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Waste Burning Heaters & Biomass Boilers


Heaters burning waste fuel have traditionally been looked upon unfavourably by Insurers owing to poor design and incorrect operation which over the years have been the direct cause of many serious fires.  Whist not exclusively, these have tended, and continue to predominate in motor garages and woodworking premises burning waste oil and timber respectively.

For sound environmental and economic reasons recent years have seen a growth in the number of waste burning heaters being marketed and this trend is likely to continue into the future. As a result, the RISCAuthority has published RC4 Risk Control – Fixed heating equipment burning waste fuel.  This replaces a previous LPC Recommendations document which was withdrawn from publication.



RISCAuthority RC4 provides informative guidance on the installation and use of heaters burning waste oil, wood burning stoves and similar devices and their associated flues. This publication should be understood and followed by Consultants on the basis of established ‘best practice’. It is important that such heaters are given the attention they deserve as far as the survey is concerned and that the appropriate details are contained within the survey report, together with risk improvements, as appropriate, based on the latest RISCAuthority document (further information specific to modern biomass heating systems is detailed separately).

In particular, attention should be given to the following:

  • Prior to installation, checks should be made with the heater manufacturers that the heater is suitable for use with the proposed fuel.

  • Heater installed, maintained and serviced by competent persons in accordance with the manufacturer’s instructions. This to include the inspection and cleaning of flues with special emphasis on wood burning stoves and the presence of soot and tar deposits.  Wood burning stoves should preferably be installed and maintained by a HETAS registered installer –

  • Heater operated at all times by trained persons in accordance with the manufacturer’s instructions.

  • Heating appliance located in a separate fire compartment or surrounded by a metal guard providing a clear space of at least 1m from combustibles. Where governed by the risk, separation distances in excess of 1m may be required.

  • In the case of wood burning stoves, no major stockpiles of wood for burning should be kept in the building within 5m of the stove.

  • The provision of an insulated flue and suitable protection and cut back where penetrating timber roof decking and other combustible elements of construction. 

  • The fitting of spark arrestors to flues from wood burning stoves is against current thinking as these can increase the build-up of soot, restricting the escape of combustion gases, resulting in an increase in flue temperature. Where a spark arrestor is fitted, it is critical that the flue is inspected and cleaned more frequently than otherwise would be the case, extending to the cleaning of the spark arrestor itself. This should be carried out at least quarterly, or more frequently as determined by a risk assessment.

  • Wood burning stoves where located externally are to be kept clear of combustible materials. A minimum spatial distance of 6m should be maintained, extended to 10m or more where a severe hazard is present.


  • Hazards associated with the storage of fuel.

  • Correct attention to oil leaks and spillages.

  • Special attention required where sawdust is used as a source of fuel.

For further details of the above and other fire safety measures, reference should be made to the full text of RC4.

Notwithstanding the developments that have been made over the years in waste heater design, there will doubtless be some old ‘rogue’ heaters in place (often of ‘homemade’ design), in a poor and dangerous condition for which requirements for immediate decommissioning and removal will often be the only solution.



In recent years significant strides have been made in the development and use of biomass boiler technology across a wide range of commercial and industrial buildings using wood in the form of pellets or chips, supported by such measures as the Renewable Heat Incentive (RHI) scheme. Eligibility for the RHI scheme is dependent on the equipment and installer having Microgeneration Certification Scheme (MCS) or equivalent certification.

A biomass system typically comprises the following components:

  • External or outdoor area (fuel delivery).

  • Fuel storage (for example, in the form of a bunker, silo or a containerised store).

  • Mechanical transport (such as augers or ‘walking floors’) to move the fuel to the boiler.

  • Boiler house (incorporating the boiler, chimney, controls etc).

  • Boiler (fired with bio fuel).

  • Heat exchanger (to convert to hot water or steam).

  • Chimney (including fans).

  • System controls.

A number of excellent publications relating to biomass heating systems are freely available:

  • RISCAuthority RC64: Recommendations for fire safety with small biomass installations.

  • Biomass heating: a guide to medium scale wood chip and wood pellet systems, published by the Biomass Energy Centre.

  • Biomass heating: a guide to small log and wood pellet systems, published by the Biomass Energy Centre.

  • Health and safety in biomass systems – Design and operation guide, published by the Combustion Engineering Association.

Biomass systems are subject to the same general health and safety principles, codes of practice, and design, installation and operation standards that apply to gas, oil or coal fired boiler systems, including, for example, the provisions of the Pressure Systems Safety Regulations 2000 (where providing steam). However, reference to the above publications reveals a number of special hazards and risk control considerations applicable to Biomass systems, some of the most common areas of which are summarised below under the main operations of a biomass system, namely:

  • Fuel delivery,

  • Fuel storage and handling,

  • Boiler operations and combustion



  • Biomass fuel is typically delivered by a lorry with accompanied hazards of vehicle manoeuvres (often in a restricted location) and tipped into a below ground hopper or onto a mechanical conveyor. Fuel may also arrive in bags or be blown into a silo.

  • Main hazards are those associated with mechanical augers and conveyors, and the risk of falling from height where large, below ground fuel receptors are involved.

  • In all cases a suitable and sufficient risk assessment should be carried out from which method statements should be produced. This key principle applies to all operations of the biomass system.

  • Pneumatic delivery increases the risk of dust and explosion and is particularly prevalent with pellets which tend to disintegrate during the delivery process. All delivery pipes should have a smooth internal bore and large radius bends to reduce the chances of pellets disintegrating. To prevent static discharge, they should also be robustly constructed from suitable material and earth bonded. Employing quality pellets with a low dust content and good mechanical durability is also an important consideration.



  • Biomass fuel storage can take many forms, such as containerised fuel stores, walking floor tip areas, bunkers with tipping walls and above ground hoppers. For operational and safety reasons, fuel storage should be located as close to the boiler house as possible.

  • Hazards associated with storage and handling of biomass fuels include:

    • Fire and explosion.

    • Issues relating to COSSH – e.g. toxic spores or carbon monoxide (CO) poisoning.

    • Mechanical failure of the storage vessel.

    • Slips and falls.

    • Injury from contact with machinery and moving parts.

  • Unless wood chips are reasonably dry, typically less than 30% moisture content, they may degrade in storage. Microbial activity can lead to piles of wood chips generating heat as they decompose. Under the right conditions fungal spores will

    develop in the chip pile and these spores will be released when the pile is disturbed; if inhaled they can cause an incurable disease commonly known as ‘farmers lung’. To avoid the formation of spores, moist or wet wood chip should be stored for as short a period as possible, and should be used within one month of being chipped. An FFP3 filter dust mask should be worn when working in close proximity with stores or deliveries of stored chips.

    In very large piles of wood chips it is possible for the heat released from microbial action to lead to an increase in temperature in the heart of the pile, sufficient to give rise to spontaneous combustion. The potential for this to occur will depend on a number of factors, including the moisture content of the chips, ambient conditions and the store design. Wood chips in long term storage should be turned regularly to prevent microbial activity and aid drying, combined with continuous temperature monitoring.

    Where determined by a risk assessment, inert gas injection or fixed fire suppression systems may be required in accordance with Section 5.9 of RC64. 

  • The HSE has issued a warning notice in relation to serious concerns over the storage of wood pellets and the risk of carbon monoxide (CO) poisoning. Full details in relation to this hazard and the precautions which should be adopted are contained in RiskSTOP Liability News: Issue1. Where determined by a risk assessment, CO alarms should be installed.

  • The handling and storage of wood pellets presents a potential dust explosion risk for which a DSEAR risk assessment should be conducted and the appropriate safeguards implemented, including measures such as earth bonding, the installation of appropriate ATEX rated electrical equipment and explosion venting (RiskSTOP Technical Bulletins 21, 23 & 42 refer).



  • A CO monitor should be installed in the boiler house (this is a requirement under Building Regulations - Part J, but only for dwellings; in all other buildings this is established ‘best practice’). This should be positioned at head height on a wall near the boiler, visible from where an operative might stand and review the plant. Installing the monitor directly above the boiler is not recommended as this might result in unwanted alarms such as when opening a boiler inspection port.

  • In order to prevent burn back from the fire bed of the biomass boiler into the fuel storage, duplicate anti-burn back devices should be fitted as specified by the manufacturer. It is common for a rotary valve to be installed between the combustion grate and the main fuel store/silo, together with an automatic mains water drench valve. In the case of the latter, it is important to ensure that the water supply is of the highest integrity and, where required, protected from frost.

  • Fires arising from an accumulation of tar can occur in the chimney and flue ductwork. Inspection and cleaning of the flue system at least annually by a professional chimney sweep is recommended practice.



  • Of paramount importance is the need to ensure that the biomass plant is installed, operated and maintained by suitably trained and competent persons in strict accordance with the manufacturer’s instructions.

  • Biomass boilers have a greater maintenance requirement than fossil fuelled boilers. A full internal and external inspection of the boiler should be carried out annually or more frequently as required, dependant on use.

  • Installation and maintenance should preferably be entrusted to a HETAS registered/MCS certified installer.

  • Some biomass boilers incorporate an inbuilt automatic cleaning provision of the heat exchanger surfaces by the use of compressed air jets or nozzles. This happens automatically while the boiler is in operation and can significantly reduce downtime and routine maintenance costs.

  • An ‘operation and maintenance manual’ and a ‘health and safety file’, as required by the Construction (Design and Management) Regulations 2015, should be available for the management and operators of the plant. This documentation must be site specific. A ‘logbook’ should also be maintained, recording, among other things, operational data, safety checks and maintenance activities.

Typical biomass boiler design

Modern biomass energy centre with external pellet silo

190kW biomass pellet boiler supplying a 200 bedroom hotel installed in two 6m steel containers, comprising a 10 tonnes pellet silo and a fully automated biomass boiler room fed via a day hopper and auger system.


Where biomass heating systems are encountered in the field, it is important that close attention is given to the type of installation, operating procedures, operator training and the maintenance regime in place, ranging from weekly visual inspection and cleaning through to complete internal and external inspection, the key information on which should be contained in the survey report.

In the event of any queries or concerns relating to a specific case, reference should be made to the RiskSTOP Technical Helpline.

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