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Wood as a fuel - Designing a biomass heating system

You will need to engage an experienced biomass heating engineer, biomass boiler supplier or another suitably qualified person to determine the technical and economic viability of a potential biomass heating project.  

Binder 300kw wood chip boiler with 6000 litre buffer tank. Swinton Park Hotel, Ripon, Yorkshire.The following information explains the factors affecting the feasibility of this kind of project so you know what questions to ask to ensure all potential issues are considered.


Woodfuel heating systems differ from conventional fossil fuel boilers in the way they are sized, operated and maintained. Most problems experienced by existing biomass boiler installations are as a result of human error in design or installation in relation to one of three key issues:

  • boiler sizing linked with accumulator tank sizing
  • fuel specification vs boiler specification
  • Fuel reception, storage, and feed into the boiler

If you are working with consultants to design your system these documents will help you understand what questions you need to ask, and when:

Biomass Heating- A practical guide for end users (CarbonTrust) provides an easy to use reference on the process and technical elements of planning, designing and installing biomass boilers.

CIBSE guidance on designing biomass heating systems

Sizing your boiler

It is important to correctly size a biomass boiler. Oversized boilers run inefficiently, leading to higher fuel consumption, maintenance costs and increased emissions. They also result in unnecessary capital expenditure compared to a correctly sized, smaller model.

  • Using actual (not estimated) meter readings calculate the annual fuel consumption. This is your baseline figure.
  • Energy efficiency first!  Start with investigating opportunities to reduce demand for heating and hot water using behavioural changes and energy efficiency measures. For example, insulation and draught proofing. The Energy Savings Trust provides a full range of measures.
  • Next, undertake a basic pre-feasibility biomass boiler study. This will review baseline energy consumption (adjusted for energy efficiency improvements), boiler size, fuel consumption and fuel store arrangement options. The overall costs will be estimated and the financial viability can be evaluated.
  • The boiler size will depend on the annual heat load, when you need heat during the year (seasonal heat profile), whether heat storage is necessary (accumulator tanks) and whether there are other heat sources (e.g. oil, gas, solar) that can be linked into the network. Biomass boilers run most efficiently when operating near to their maximum output.

 The biomass decision support tool (Carbon Trust) - guidance on sizing your boiler.

Using accumulator tanks

Log boiler and accumulator at Rosliston Forestry CentreAccumulator (hot water) tanks, otherwise known as thermal stores, help biomass boilers to operate at constant and more efficient loads, instead of frequently starting up and shutting down in response to varying heat demand. Functioning as a heat store the accumulator tanks meet space heating and hot water needs. This has the effect of smoothing out the heat demand profile for the biomass boiler allowing it to operate more efficiently.

The size of your accumulator tanks should be calculated as an integral part of sizing your biomass boiler to ensure the correct balance between heat generation and storage.

Working with multiple fuel and boiler types

In some cases it will be necessary to run a fossil fuel boiler alongside your biomass system to ensure there:

  • are existing fossil fuelled boiler/s on the site which could be used to provide heat during periods of low or high load, so your woodfuel boiler can be used at optimal efficiency
  • is a ‘base load’ of demand which is constant throughout the year which could be supplied by the woodfuel boiler, with a fossil fuelled boiler installed to meet peak demand in winter months
  • is a constant load that could be supplied by the woodfuel boiler through autumn, winter and spring. Peak load in winter could be supplied by an existing oil or gas boiler running in parallel to the woodfuel boiler. Low summer load could be supplied by the more flexible oil or gas boiler/s.

Which woodfuel type

  • Conventional logs work well in batch boilers but usually require manual loading. Well suited to smaller scale systems, e.g. domestic properties.
  • Woodchip systems are more suited to large heat loads. Space for fuel storage and delivery is needed. It is possible to use fuel from your own woods and or the local woodfuel supply chain 
  • Wood pellet systems are suitable for large and small scale systems. They need less fuel storage space than woodchip systems, but fuel supply is usually not sourced locally

Whatever fuel type is chosen it is crucial to remember that all boilers are designed for a particular fuel specification which will include moisture content, size, etc. The fuel specification will be included in the boiler manufacturers guidance and will normally be shown on the Manufacturers Plate on the boiler itself (see example below which states that the fuel is pellets to the specification ‘EN1496-2’ of 6 millimetre diameter). Modern boilers are designed to burn their fuel very efficiently but if the incorrect specification of fuel is used they will burn inefficiently and are likely to break down. The use of the incorrect fuel is also likely to invalidate the manufacturers warranty.

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Designing a woodfuel store

woodchip deliverPoorly sized, located or designed woodfuel stores result in increased cost of delivery, inconvenience and system breakdowns.



wood chip store A discussion with local fuel suppliers about their vehicle and delivery options will help to inform fuel store design. Most commonly, logs will be delivered in bags, pellets will be blown and chip will be tipped, variations may be available according to specification.

Sizing the fuel store

  • The different energy densities of woodfuel determine the fuel store size. The amount of space required for storage is a trade off between the cost of storage construction, number of heating days of fuel required between deliveries.

  • Ensure you have space to store enough fuel for consumption between winter deliveries when your heat demand will be highest.

  • Ensure you include a buffer to deal with unexpected delays in delivery e.g. due to adverse weather conditions.

Access to the fuel store

  • Access to your fuel store for deliveries is critical; it may determine where your boiler house is located.
  • The fuel store needs to be adjacent to the boiler to enable direct fuel feed. Simple fuel transfer ensures low maintenance and trouble free transfer of fuel from store to boiler.
  • Ensure that the delivery vehicles you are likely to use can access the store easily – check nearby access roads for vehicle weight or height restrictions (e.g. for bridges) as well as on site access.
  • Check that there is space for the delivery vehicle to turn on site.

 Above or below ground

Below ground woodchip bunkers offer great flexibility but are expensive to construct. Above ground stores can restrict range of vehicles able to deliver fuel and necessitate ramp construction to enable tipped delivery.

Maintaining the system

Woodfuel boilers generally require a small amount of maintenance, removal of dust from sensors and removal of blockages from the feed system. It is useful to identify an enthusiastic person who will carry out day-to-day maintenance. They will need training and guidance. The installer has a duty to explain what is user maintenance and what requires a technician.

Lack of simple maintenance is a common cause of problems when woodfuel systems are first installed – make sure you know what is needed and that there is someone actually doing it.

Also see:

Biomass Heating: A practical guide for potential users  (Carbon Trust)

CIBSE guidance on designing biomass heating systems


Last updated: 17th November 2017
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