What is Biomass?

Biomass is biological fuel, burnt to produce energy. Usually we mean wood, but it could include straw, miscanthus, oil-seed rape pellet, etc. Wood is most commonly in the form of pellets, chips, logs or briquettes.

The first and most important decision to make is the form of fuel to be used. At the domestic scale we can eliminate wood chip as it has handling, transport and storage problems that are only successfully dealt with on a large scale.

Briquettes are essentially the same as wood pellets but are the size of a small log (a pellet is 6mm in diameter and 10-25mm long — the diameter governed by quality standard regulations) and will burn in the same boiler as a log.

In terms of living with a biomass boiler, the decision will revolve around manual or automatic operation. A log or briquette boiler needs loading manually at least once a day and often more, and ash will need to be emptied weekly. A wood pellet boiler can be automated — the only manual intervention being emptying ash once a month or so.

The Options

Stoves: Room heaters that use logs, briquettes or pellets.

Stove boilers: Essentially a stove but with a back boiler to heat water for central heating and domestic hot water. Bear in mind that you need hot water year-round; will the stove be running in the summer? Pellets tend to be a good fuel for these stoves as the output is more controllable than logs.

Boilers: Operate in largely the same way as an oil-fired boiler, in that the fuel hopper is loaded and the boiler takes over from there.

How Much Will it Cost to Buy and Install?

The cost of biomass boilers varies far more than oil or gas boilers. The cost will also vary with the type of boiler, the level of automation and sophistication, as well as with size and quality.

A standard log-burning stove costs around £500 and a stove boiler will cost £3,000. If you are after a biomass boiler, a log boiler costs around £5,000 and a wood pellet boiler £15,000. However, fully automated wood pellet boilers with an automated fuel storage and delivery system could cost £25,000.

Installation can be expensive too for biomass boilers — up to another £5,000–£10,000 on some systems.

What Will I be Burning?

Biomass fuel comes in four common forms:


They are made from forestry waste and look more like flakes, variable in size. They are used in automatic loading applications and typically where there is a very high heat load — the reasons being that chips are about 1.1p/kWh cheaper than pellets but also good for automated systems. Boilers suitable for chips are, however, bigger and more expensive than wood pellet and log boilers, and thus generally not used in residential settings.


Wood pellets are available bagged – typically 10kg, 12kg, or 15kg – or by the pallet load, delivered to the road outside your house, or in bulk delivered directly to your fuel store.


An industry has grown up in the last few years supplying logs. They are usually, but not always, kiln dried.


Effectively a manufactured log, made in largely the same way as a pellet, but bigger. They are as large as a log and used in the same applications, but provide a higher heat output per kilogram.

Tip: With all biomass fuel, quality is the key and the issues to look for are moisture content, bulk density and ash. In all cases, the lower the better.

What About a Mixed System?

Biomass could work with another heat source. Solar thermal and air-source heat pumps are both relatively cheap and both work best in spring and summer. A pellet stove boiler could take over main duties during the rest of the year.

Using a wood pellet stove boiler with solar thermal back-up to supply underfloor heating will achieve low running costs.

How do I Find Fuel?

An online search on wood pellets, logs or briquettes will throw up hundreds of potential suppliers. A drive around your own locality is likely to find a few roadside firewood advertising boards, typically at farm gates.

So far as wood pellet is concerned, always looks for ENplus accreditation as this indicates quality and Biomass Supplier List (BSL) accreditation so that you can qualify for RHI. Other issues are:


Moisture content (MC) is the key issue with all biomass fuel. Some kiln-dried log suppliers advertise as having 20% to 30% MC. That is what could be called poor quality as almost a third of the log you are putting in the stove is water. Logs need to be below 20% and pellets below 10%. Good quality manufacturers, like Verdo Renewable, get MC down to 8%.

Beyond MC ash, dust, density and durability are all quality measures for pellets and briquettes. Ash content is key and Verdo claim to have this down to 0.7%.


Not only do you need reliable quality in a product, but also consistent availability. The reality is that we always need a delivery of fuel within a few days of noticing we need it. The supply needs to meet that requirement.

Biomass Supplier List

A list maintained by Ofgem of suppliers that manufacture or supply wood from a sustainable source. To claim RHI, it is essential that your supplier is on this list.

What Will it Cost to Run and What About the RHI?

  • Briquettes cost around £280/tonne, providing 4,500kWh. This averages our at a cost of 6.2p/kWh.
  • Pellets (bagged or bulk) will cost £220/tonne. Providing 4,800kWh/tonne, they average a cost of 4.6p/kWh.
  • Logs vary but you will be paying between £200–£250/tonne for 20% moisture content (MC) logs. They will provide 3,500kWh/tonne, giving an average cost of 7.14p/kWh.
  • Chips with a 20% MC cost around £150/tonne. Providing 4,100kWh/tonne this works out at an attractive cost of 3.66p/kWh.
  • For comparison, natural gas is now about 6.1p/kWh and 10p/kWh for heating oil. 

The difference comes with the Renewable Heat Incentive. The domestic scheme started in April 2014 and the rate was 12.2p/kWh. However, rates have been halved and as of October 2015 the rate is 6.43p/kWh.

Assuming a 200m² house is built to current Building Regulations standards, the deemed space heating load is likely to be some 11,000kWh plus, say, 4,000kWh for hot water. A total of 15,000kWh will cost around £645 per year for a wood pellet boiler (assuming 93% efficiency) and will return some £964.50 in RHI payments every year, tax free and index linked, for seven years.

How Much Space do I Need?

The boiler itself will be about the size of a four-drawer filing cabinet, but it is fuel storage that will be the big issue. Wood pellet boilers will fit in most utility rooms, but a log boiler is a fairly messy beast and you will probably want to house it within a dedicated plant room instead.

In either case, the fuel store needs to be close by to facilitate easy loading.

  • Wood pellets are available in bags or loose in bulk (cheaper).
  • Bags are generally sold in pallet loads of 960kg which is broadly 1m³; bulk deliveries will need at least 8m³ (a 2x2x2m cube).
  • A bulk store needs to be reasonably close to the road. Bulk deliveries will be made by lorry and the wood pellets blown into the fuel store.

The wood pellet delivery system from the fuel store to the boiler will be either vacuum or a screw-type. The store can be supplied by the boiler manufacturer or it can be made bespoke. An option where space is tight is for a below-ground store and an ‘onion’ septic tank can be pressed into service for this.

biomass fuel store and boiler set up diagram

Viessmann’s suggested fuel store and boiler set-up

What About Maintenance?

Maintenance will vary with the level of sophistication and is an issue that needs to be directed to the supplier and clearly understood for any particular machine.

In broad terms, boilers will need an annual maintenance visit from the installer but, otherwise, log boilers are likely to need regular (perhaps weekly) cleaning and de-ashing, while with wood pellet boilers that could be at three monthly intervals. Each machine does have very individual requirements.

Biomass: My Experience

Editor Jason Orme installed a biomass boiler in summer 2015. He reveals his key learnings:

  • Think carefully about the positioning of the boiler if you will be using bagged pellets. At 15kg they are quite light but you want to store them next to the boiler, and that’s not practical in most utility rooms.
  • Choose one with a big hopper (200kg), otherwise you’ll be filling it all the time. I intend to upgrade to an automated feed but tipping six to seven bags in once a fortnight is hardly onerous.
  • Quality is key. We went for a Mercedes rather than a Dacia, using local company Eulinx to install the system, based around a Windhager BioWIN 2. Total cost was around £20,000.

How do I Choose A Boiler?

Reputation: How long has the manufacturer been making boilers, and how many have they installed in the UK?

Controls: Does the control system address the whole house or individual zones? Does it have remote monitoring and can it be controlled from a PC or App?

Automation: How much work will the user have to do to keep it working.

Burner control: The quality of the burner determines the efficiency of the boiler. Good boilers automatically control the air flow over the burner, ensuring it always operates at optimum efficiency and allowing the heat output to match the demand (called modulation).

Efficiency: Don’t choose a boiler with less than 90%.

Fuel loading: All but fully automated systems need manual loading with fuel. The size of the hopper or combustion chamber will determine how often the owner has to load it.

Maintenance: How much work does the user have to do in terms of cleaning — some are self-cleaning, some partially so and some entirely manual. All boilers need servicing but it should cost no more than a gas or oil boiler.

Warranties: Good boilers will have at least a two-year warranty (parts and labour) and some offer up to five years.

Compliance: Microgeneration Certification Scheme (MCS) accreditation is necessary for the RHI, but actually does not tell us how good the machine is.

Finding a Good Biomass Installer

A good installer will have the following attributes:

  1. Be a plumber. It’s a boiler so it needs a plumber to install it.
  2. Be HETAS registered — just as a gas boiler installer needs to be Gas Safe registered and an oil boiler installer needs to be OFTEC registered.
  3. Work within an acceptable budget range. The price of the boilers varies with the manufacturer (amongst other things) and a good installer will work with equipment in your budget range.
  4. Have familiarity with equipment from more than one manufacturer. This is necessary to ensure they have a breadth of knowledge.
  5. Have a list of reference sites. Speak to them and be sure the installer really did do a good job and is not just making up names and addresses.
  6. Finally, the installer should not intimidate or be dictatorial. You need to be able to discuss your needs and understand the answers.

Do I Need a Hot Water Cylinder?

Hetas regulations (hetas.co.uk — a good place to search for installers) recommend that a log boiler has a hot water storage capacity of at least 50 litres per kW output. So a 20kW log boiler will need a 1,000litre cylinder. This is because once a log boiler is loaded with logs, it cannot easily be shut down.A wood pellet boiler burns (broadly) one pellet at a time so it does not need a large cylinder. A normal domestic cylinder is usually sufficient in most instances, but it is wise to take advice on your requirements from an installer.

What About a Back-up in Case it Fails?

There is no more chance of a biomass boiler failing than there is of any other boiler. In fact, a biomass boiler will retain its efficiency (more than 90%) for maybe twice as long as a gas or oil boiler. One solution that many people like to consider, however, is to retain their existing oil or LPG set-up as a back-up.

Tim’s View

“Like all renewable energy systems, biomass is a good idea in the right place. And from a financial view, the right place is probably not the average modern house. In that case, the net RHI receipts are unlikely to meet the capital cost of a biomass boiler. A 500m² or 600m² barn conversion is a different proposition however — where the higher heat demand is better suited to biomass and the capital cost is more readily offset with RHI.”

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