the fuel’s extraction, processing and transportation.


Photos: logs*, Horia Varlan; wood chips*, Jake Rome; wood pellets, Keving Lindegaard. (* = reproduced under creative commons)

  • Logs require little processing except for seasoning (drying out). This process can take up to 3 years and brings their moisture content down to about 20% meaning the logs burn hotter and produce less smoke. Logs are usually cheaper than other types of wood fuel but need more space for storage and can only be used in manual-feed boilers or stoves. 
  • Wood chips are sourced from forestry ‘thinnings’ or made from untreated waste wood. Wood chips are typically used in larger heating systems such as those found in schools or blocks of flats. When they are used in smaller (e.g. domestic) systems the chips must be produced to a standard size and with low moisture content. 
  • Wood pellets are made from by-products such as saw dust and have a low moisture content of between 8-10%. As a consequence they are more energy-dense than logs or chips and require about a third of the storage space. The uniform shape makes pellets ideal for automated systems. 

Cost & savings

The initial cost of installation depends on the size and type of system: a log burning stove might be in the region of £2,000; a stand-alone pellet stove around £4-4,500; and an auto-feed pellet boiler for a typical family house around £11,500. You should allow for a further £100 per year for annual maintenance for a boiler.

The financial and CO2 savings you will make from your biomass system will vary depending on what fuel you are currently using for heating your home. If you’re using electricity or oil biomass will save more than if you’re using mains gas. And ensuring your property is fully insulated will reduce your heating demand and mean you can fit a smaller and cheaper system.

The following table shows potential financial and CO2 savings for a 3 bedroom semi-detached property with loft and cavity wall insulation:

Fuel replacedSavings £/yearSavings CO2/year
Mains gas £100 3000kg
Oil £280 4000kg
Coal £300 7500kg
Electricity £580 7500kg
LPG £720 3500kg

An additional financial benefit is the Renewable Heat Incentive (RHI) through which you will be paid for each unit of heat that your renewable heat technology produces. But, in order to be eligible, both the system and installer must be registered with the Microgeneration Certification Scheme.

Installers that are also members of the Renewable Energy Consumer Code scheme will have agreed to stand by a consumer code that covers contracts, completing the order and after-sales activities. If your installer is registered with a Competent Persons Scheme such as HETAS, your system will comply with all regulations; if not, your local council’s building control department will have to check and sign-off the installation.

Other things to consider:

Biomass heating systems need air to circulate around the unit, so adequate air supply and ventilation are essential. This may require a flue and additional vents. An existing chimney flue may need to be strengthened or lined.

If you’re going for biomass you’ll need a reliable supply of wood fuel. And if you are worried about environmental impacts then you need to check that the pellets, logs or chips come from a sustainable source, and using a local supplier will reduce emissions from transportation.

Biomass systems and their flues must meet current UK building regulations, and in designated smoke control areas (e.g. many towns and cities) only certain exempted boilers and stoves can be used. You can find out about these restrictions from your local authority.

Biomass boilers and stoves don’t generally need planning permission as long as the flue sticks up less than 1m above the roof height and is not on the home’s ‘principal elevation’. But if you live in a listed building, a conservation area or a World Heritage Site this may not be the case, and you should contact your local planning department.

The video below looks at some things to consider before embarking on a biomass boiler project (for your house or as a community).

 


Photos: logs*, Horia Varlan; wood chips*, Jake Rome; wood pellets, Keving Lindegaard. (* = reproduced under creative commons)

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