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Biomass Heating Options

Biomass describes anything which grows, but most of the time biomass heating refers to the burning of wood products, for example logs, wood pellets, wood chips or briquettes. As long as something grows in the place of a harvested tree, as almost invariably does, there is virtually no adding of greenhouse gas to the atmosphere in the long run, unlike when burning oil, coal or gas (which also require much processing and transportation).

Types of Biomass heating:

Room heaters

1)      Wood burning stoves – An open log fire is made hugely more efficient if replaced by a log stove, delivering perhaps 70% of the wood’s heat energy to the room rather than 20-30% and drawing far less cold air into the building as flow up the chimney is very restricted, reducing draughts.

2)      Automated wood pellet room heaters – these are hand-filled every few days with wood pellets and controlled by timer and thermostat to warm the room as and when required. They have a visible living flame but are very quiet and can also heat radiators and/or a water tank (although they then need refilling much more frequently).

Central heating systems      

1)      Log, or ‘batch’ boilers – these are filled with very dry logs or strawbales once to three times daily to quickly heat a large tankful of water (a thermal store) which in turn supplies the radiator circuits in response to a timer/programmer and thermostats, for as long as the stored heat lasts.

2)      Automated chip or pellet boilers – these have large fuel silos within 15m of the boiler, drawing the fuel in as required and behaving otherwise like a conventional oil or gas boiler. Some can only run on pellets, often fed through flexible pipe using a suction pump, whilst others can use chips or pellets, with a heavy duty auger between silo and boiler.

What are the differences between wood pellets, wood chips and logs?

The principal pros and cons of the three fuel types are as follows:

Logs – log systems have fewer wearing parts and are simpler and cheaper to install. The downside is that handling the logs requires MUCH more labour time on a daily basis. This is OK if the house to be heated is of small-to-average size, or if you have a constant supply of free labour on site, but it is the reason why many such systems are regretted.

Chips – The advantages of chip systems are that they require much less regular intervention than log systems – they can be left alone for up to a month at a time – and that they can be supplied from local sources, making you independent of world energy markets and price fluctuations. They are ideal for landowners with commercial forestry near their homes as use of low value wood for heating can justify the cost of thinning and other beneficial forestry management operations. Chip systems suit large heat demands such as very large houses and/or groups of dwellings and/or commercial buildings which can be served from a shared boiler in a ‘District Heating Scheme’, as large fuel cost savings are necessary to justify the high cost of the fuel handling equipment.

Pellets – wood pellets are a highly processed product, very dry and dense, which flow a bit like a liquid and take up a quarter of the space of chip with the equivalent heat content. As a result the storage and handling equipment is much cheaper and more compact which makes it more suitable where heat demands are more modest and/or there is less space available for storage or for the manoeuvring of delivery vehicles. A blower delivery lorry must get to within around 15m of the pellet store, whilst a chip trailer needs to be able to tip its full load over a lip into a hopper within a few metres of the boiler house.

Is my house suitable?

When considering which biomass boiler system is most suitable for your home, the main issues to consider are: –

  • Fuel supply – The fuel supply must be reliable for the foreseeable future. Logs and chips are too bulky to be transported more than a few miles economically, whilst pellets can be shipped around the world making them a global commodity, without much effect on their bulk cost or environmental impact. Obtain quotes for fuel delivery, chipping, etc, as appropriate, and discuss delivery methods, width or height restrictions, turning space requirement and even cleaning/de-ashing services, before making any purchasing decision. Wood fuel suppliers must guarantee minimum quality standards relating to moisture content, particle size, etc.
  • Heat cost – the relative costs of the different heating fuels is estimated in the table below, compared with equivalent costs, allowing for boiler efficiency and servicing costs for a 3-bedroom detached house in Scotland, from the April ’13 Sutherland Tables.
Fuel Type Cost Per KwH Cost Per Year
Wood Pellet 5p £1,068
Wood Chip 3.5p £1,141
Logs 4.5p £976
Mains Gas 7.4p £1,356
Oil 9.0p £1,881
Night Storage Electricity 8.1p £1,748
LPG 12.5p £2,656

How much does it cost?

The cost of a professional biomass boiler installation is dependent on the property, fuel choice and location and ranges from about £600 to £1,200 per kW of peak heat output, excluding the cost of the heat distribution system (eg. underfloor heating).

The installed cost of a typical 40kW system, for example, would be in the region of £36,000 plus the cost of the distribution system. The price per kW gets lower as systems get larger.

What are the advantages?

By harnessing a sustainable resource, a biomass boiler has much lower CO2 emissions. A good system causes at least 90% less CO2 emission than an equivalent oil boiler.

A well-designed system in an energy-efficient dwelling will cost less to run than any other at current fuel prices. An approximate running cost comparison per kWh and per year for a typical 3 bed detached house in Scotland are shown in the table above. (Note that, as explained above, a chip boiler would not be appropriate for such a building on its own due to high fixed capital costs for small installations).

Using a local heating fuel source can insulate you from the sometimes wild fluctuations in heating costs experienced by oil and gas systems, caused by political and other influences on global markets.

Maximising heat efficiency with wood briquettes

Verdo Wood Briquettes and UK Heatlogs.

They burn hotter and cleaner, are cheaper to buy, and much easier to store and handle – so why do so few people with open fires and wood-burning stoves use recycled wood briquettes to heat their home?
Big in Europe, but still largely untried by many fire users in the UK – particularly in the south – those selling them claim that once you have tried briquettes, you’ll never go back to hauling piles of logs off your drive.
Briquettes deliver around 50% more heat for each pound spent than logs. They also have strong environmental credentials as they are made from waste wood produced as part of the furniture or other wood-related businesses – or in some cases collected from skips. Burning wood is generally considered a low-carbon alternative to fossil fuel because trees absorb carbon as they grow.
Briquette producers take sawdust and other shredded wood, apply 10,000psi (pounds per square inch) of pressure, and out pop briquettes. One briquette can last up to four hours, and big users will find they save around £150 a year compared to buying conventional logs.
Wood briquettes are cleaner and easier to store, and take up around half the space of a log pile. Until you have tried one you won’t believe how much heat they can deliver.
Logs sold in the UK generally have much higher levels of moisture – 20% in well-seasoned wood.
You have to keep them in a dry place as they expand rapidly if they get wet. When they burn they leave around 1% of their original volume as ash, meaning you don’t have to empty the stove as often.

Which wood burns best?

Wood is the natural sustainable choice of fuel for domestic fires – in use since the first fire many millennia ago. When we warm our homes with wood, we participate in a natural cycle and an ongoing continuum of activity that we share with ancient ancestors.

I am amazed at the number of country people who don’t have fires, whether open log fires or woodburning stoves, because they are “too dirty” or “too much work”.

In fact the procedure of building and lighting the fire is one of my favourite jobs of the day, and I love handling and preparing the firewood. I am not one to pursue a life where all comfort comes from the flick of a switch.

The ability to burn wood for heat in your home gives you more freedom and options for fuel. You are no longer dependent on large energy utilities who may or may not be able to supply power and fuel

Environmental Issues

When we burn wood we are releasing solar energy, in the form of heat that has been stored in the wood as chemical energy. The process of photosynthesis converted solar energy, water and carbon dioxide into oxygen and the organic molecules that form the wood, half the weight of which is carbon.

So burning wood is just the quick reversal of this process, liberating the sun’s heat when we need it most.

Unlike the burning of fossil fuels like coal, gas or oil, burning firewood releases no more greenhouse gases (carbon dioxide) than would be produced were the wood to simply rot on the forest floor. If we are responsible in the ways we grow, cut, and burn our firewood, wood burning can actually be a good choice for the environment.

So burning wood is a good choice from the greenhouse gas point of view but what about other pollution – surely all that smoke can’t be good?

Smouldering, smoky fires that produce a plume of blue-grey smoke from the chimney are the main cause of wood heat-related air pollution. Smoke is made up of many tiny airborne particles and wood smoke can be harmful when it is inhaled. In some countries wood smoke has become a major air pollution problem and this has led to both local regulations and more efficient wood-burning appliances.

One thing to make clear at this stage is that if you are burning the right wood in the right way then there shouldn’t be much smoke. As you probably know from bonfires, a slow, wet fire produces lots of thick smoke – in the fireplace we are aiming for a quick, hot, dry burn producing very little smoke. Another thing to bear in mind is that a smoky fire is an inefficient one – we want all the released energy to heat our home – not to go up the chimney in the form of complex particles. Carbon dioxide, the product of a clean, hot burn, is a colourless non-particulate gas, so a hot fire with minimal smoke is an efficient energy-converter with less pollution.



Alder Produces poor heat output and it does not last well. Poor
Apple A very good wood that bums slow and steady when dry, it has small flame size, and does not produce sparking or spitting. Good
Ash Reckoned by many to be one of best woods for burning, it produces a steady flame and good heat output. It can be burnt when green but like all woods, it burns best when dry. Very good
Beech Burns very much like ash, but does not burn well when green. Very good
Birch Produces good heat output but it does burn quickly. It can be burnt unseasoned, however the sap can cause deposits to form in the flue with prolonged use. Good
Cedar Is a good burning wood that produces a consistent and long heat output. It burns with a small flame, but does tend to crackle and spit and the sap can cause deposits to form in the flue with prolonged use. Good
Cherry Is a slow to burn wood that produces a good heat output. Cherry needs to be seasoned well. Good
Chestnut A poor burning wood that produces a small flame and poor heat output. Poor
Firs (Douglas etc) A poor burning wood that produces a small flame and poor heat output and the sap can cause deposits to form in the flue with prolonged use. Poor
Elm Is a wood that can follow several burn patterns because of high moisture content, it should be dried for two years for best results. Elm is slow to get going and it may be necessary to use a better burning wood to start it off. Splitting of logs should be done early. Medium
Eucalyptus Is a fast burning wood. The sap can cause deposits to form in the flue and can increase the risk of a chimney fire if burned unseasoned. Poor
Hawthorn Is a good traditional firewood that has a slow burn with good heat output. Very good
Hazel Is a good but fast burning wood and produces best results when allowed to season. Good
Holly Is a fast burning wood that produces good flame but poor heat output. Holly will burn green, but best dried for a minimum of a year. Poor
Hornbeam A good burning wood that burns similar to beech, slow burn with a good heat output. Good
Horse Chestnut A good wood for burning in wood stoves but not for open fires as it does tend to spit a lot.  It does however produce a good flame and heat output. Good (For stoves only)
Laburnum A very smokey wood with a poor burn. Poor do not use
Larch Produces a reasonable heat output, but it needs to be well seasoned. The sap can cause deposits to form in the flue with prolonged use. Medium
Laurel Burns with a good flame but only reasonable heat output. It needs to be well seasoned. Medium
Lilac Its smaller branches are good to use as kindling, the wood itself burns well with a good flame. Good
Maple Is a good burning wood that produces good flame and heat output. Good
Oak Because of its density, oak produces a small flame and very slow burn, it is best when seasoned for a minimum of two years as it is a wood that requires time to season well. Good
Pear Burns well with good heat output, however it does need to be seasoned well. Good
Pine (Including Leylandii) Burns with a good flame, but the resin sap can cause deposits to form in the flue and can increase the risk of a chimney fire must be well seasoned. Good (with caution)
Plum A good burning wood that produces good heat output. Good
Poplar A very smokey wood with a poor burn. Very poor
Rowan Is a good burning wood that has a slow burn with good heat output. Very good
Robinia (Acacia) Is a good burning wood that has a slow burn with good heat output. It does produce an acrid and dense smoke but this is of course not a problem in a stove. Good  (For Stoves only)
Spruce Produces a poor heat output and it does not last well. Poor
Sycamore Produces a good flame, but with only moderate heat output. Should only be used well-seasoned. Medium
Sweet Chestnut The wood burns ok when well-seasoned but it does tend to spit a lot. This is of course not a problem in a stove. Medium (For Stoves only)
Thorn Is one of the best woods for burning. It produces a steady flame and very good heat output, and produces very little smoke. Very good
Willow A poor fire wood that does not burn well even when seasoned. Poor
Yew A good burning wood as it has a slow burn, and produces a very good heat output. Very good



Carnethy Woodfuel – Tree Facts and Figures

Trees are the longest living organisms on the planet and one of the earth’s greatest natural resources. They keep our air supply clean, reduce noise pollution, improve water quality, help prevent erosion, provide food and building materials, create shade, and help make our landscapes look beautiful.

Here are some more thought-provoking facts and figures about our oldest citizens and living treasures…trees!

  1. Trees receive an estimated 90% of their nutrition from the atmosphere and only 10% from the soil.
  2. Trees grow from the top, not from the bottom as is commonly believed. A branch’s location on a tree will only move up the trunk a few inches in 1000 years.
  3. No tree dies of old age. They are generally killed by insects, disease or by people. California Bristlecone Pines and Giant Sequoias are regarded as the oldest trees and have been known to live 4,000 to 5,000 years.
  4. Mature trees can add 10% to your property value.
  5. Some trees can “talk” to each other. When willows are attacked by webworms and caterpillars, they emit a chemical that alerts nearby willow of the danger. The neighboring trees then respond by pumping more tannin into their leaves making it difficult for the insects to digest the leaves.
  6. Knocking on wood for good luck originated from primitive tree worship when rapping on trees was believed to summon protective spirits in the trees.
  7. Trees can induce rainfall by cooling the land and transpiring water into the sky from their leaves. An acre of maple trees can put as much as 20,000 gallons of water into the air each day.
  8. Trees properly placed around buildings can reduce air conditioning needs by 30%.
  9. A mature tree removes almost 70 times more pollution than a newly planted tree.
  10. Each average-sized tree provides an estimated £4 savings in annual environmental benefits, including energy conservation and reduced pollution.
  11. A single tree produces approximately 260 pounds of oxygen per year. That means two mature trees can supply enough oxygen annually to support a family of four!
  12. One tree can absorb as much carbon in a year as a car produces while driving 26,000 miles.
  13. Over the course its life, a single tree can absorb one ton of carbon dioxide.

Carnethy Woodfuel primary hardwood species

All our hardwood logs are sourced locally from Edinburgh, the Lothians and Borders and are seasoned for at least 12 months and sold with an average moisture content of 22%. We have chosen to sell hardwood logs only based on their excellent burning efficiency and heat output.

Our supply of wood has been sourced responsibly and is obtained primarily from dead or dangerous trees that require felling.

Hardwoods are generally better for burning than softwoods. As a rule of thumb hardwoods are produced by slow-growing deciduous trees and therefore the logs have a greater density than the faster growing softwoods from evergreen trees.

In summary, hardwoods tend to have broad leaves, while softwoods tend to have needles and cones.

As a rule of thumb, wood which is well seasoned makes a distinctive ‘clack’ rather than a dull ‘thud’ when knocked together. It will also feel much lighter than an unseasoned log.

Other indicators of a seasoned log include the bark peeling away and cracking and splitting of the wood around the outside. It should be stacked off the ground with plenty of space between the logs to allow air movement and with the top covered to keep rain and snow out.

Seasoned wood should give you approximately 50% more heat than the equivalent unseasoned log.

Our hardwood logs that make up our bulk bags generally contain the following mixed species:

1. Beech

This makes a great well burning log although it does contain a high water content and can take longer to season than some other log varieties. Beech rarely throws sparks and makes good embers. Beech trees grow in a variety of different soil types and can grow up to 100 feet tall.  A mature beech tree develops a huge canopy which carpets the forest in a dense shade making it difficult for other seedlings to grow.

2. Birch

These logs burn fairly quickly but provide a good heat output, bright lively flames and a pleasant smell. Birch are usually small to medium sized trees that grow in lowland areas and have shallow root systems. These trees are probably best known for their unique bark

3. Sycamore

This common European tree makes a great wood burning log with a moderate heat output and good flame.

4. Oak

Generally considered one of the best hardwood logs which burns slowly and produces an excellent heat. The oak species consists of about 600 different types and can live for 200 years and grow as tall as 100 feet. The fruit of the oak tree is called the acorn. The trees start producing acorns when they are 20 years old. By the time the tree reaches 70-80 years old, the tree will be producing thousands of acorns

5. Rowan.

This tree produces lovely red berries in the Autumn producing slow well burning logs.

6. Ash.

Widely regarded as the most efficient burning wood type, with low smoke and an excellent flame pattern providing plenty of heat and little residue. A freshly cut piece of ash has a moisture content only slightly higher than seasoned ash.  This allows the ash tree to be safely used immediately after harvesting.

Never use wet or unseasoned (green) wood as this will cause a lot of smoke and a very disappointing fire. It could quickly result in the build up of soot and creosote which could easily cause a chimney fire.

Burning wet or unseasoned wood will also reduce the effectiveness of a wood burning stove and very likely to result in a staining and blackening of the glass.

Changes affecting the Renewable Heating Incentive (RHI)

New biomass sustainability requirements

  • The UK government will introduce new sustainability criteria for installations using biomass fuels under the Renewable Heat Incentive on 5 October 2015.
  • From 5 October 2015 biomass fuel used by RHI participants must be sourced from a supplier on the Biomass Suppliers List – at the time the fuel was purchased. This is a list of suppliers of sustainable biomass fuel.
  • It is advisable to check whether your fuel supplier is registered on the Biomass Suppliers List before entering into any long term supply contract.
  • Not all fuels supplied by suppliers on the BSL are sustainable as a supplier may supply more than one type of fuel. You should  check with your supplier, or prospective supplier, which of their fuels are registered

What could I earn using RHI?

RHI cash payments are made quarterly over seven years. The amount you receive will depend on a number of factors – including the technology you install, the latest tariffs available for each technology and – in some cases – metering.

What technologies can I claim RHI support for?

  • Biomass (wood fuelled) boilers
  • Biomass pellet stoves with integrated boilers providing space heating
  • Ground to water heat pumps
  • Air to water heat pumps
  • Solar thermal panels (flat plate or evacuated tube only) providing hot water for your home

Air to air heat pumps, all log stoves, pellet stoves without back boilers and hybrid PVT are not supported by RHI.

Water source heat pumps  can potentially be eligible for the Domestic RHI – they are included in the definition of a ground source heat pump.

Certain cooker stoves and certain high temperature heat pumps may also be eligible.

All systems must also be listed on the Domestic Renewable Heat Incentive Product Eligibility List –

Who can apply for RHI?

  • Owner-occupiers, self-builders, private landlords and registered providers of Social Housing who have installed an eligible technology can apply for RHI support (provided they meet eligibility criteria).
  • Single domestic dwellings are covered.
  • RHI support is not available to new build properties (other than self-build projects).

You must apply within one year of the commissioning date of your system.

How do I apply for RHI?

You can apply for RHI via Ofgem’s website . Providing you have all the relevant information to hand and your application does not require a manual review, you should receive an immediate decision. To check if your application will need manual review, visit

If you are unable to apply online then you can contact Ofgem via their Domestic RHI Applicant Support Centre on 0300 003 0744 Monday to Friday from 09:00 to 17:00.

To apply you will need

  • MCS installation certificate number for the heating system
  • Energy Performance Certificate (EPC) number
  • Green Deal advice report number (unless you are a self-builder or a social landlord)
  • Bank details


The table below summarises the latest tariffs available for each technology:

Air source heat pumpBiomassGround source heatpumpSolar thermal
Tariff (p/k Wh renewable heat) (Applications submitted between 1 Oct. 2015 and 31 Dec. 2015 incl.)7.426.4319.1019.51
  • These tariffs have been set by the UK Government at a level designed to compensate for the difference between costs of installing and operating renewable heating systems and fossil fuel systems, including non- financial costs such as disruption, on the basis of 20 years of heat produced. Fossil fuel costs used are those for off-gas households.
  • Ofgem will make payments quarterly for seven years. Normally the heat required to heat the property will be deemed (estimated) and payments will be based on this amount.
  • Biomass –  renewable heat generated by biomass will be based on an estimated heat demand from an EPC
  • Heat pumps –  renewable heat generated by heat pumps will be based on an estimate of the heat demand from an EPC combined with an estimate of the heat pump’s efficiency
  • Solar thermal systems – renewable heat generated by solar thermal systems will be based on the estimate of system performance completed as part of a Microgeneration Certification Scheme (MCS) installation.

Elements affecting payment

  • Once you are receiving domestic Domestic RHI payments, the rate you get will change annually in accordance with the Retail Price Index (RPI).
  • The RHI scheme uses a ‘degression’ system designed to manage the scheme budget available for the domestic RHI. From time to time, the tariff for a technology will be reduced (for new applicants only) if the total amount being claimed in total for that technology reaches a certain level. Anyone who is already claiming domestic RHI will not have their tariffs reduced through degression.
  • If you have already received a grant from Government or public funds (such as the Renewable Heat Premium Payment), then the amount received will be taken into account when calculating your domestic RHI payments.