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Biomass Boilers – How it Works

Biomass is energy stored in living organisms, usually derived from the photosynthesis process. Both animal and vegetable obtained material can be a source of biomass. Biomass is carbon based, it also contains other naturally organic molecules including oxygen and hydrogen. The carbon used to create biomass is taken from the atmosphere in the form of carbon dioxide (CO²) found in plant life. This plant life may be consumed by animals therefore converting into animal biomass. While carbon is growing, biomass takes it out of the atmosphere and returns it when it is being burned. Biomass can be constantly replenished if managed on a sustainable basis. Consequently there is no net increase in the level of CO2 omissions.

Types of Fuel:


Logs: Acquired from trees with little handling involved. In order to ‘season’, the logs need to be left covered for a period of 1-3 years for the moisture level to fall from around 50% to 20% making them lighter, the drier the logs the hotter the fire, it will also produce less smoke and tar.


Pellets: Made from the processed by-product of wood manufacturing and saw mills. Pellets require less storage than chips and logs, around a third of the space, they are denser and drier at 8-10% moisture content. As they require little space this makes them ideal for projects where space is limited. Pellets are also ideal for automated domestic heating systems as they have a uniform shape and a higher energy density.


Chips:Like pellets these are derived from saw mills and clean untreated waste timber. Ideal for larger scale projects such as community heating systems. They need to achieve high uniformity and have low moisture content for problem free use in smaller systems.

Types of Technology:

Stoves: Pellet and log stoves have a high efficiency level of around 60-80%. The higher the level, the less fuel will be needed to heat a house. Pellet stoves automatically feed the pellets from an integral storage space. When at full capacity, pellet stoves can hold enough fuel to last 2 days and are generally used to heat the room where they are placed. Higher output stoves can come equipped with space heating and an integral back boiler to provide hot water. When being used to heat water, the fuel for an integral storage space will be consumed quicker than a stove used primarily for heating a single room. Selected stoves can come with a controllable fan that can be used to regulate heat output.

Pellet stoves generally have an electrical fan in a sealed unit with an ignition to light them electronically. In case of power failure, certain stoves come with a back-up power supply. Stoves and their flues must comply with the terms of Part J of the Building Regulations (England and Wales) or Part F in Scotland or Part L in Northern Ireland.

Log stoves are most beneficial in rural areas, with a regular supply of wood fuel. New innovative low energy housing can heat an entire house with one stove located in the most popular room of the house.

Viessmann Product

Log boilers: Will not need stoking as frequently as a log stove but are loaded by hand. Domestic models range from 20 – 50kW and are generally located in a separate boiler room. Log boilers are intended to work alongside a large heat store known as a buffer tank or accumulator. This improves its efficiency by allowing it to operate for shorter periods at higher loads. If the buffer tank or boiler is sizeable enough they should only need loading every 1-3 days.

Pellet boilers: Selected boilers come equipped with an integral pellet hopper, this is repeatedly topped up automatically from larger store outside of the house. To supply fuel for a week or more some boilers have an integrated silo. ‘Flexibags’ or external silos can hold fuel for 2-12 months. Generally installed in utility room and cellars, domestic models range from 8-30kW.

Chip boilers: Do not work effectively below 50kW and work in a similar way to pellet boilers. Therefore they are not suitable for single domestic use, however a good option for community heating schemes and larger buildings.

Combination boilers: Selected specialised boilers (not to be confused with gas combination boilers) can use a mixture of wood fuels e.g. sawdust, wood chips, logs. If you have your own various fuel resources, they can be an attractive option.

Is wood derived biomass suitable for my property?

A wood fuel heating system can be effectively integrated into an existing hot water and central heating system. Choosing the most beneficial wood fuelled system depends on a range of factors.

Ventilation and Flues: To disperse the combustion gases you will need a permanent vent to provide sufficient air for the appliance and a suitable flue. Older houses with existing chimneys may be suitable, but the chimney may need to be lined.

Space: Similar to fossil fuels such as coal and oil, wood fuels provide heat by burning however it has a lower energy content, therefore more wood is needed to burn the same amount of coal/oil so more space is needed to store the wood. The storage area needs to have easy access for deliveries and most important be dry and moisture free. For deliveries in the coldest time of year the storage space needs to have space for the fuel you think you will use in between deliveries and a little more. A wood fuelled appliance cannot be wall mounted and will often be larger than its oil or gas equivalent.

Availability of local supply: You will have to make sure you have a choice of suppliers in your local area to ensure a reliable supply. The numbers of wood suppliers are constantly increasing and generally available everywhere.

Permission: All wood heating systems have to comply with Building Regulations. If you live in a ‘smoke control zone’ ‘exempted’ appliances are required. For more information visit www.smokecontrol.defra.gov.uk Wood burning boilers and stoves do not generally require planning permission however exemptions include: if the flue exceeds 1m above the roof or it is installed on the principle elevations and visible from a road, in buildings in conservation areas and world heritage sites.

Maintenance: Wood fuelled boilers, room heaters and stoves need to be kept clean and swept on a regular basis to remove ash. The ash bin will need to be emptied on a weekly basis. A log fire requires ash removal before every useSelected appliances especially boilers will have a built in self-cleaning system; this will collect ash from the heat exchange tubes and combustion gate. Some boilers have a mechanism to compress the ash which reduces the amount of times the ash bin needs emptying. The boiler will need to be shut down periodically whilst being cleaned by hand if it does not have an automated ash cleaning mechanism. If the ash is not cleaned out regularly, the build up will adversely affect the combustion conditions and consequently lead to boiler failure.

The only maintenance requirement for the cleaning for the heat exchanger and the automatic ash removal will be an annual maintenance check and occasional ash removal.

A flue pipe and a chimney or wood burning stove must be swept regularly to prevent blockages and remove all soot deposits. HETAS advise that this should be done ‘at least twice a year, preferably before the heating season to check that the flue has not been blocked by bird’s nests for example, and also at the end of the heating season to prevent soot deposits from resting in the chimney during the dormant period’.

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