The last few years have been a bit of a rollercoaster ride for biomass. There’s been a huge increase in the number of woodburning stoves, a bit of an increase in boiler sales, followed by a downturn as the renewable heat incentive (RHI) tariff tumbled.

However, biomass seems to tick most boxes. The fuel is cheap, sustainable and readily available, and the technology is robust with a long life.

What is Biomass?

Biomass is any organic material that can be used as fuel. It includes rapeseed pellet, straw and miscanthus, but is generally taken to mean wood. It’s considered a carbon-neutral fuel, but in reality CO2 is emitted in its transportation, and in some cases production.

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 manufactured from compressed wood dust, and the fuel has a heat output of 5,000kW/tonne, compared to 2,000-3,000kW/tonne for logs and chips. Generally pellets have a moisture content of around 8%. They 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 is a Biomass Boiler?

Log and wood pellet boilers are the only real options for domestic projects.They 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.

Pellet boilers are the most common as they offer higher levels of convenience, close to that of an oil-fired boiler.

  • Cost will vary from around £5,000 for a fairly basic machine with manual fuel loading to £20,000 for a sophisticated machine with automated fuel delivery
  • Like oil-fired boilers an annual service is required, but the only other maintenance is ash removal — usually a monthly activity that provides good nutrients for the garden

There are a few types of log boiler available, but look for gasifying batch boilers.They operate at much higher temperatures, gasifying the logs, increasing efficiency to over 90%.

  • Prices vary from £5,000 to £20,000 plus installation
  • As logs are loaded in batches the boiler only tends to require loading once a day, or perhaps on alternate days. Ash will need to be emptied weekly
  • The heat output of a log boiler and heat demand of the house is balanced around the size of the hot water cylinder, which is generally much bigger than for a gas or oil system. Ideally a single ‘burn’ of a batch of logs is sufficient to heat the hot water cylinder and that in turn is sufficient to meet the demands of the house until the next day’s burn

What is a Biomass Stove?

Biomass stoves are room heaters that use logs, briquettes or pellets.

Stoves with back boilers can also be specified to meet central heating and domestic hot water demand. Prices can reach £5,000 for a fairly sophisticated pellet stove with back boiler.

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.

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.

How and Where Do I Buy 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.

How Much Space Do I Need for a Biomass Boiler?

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

How Do I Choose the Right Boiler?

Consider the following:

  • 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% efficiency.
  • 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 are partially so and some are 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.

Choosing between a stove and a boiler is relatively straightforward. In situations with a lowish heat demand, a wood pellet stove boiler can be a good option but otherwise a stove has to be considered as highlight heating.

If a boiler is needed then choose the preferred fuel first. Economically there is little justification for a log boiler, unless you have an abundance of free or low-cost fuel available. 

Could I Choose 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.

Biomass Boiler Maintenance Requirements

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.

Do I Need a Hot Water Cylinder?

Hetas regulations ( — 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.

Renewable Heat Incentive

Biomass boilers qualify for the renewable heat incentive (RHI), the government scheme whereby those with renewable heating technologies are paid back for the heat they generate. Wood pellet fuelled stoves and stove-boilers also qualify.

Log burning stoves, though, do not qualify.

The current RHI tariff is 6.74p/kWh. If we assume a heating demand of 20,000kWh per year – about the minimum for a biomass boiler – the running cost would be around £1,200 per year with an RHI return of around £1,350 per year. This is a ‘profit’ of around £150 per year for seven years.

But gaining that ‘profit’ will require an investment in excess of £10,000 for the boiler. A wood pellet stove-boiler could be as little as £2,000 and would return the same ‘profit’.

An oil boiler, by comparison, might cost £5,000 including an oil storage tank, and will cost around £1,100 per year to run (for now) — obviously with no RHI return.

Is Biomass Right for my Home?

  • The economics around domestic-scale biomass boilers has changed a lot in recent years. The price of wood pellets is now about the same as heating oil, higher than natural gas, and the capital cost is still very high
  • The domestic RHI tariff has nose-dived, but it is good, reliable technology that offers net-zero CO2 emissions and fuel price rises that are usually much lower than fossil fuels
  • In big, hard-to-heat properties biomass offers a good option but in more typical new builds or conversions, other options are more economical

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

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