Module 82: Biomass boilers for low lifetime costs

This module examines the elements that determine the real costs associated with a biomass system, and key considerations when specifying a biomass boiler

Drivers such as the Renewable Heat Incentive (RHI), changes to Building Regulations and increasing environmental awareness have combined to accelerate the adoption of biomass boilers for low temperature hot water (LTHW) production. With a high capital cost compared with traditional oil or gas boilers, there is uncertainty as to the real long-term costs of installing biomass. Recent evidence has suggested that poor design and installation – and lack of maintenance – contribute significantly towards the lifecycle cost (LCC) of the installation.1

This CPD will examine some aspects that determine the real costs associated with a biomass system, and identify key areas to consider when specifying a LTHW biomass boiler.

A recently published Department of Energy and Climate Change (DECC) study1 showed that the average biomass system efficiency is around 66%, whereas manufacturers will factory test boilers typically above 92% efficiency, highlighting that it is the design, installation and operation that is affecting the reported overall system efficiencies. The CIBSE application manual for biomass heating (AM15:2014) identifies that there are many factors – aside from the boiler itself – that must be considered.

These include: boiler sizing; flue design; fuel specification and store design; hydraulic design; system monitoring; and operation and maintenance considerations.

Boiler sizing, selection and flue arrangements

It has been reported1 that installations have been sized so that the biomass boiler output optimises RHI income, rather than specifically meeting the pattern of heating loads. This may be driven by the significantly higher capital cost of biomass compared with fossil-fuel boilers. A biomass boiler does not work in the same way as a fossil-fuel boiler, and the design specification should account for the differences – CIBSE AM15 is an excellent resource for determining how to integrate biomass with fossil fuels.

The potential heat-demand profile is the basis of correct boiler size selection. For retrofit projects, historical use data is often used – ideally in the form of half-hourly heating data. Where this is not available, an appropriate model or simulation (as specified by EN ISO 137902) can be used.

With this data, the designer can determine the base and peak loads in both summer and winter, so that the boiler is sized based on an appropriate operational regime (peak load or base load). The Biomass Decision Support Tool (free to download from the Carbon Trust website3) is aimed at assisting with correct sizing.

Biomass boilers respond slower than fossil-fuel boilers, so require adequate time to heat up/cool down. Any associated gas boiler must not operate too early – the biomass boiler is often designed to act as the ‘lead’ boiler.

Installing a buffer vessel is essential, unless there is a minimum constant load of around 30%, as most biomass boilers have turn-down ratios of between 2:1 and 3.5:1.4 Biomass boilers cannot operate effectively with short cycling periods. This can have a very dramatic affect on overall boiler thermal efficiency – much more than for a gas boiler. A buffer vessel will improve this, but rules of thumb should not be used for sizing, as capacity ratios vary considerably depending on the particular installation factors. For example, for pellet-fuelled boilers, this could range from 10 to 60 litres per installed kW biomass boiler load.4

When selecting a boiler size – whether a single biomass boiler or a biomass/fossil fuel combination – practical and economic considerations such as capital costs, RHI tariff break points and physical constraints on site should be taken into account. A biomass boiler sized to 50% of the peak demand will normally be able to provide 80-85% of the building’s annual heating needs.3 Sizing a biomass boiler to 100% of the peak load is unlikely to be a cost-effective strategy, as it will operate at full output for less than 1% of the year. It is much more efficient and cost-effective to install two smaller biomass boilers, or a single one alongside a peak-lopping fossil-fuel boiler. However, integration of the controls needs careful consideration – conventional building management system methodology alone is unlikely to be appropriate. When including fossil-fuel boilers, they should connect after the biomass boiler/thermal store with a ‘series injection’ connection (as described in AM15 section 7.4) to ensure the highest possible biomass utilisation. The biomass control must meet the demands of normal operational requirements, as well as maintaining biomass system efficiency, ensuring greatest utilisation of the biomass boiler and thermal store, and reducing the use of the peak-lopping fossil-fuel boiler.

The flue arrangement for the boiler has a significant impact on system performance. It must produce adequate draught to remove, and then appropriately disperse, gaseous products of combustion and – critically – ensure that they do not enter occupied spaces. A flue that is too short can cause the induced draught fan to provide increased flow, resulting in excessive fuel burn, boiler overheating and increased nitrogen oxide (NOx) and particulate matter (PM) emissions. Boiler life is shortened and boiler efficiency reduced. Conversely, a correctly sized flue without a draught stabiliser can cause the same problems. Flues should be carefully considered as an integral part of the design process, as described in AM15 section 10.

Fuel specification and store sizing

A frequent cause of failure in a biomass boiler or fuel-feed system is the incorrect use of fuels. The exact requirements for the fuel vary between boiler manufacturers, models and fuel-feed system; however, fuel should be supplied to BS EN ISO 172255 (and, previously, BS EN 14961). Industrial boilers may be configured to run on atypical fuel types but, in this case, the manufacturer should be contacted and fuel testing would normally be required.

Pellets require moisture content of <10%, while wood chips can vary typically between 15% and 40%. On some boilers, the rated output is only achievable up to fuel moisture content of 25%, even though they are capable of burning wetter fuels. The reduction in output will be approximately 8% for every 5% increase in fuel moisture content – and this will also cause incomplete gasification and oxidation, so producing black smoke and tar accumulation. These factors should be taken into account when calculating payback. Higher moisture content wood chip can also form a ‘bridge’ around augers rather than feed into them, starving the boiler of fuel, despite the fuel store being full.

Fuel prices are lower for bulk deliveries and often cheaper if unloading times are short. Generally, a larger fuel store – despite the higher capital expenditure – offers lower lifetime costs. Larger fuel stores also allow increased flexibility with delivery intervals. Fuel stores that allow for quick tipping of deliveries – such as underground stores,compared with auger-based feeds – mean reduced standing time for the delivery driver, so lower fuel costs for the client. Vertical augers may also require a significant power supply that can greatly increase electrical consumption, so adding to the cost of fuel. The consequence of differing delivery regimes can significantly alter the LCC of a biomass installation. For example, a bulk transport truck consistently delivering full (seven tonne) loads of wood pellets to customers can be less than 40% of the delivery cost for that same truck delivering small loads.6

Many boilers can operate on either wood chip or pellet, though they are likely to require recommissioning if the fuel changes by more than 5% compared with what was originally used. By selecting a boiler and fuel-feed system that can operate on a range of fuels, the operator has a degree of protection against price rises and is able to switch between fuel types to maintain low operating costs.

For smaller systems, fuel levels may be monitored visually, but larger systems can have automated feedback-using sensors, weighing pads or other similar devices. This often triggers an alert – in the form of an email or text message – direct to the fuel supplier when the level is sufficiently low to require re-ordering of fuel. So the level is maintained with little intervention by the end user and there is a low chance of the boiler running out of fuel.

Hydraulic design

The hydraulic design is a major factor ensuring a high level of system efficiency and fuel usage. The design should take into account the specific requirements from the manufacturer, as well as relevant industry standards. When integrating additional boilers with biomass, issues can often occur if the gas boiler comes into operation too early. It is typical to see a comparatively small output biomass boiler installed alongside a much larger gas one (as in the system in Figure 1). If the hydraulic design and pump and piping layout is not properly considered, the smaller boiler may not be able to deliver its full potential of heat to the system.

Figure 1: This biomass boiler and the modular gas boilers are supplying 180 residential homes (Image source: Rural Energy)

Care should also be taken to arrange the pipework to reduce the amount of turbulence in the thermal store and allow it to stratify effectively. The best type of store piping arrangement – such as two-port, three-port or four-port – can vary from project to project. CIBSE AM15 provides examples of preferred arrangements to utilise the biomass boiler fully.

Specifying the correct equipment can greatly affect the operational costs of the system. Components should be selected not only to be safe and fit for purpose, but also to reduce associated electrical loads. Pipework and pumps should be correctly sized, with the pumps appropriately controlled. All pipes and stores must be insulated to a suitable standard – the thermal store will require more insulation than a normal calorifier, as the standing losses need to be very low to ensure that hot water maintains a sufficiently high temperature to remain useable for long periods. Ideally, plate heat exchangers should be selected to have a low internal pressure drop; a typically used value is less than 35kPa, but a lower pressure drop reduces capital expenditure on pumps, as well as lowering operations cost. Pumps are available with integral sensors to monitor the flow and return temperatures, modulating speed in response to load demands.

System Monitoring

Remote system monitoring allows the client, manufacturers and installers to inspect the boiler and system performance readily. This can be used pre-emptively to address issues and enable preventative maintenance programmes to maintain boiler availability and performance. A useful monitoring system is able to track the fuel used, as well as the heat produced by the boiler and delivered to each separate load, both allowing tracking of heat demand and identifying areas of concern. The manufacturer’s ability to access and control the boiler remotely can expedite the identification of faults and, potentially, allows faults to be corrected without having to call out a site technician. Commonly, a wired or wireless internet connection is required for the boiler house to connect the boiler and monitoring systems. Many institutions are concerned with data security and only allow limited use of this connection. The alternative is to use a system that works with a mobile phone signal – and, for areas with uncertain signal strength, devices can be used that switch between mobile telephony providers to use whichever network’s signal is strongest.

Operational considerations

Biomass boilers require weekly checks and cleaning by the operator to maintain efficiency. If there is no-one available on site, this may be through a service contract. Although there is often a temptation to place boilers in as small a place as possible – such as shipping containers – to keep initial costs down, there need to be suitable access hatches to allow cleaning to occur quickly and easily.

All financial models should take into account the cost of servicing and maintaining the biomass boiler, fuel-feed system and fuel store. Suppliers should be able to provide an idea of typical costs or replacement parts that may be required. Some boiler manufacturers also offer five, 10 or 20-year maintenance warranties on their products, which eliminate the unknown cost of call-outs and repairs for the main lifetime of the heating plant.

When specifying a boiler, not only must it perform efficiently, but it should also be easy for the end user to control and maintain. Some boilers require far more user intervention and manual cleaning than others – so unless the person who will be operating and maintaining it is prepared to undertake this work, these types of boilers must not be considered.

A well designed, operated and maintained biomass heating system can provide a high level of system availability and efficiency. It will require increased initial planning and investment, and will achieve very good comparative lifecycle costs when it is used in conjunction with the RHI. The owner and operator play a key role in the process, and must be included in the development and application of the system. Along with clear written guidance and training, they are vital for effective biomass system commissioning, handover and lifetime operation.

Figure 2: A biomass boiler installation showing the fuel feed (in the foreground), through to the integrated bins (on the left) that automatically remove the ash from the combustion chamber (Source: Rural Energy)

© Tim Dwyer, 2015.

Further reading:

For authoritative guidance on EU F-gas regulations see information sheets downloadable from Gluckman Consulting


  1. Luke S, Desk-based review of installation and performance practices of biomass boilers, Department of Energy & Climate Change (DECC), 2014.
  2. EN ISO 13790:2008 – Energy performance of buildings – Calculation of energy use for space heating and cooling, 2008.
  3. Biomass Decision Support Tool,, accessed 8 September 2015
  4. CIBSE AM15, Biomass heating, 2014.
  5. BS EN ISO 17225:2014. Solid biofuels. Fuel specifications and classes.
  6. EN 378:2008+A2:2012, Refrigerating systems and heat pumps – Safety and environmental requirements.
  7. New biomass sustainability requirements for the Renewable Heat Incentive – DECC, February 2015.
  8., accessed 22 August 2015.