A Guide to Using Wood as a Fuel Source

Fireplace.co.uk Advice Sheet A guide to using wood as a fuel source

Although, this article principally focuses upon the use of logs as a domestic fuel source it will hopefully be rewarding, and interesting to the reader to first outline the environmental benefits of choosing wood as a fuel, before moving on to introduce the different forms of wood products, which are available as an alternative to logs as a sustainable, renewable, low or carbon neutral fuel also see The Environmental and Sustainable use of Timber

Wood is a far more environmentally friendly fuel than the fuel alternatives of oil, coal, or gas but needs to burnt in an efficient wood burning stove and the fuel harvested from a sustainable source to provide a carbon neutral way of heating you home  

A tree respiratory process absorbs carbon monoxide from the environment. Once the tree is felled and used as a wood or log fuel the carbon dioxide is released back into the atmosphere.  If wood is burnt efficiently on an efficient wood burning stove less carbon dioxide is released back into the atmosphere than if the tree was simply left to decay and rot.

Unlike the alternative fuels of gas, oil and coal fuel wood can be much more readily and quickly replaced by the planting of new trees. It is environmentally responsible and we at fireplace.co.uk would argue a duty to ensure your wood supply is sourced from a reputable sustainable source and carries the Woodsure accreditation or similar mark guaranteeing that the wood is harvested from a sustainable supply. This will help ensure the carbon neutrality of your wood fuel and ensure its supply for the next generation.    

We can not stress enough that wood needs to be burnt efficiently, in an efficient stove and that the wood must have a moisture content of no greater than 20% and therefore seasoned and stored correctly. For more details on using wood as a fuel see fireplace.co.uk knowledge hub theme using wood as a fuel.

 

Abbey with stove using wood as a fuel source

The more technologically minded amongst us might argue that a log fuelled open fire, or log burning stove does not posses the immediate glamour of glistening solar panels, whirling wind turbines, or geothermal ground source heat pumps. In counterbalance, the more romantically inclined may suggest the use of wood, as a fuel to heat our homes is equally, if not more, seductive bathing us and our homes in a warm glow. In choosing an open fire, or log burner as the primary or secondary source of heat for our homes we are recapturing the traditions, and following in the footsteps of our ancestors, and they knew a thing or two about keeping warm having survived the ice age in little more than a furry hearth rug, a spear, a stone cave and a fire ( to find out more about the history and humankinds relationship with fire click the link ).  The use of wood as a fuel is older than civilisation itself and was only limited by our earliest of ancestors’ ability to generate a spark. It is merely over the past two hundred years or so, since the onset of the industrial revolution, that wood has been replaced as the traditional domestic fuel by the fossil fuels of coal, oil, and gas.  The year on year increasing cost of gas and oil, our growing awareness of the earth’s finite resources, and the environmental damage caused by the burning of fossil fuels, has re-ignited an interest in wood as a sustainable, renewable, low or carbon neutral alternative.  

The laws of physics state Energy cannot be created or destroyed; it can only be changed from one form to another.  When we burn wood we are in effect releasing the sun’s energy stored in the wood as heat. The process of photosynthesis converts the sun’s energy, alongside water and carbon dioxide into oxygen and the organic molecules which make up the tree, 50% of which by weight is carbon in the form of carbohydrates of one type or another.  When wood is burnt the energy is released in the form of heat and the carbon oxidises and is released back into the atmosphere as carbon dioxide .  As long as wood fuel is harvested from sustainable woodland, where replacement trees are planted or existing trees are pollarded, the carbon dioxide released back into the atmosphere by the burnt fuel is reabsorbed by the new growth or the replacement tree. The carbon dioxide is reabsorbed via the trees respiration process and oxygen released back into the atmosphere.  This creates a neutrally closed carbon cycle.  Furthermore, if the tree or wood is left to decompose on the ground it would still release similar amounts of carbon dioxide into the atmosphere as if it is burnt.

In the context of fuels within the UK domestic market the term biomass fuels predominantly refers to wood in one form or another.  Wood as a fuel is available in the following forms.

Wood Chip: A lower grade fuel consisting of literally chipped wood typically being less than 45mm in size. Wood chip by its very nature is a low energy density fuel (See Fig 1: Colum 2) and often has a moisture content of approximately 30%.  Special stoves are required to burn wood chip as a fuel.

Wood briquettes: These are usually made from the waste from wood mills and other wood processing plants. The wood waste is forced at extremely high pressure through a die or mould.  The pressure causes the lignin in the sawdust to soften and bind together producing a round briquette of between 50-75 mm diameter and 130-150mm in length.  Briquettes are available in two forms one with a hole through the middle, and as a solid block. The briquettes with a hole in the middle are made using a screw press, the hole being where the screw has passed through the material. Solid briquettes are manufactured using a piston press to squeeze the sawdust together. The briquettes with the hole running through the middle have the advantage that the cavity increases the surface area of the burn. Wood briquettes can be used in conventional stoves or open fires as an alternative to logs or fossil fuels.

Wood Pellets: Again, constituted out of the waste products from saw mills and other wood processing plants.  Pellets are manufactured in a similar way to briquettes and share similar characteristics but are much smaller in size typically being 6mm in diameter and 40 mm long. You will need a pellet wood burning stove to use this type of fuel, as the fuel is automatically delivered to the fire box via a hopper. It is possible to obtain specialist kits to convert some former oil fuelled appliances to wood pellet fuels.

The quality of wood briquettes and pellets differs from one manufacturer to another in terms of the woods used, whether they are made of sawdust, or chips and shavings, if and what binding agents are used, and to what density they are compressed. Moves are being made at a European level to develop industry standards covering the manufacture, the properties and the evaluation of biomass fuels, which will include wood briquettes and pellets CEN/TC 335 whilst specific fuel specifications and classes are already defined in CEN/TS 14961. Although, briquettes and pellets are commercially available made from other biomass materials such as straw these are not suitable for domestic use due to their formation of corrosive clinkers and ashes. Wood briquettes and pellets have low moisture content typically ranging between 8% - 10% and due to their compression (approximately, 1,000 Kg per cubic meter) a high energy density that is energy or heat content by volume (See Fig 1: Colum 2).  

Grants are available to assist non domestic and domestic customers with the capital costs of installing specialist stoves to burn wood chip and wood pellets in our homes. At present grants are available for the home user from the Renewable Heat Premium Payment scheme until the plans to extend the Renewable Heat Incentive scheme beyond the non domestic user and include the domestic customer reach fruition.

Wood Logs: Produced from trees cut and split into pieces approximately 40 -150mm round and 200-500 mm in length.

Fig 1 using wood as a fuel source

Before purchasing logs there are a number of issues you will need to consider and they can broadly speaking be broken down into to two themes.   First the logistics which underpin using logs as your chosen fuel and secondly the quality of the fuel itself. Taking the former first you need to ascertain how and where you going to obtain your logs and how much time you are willing to invest in your fuel. If you’re lucky enough to live in a wooded rural area you may find enough blown down boughs, branches  and even trees by the roadside to keep you supplied in wood but do remember they belong to somebody. It is always advisable to ask the local land owner before removing any storm damaged offerings you may find.   It can be worth while approaching local timber mills or processing plants that may be happy enough for you to takeaway their waste, limbs and branches which are too distorted for them to process or their off cuts.  It is much more likely they will expect payment but they can provide an economical source for your fuel. Similarly, track down your local tree surgeons, whom may for a small remittance be quite happy to drop off trailer loads of logs but they are likely to be in the round. That is not split or cut to your required size which can be quite hard, if satisfying work. Also bare in mind the wood will be green and require stacking and seasoning.

If you can not find a free source of logs or would rather miss out the hard work of manhandling cutting, splitting and stacking your logs then you need to find a log merchant. Try and secure a local log supplier as this cuts down the environmental cost of transporting the logs and therefore the financial cost. Anyway, it is not a good idea to transport logs from one area to another due to the spread of invasive tree killing bugs or diseases. Scientists have found many of the tree killing or damaging bugs and diseases travel relatively small distances if left to their own devices but if infected firewood is moved great distances then so are the bugs and diseases. Buying locally will also give your local economy and woodland a much needed boost. It is a good idea to ask friends and neighbours whom they might recommend.  Always, if at all possible go and have a look at the wood you are buying.  Do not be afraid to go and visit the wood yard and build up a relationship with your supplier, it will pay dividends later. Have a look at the logs you are buying are they seasoned, stacked well, what woods are there, hard or soft. Is the wood clean, not covered in mud or worse?  Are the logs cut to the size you require, it is recommended that a log is approximately two to three inches smaller than your stove pr hearth and three to five inches in diameter.  This not only helps to maximise heat output but makes handling the logs much easier.  It is not a lot of fun attempting to fit an oversized log into the fireplace or manipulate it into the log burner. If possible and if you have the storage spaces buy your logs in the spring so you can stack and season the logs yourself over the summer.

Wood is delivered in one of two or three classes’ softwoods, hardwoods or mixed. Hardwoods are   harvested from slower growing deciduous broad leaf trees and include Ash, Oak, Birch, and Beech. As deciduous trees grow much slower the annual growth rings of the tree are smaller and more tightly packed across the log. As a result hardwoods are much denser and therefore heavier by volume than their softwood cousins of equal moisture content. Softwoods include pine, spruce and conifer and are coniferous or evergreen.  Hardwoods as a consequence of growing more slowly contain more of the suns energy for a given volume than softwoods of equal moisture content and are therefore more expensive.  Surprisingly, when measured by weight, if both softwoods and hardwoods have the same moisture content softwoods have a similar, if not slightly higher, calorific value (energy content) due to their greater resin content (see Fig 2 Colum 1, Kilowatt hours per Kilogram) but as hardwoods are approximately twice as dense and therefore twice as heavy as softwoods, hardwoods by volume contain more energy or heat (See Fig 2: Col 2, Kilowatt hours per Stacked metre3).  In other words a ton of hardwood logs can take up approximately less than two thirds of the volume of a ton of softwood logs (See Fig 2: Colum 3) and therefore by volume contain more energy or heat but by weight logs of the same moisture content regardless of species have remarkably similar energy content (Calorific value) or heat output (See Fig 2: Colum 1 and 2)

 fig 2 using wood as a fuel source

It is estimated, that if you are burning predominantly softwoods, having already established that less dense softwoods weight for weight will produce similar heat outputs as hardwoods when both are properly seasoned, you will need on average 49% more fuel by volume. Softwoods will produce faster burning fires and are capable or raising the temperature in the room faster than the slower burning hardwoods. This makes softwoods particularly useful in early spring and autumn. You need to take what type of wood you have access to and the burn rate into account in planning your storage area and more importantly when purchasing your stove. If you are mainly going to burn softwoods as they are what available in your area you might want to consider moving up to the next size or model of stove than the one indicated solely by your heating needs requirement. This will enable you to cut down the re-fuelling intervals, and help you to maintain the heat output you require (See Fig 2: Colum 3, for species comparisons by volume, stacked M3).  For more information relating to burn times and to get the best from your fuel see How to Acheive a Welcoming, Efficent, and Healthy Fire in the Home and for motr information on energy output, weight and volume of logs see Species of Hard and Soft Woods Burn Qualities by Weight and Mass.

Within the UK it is a legal requirement,and as the evidence so far indicates it makes far more sense to buy and sell logs byvolume rather than by weight. A green or freshly cut log can weigh in excess of 60% more than a seasoned dry log (See Fig 3: Kg per m3 by % Moisture Content).

Fig 3 using wood as a fuel source

This difference in weight is directly attributable to the moisture content of the log.  Logs are traditionally bought and sold by a measure of volume known as a cord (4x4x8 = 1283 ft). This takes into account the variations in weight through the moisture content of the logs.  Increasingly in the UK logs are sold by the truck, trailer or meter cube builders’ bag so knowing in what volume your log merchant supplies their logs enables you to make cost comparisons with other merchants and suppliers. You need when purchasing logs to take into consideration how the logs are stacked or packed. There are inevitably more air pockets and therefore less logs in loosely stacked or packed than more tightly stacked and packed log piles. Do keep in mind that a builder’s one tonne dump bag is rarely a meter cubed and more likely hold 0.6 to 0.75 of a cubed meter when filled with logs  A moisture content of 20%-25% or less, in other words seasoned logs are ready to burn and a builders bag is more likely to weigh in at 200-250 kilogram’s (1 /4  to 1/5 th of a metric tonne) rather than  the  metric tonne some traders will tell you.  When sourcing your supplier and fixing a price factor in that a loosely packed builder’s bag can translate into 0.6 of a cubed meter or less when stacked.  A trailer or truck load of logs is obviously relative to the size of the truck or trailer, so appraise the load carefully.  It is always worthwhile spending a little time stacking your logs, not only enabling your logs to season correctly, as this will let you evaluate between loads and you will soon build up an eye to judge whether a deal is good or not.


When comparing the data across Figs 3 and 4 it is irrefutable it is the moisture content rather than the species of tree the logs are harvested from, which is by far the biggest determining factor of the heat output, or calorific value, of a log. As an industry standard the moisture content of a log within Britain is usually expressed as percentage of the weight of the wood. Green wood can have moisture content as high as 65% and ideally logs should not be burnt until their moisture content is down to 25% or less.  It is easily feasible to bring the moisture content of green or freshly cut logs down to 20% or less by air drying the logs, that is to say seasoning them outside, cut and split and letting the air and sun do the job over time. It is recommended that you store and season freshly cut wood for at least a season. Bearing this in mind it is worthwhile buying your logs in the spring ready for the next winter and even better to season them for a whole twelve months if possible.  

 

Fig 4 using wood as a fuel source

When a tree has been harvested or felled can affect how long it will need to season.  Logs felled in the spring and summer, particularly in the case of deciduous trees, will hold more sap and therefore moisture than logs harvested in the late autumn or winter, and need longer to season.

A wood fire is fundamentally a complex chemical reaction. In the initial stage any excess moisture within the log has to be ‘burnt off’ by the fire and evaporated. All this takes energy from the fuel, energy you will not feel the benefit of as heat. A freshly cut log can contain as much as 65% of its weight as moisture and this will need to be vaporised and evaporated before the second phase of the chemical reaction can start. That’s why wet or green wood hisses and sizzles and the water vapour can be seen disappearing up the chimney as steam and non flammable carbon dioxide.  The wetter the wood, the higher the moisture content, the more heat energy is lost (See Fig 4).  The second stage of the fire’s chemical reaction starts as the temperature of the fire reaches beyond the boiling point of water (100 degrees celsius) until at approximately 200 degrees celsius the cellulose in the wood starts to breakdown giving of volatile tars and gases.   If the conditions are right these tars and gasses ignite raising the temperature of the fire, releasing further tars and gases and a self perpetuating chain reaction is started raising the heat of the fire further. This second stage of a fire where combustible gases are released from the fuel is also referred to as pyrolysis and is characterised by bright yellow flames you associate with log fires. At the point the fire is in effect fuelling itself with log gas and it is at this point the log fire emits radiant heat that is the heat you feel in the room. For further information see The Science of Fire.

Excess smoke from a log fire is an indication of an inefficient log fire. Either the wood is unable to reach the temperature required for pyrolysis to take place as it is too wet or green. The heat generated within the fire rather than lifting the temperature of the fire and fuel for the second stage to take place is displaced into burning off the excess moisture within the wood. This prevents the fire from reaching the temperature required for the cellulose to breakdown and pyrolysis to take place. Logs with moisture contents in excess of 70-80% have insufficient energy to support a fire and will not burn. Secondly, If there is insufficient oxygen entering the fire this will prevent the gases and tars released from the fuel to ignite and pyrolysis to take place.  For either reason much of the potential heat energy in the wood disappears up the chimney as pollutant tars and gases.  By contrast an efficient log fire, fuelling itself with the wood gases of the second stage reduces the emissions carried up the chimney to carbon dioxide; a non particulate gas that is in the case of sustainably harvested logs environmentally offsets itself. The third and final stage of the chemical reaction which is a fire happens when the fire temperature reaches in excess of 450 0 C the third and   the carbon or embers of the log ignite and continue to burn with little smoke or flame emitting considerable heat. These three stages of a fire are not discrete happenings and divorced from each other within a fire but are happening simultaneously as the logs enter different phases of the chemical reaction at different times. This is clearly witnessed and part of the magic of  sitting in front of the hearth, or log burner, in the relaxing and enjoyable pass time of fire watching, mesmerised by the glowing hot embers, watching the dancing colour changing flames and the swirling shroud of almost invisible smoke disappears up the chimney.

Pine Fire Surround using wood as a fuel source

The problems of a poor fire are further compounded, as wet wood burns slowly, and with little heat consequently, the flue or chimney does not reach its proper temperature resulting in a poor draw which hampers both the combustion of the fire through insufficient oxygen being drawn through the fire and the poor evacuation of the smoke and gases. The smoke, gases and tars produced from burning a low heat fire of green or wet logs unable to reach the second stage is acidic and released as a steam in the form of corrosive creosote, which will eat into the walls or lining of a chimney. This creosote, if left to gather will coat the inside of the chimney or flue with a tarry substance creating a potential fire hazard. It is recommended that a log fire chimney is swept twice a year, to remove any soot or tarry deposits that may accumulate removing any potential fire hazard, and to ensure a good draw is maintained through the chimney.

You do not need to buy an expensive moisture meter to determine whether your logs are dry or not with very little experience it is very easy to tell if a log is green or seasoned. A seasoned log will be significantly lighter then a green log. There will be radial cracks across the face and back ends of a well seasoned log and the bark will be flaky or will peel off easily. As a log dries its colour will change to a light greyish or white colour and if you bang two logs together they will sound hollow not unlike a cricket ball hitting a cricket bat. The general rule is the drier the logs the higher the pitch of the ‘clunk’.  If you do decide to venture down the route of buying a moisture meter remember, to when checking the moisture content, either before purchasing or before using your home seasoned logs to take the reading from the centre of the log by splitting a randomly selected log, as the ends of the logs will dry first.

Once you have taken delivery of your logs, where are you are going to store them? Are the logs green or seasoned? Do you have plenty of room to stack and store the logs? As a general rule the bigger the load the cheaper the logs to buy. The same is true of green wood, the greener the wood the cheaper it should be to buy.

Logs need to be left open to the wind and sun but protected from the rain to store and season properly. There is contradictory evidence that in the earlier stages of the seasoning process exposure to rain helps to wash out the sap within the logs and it does seem to help. It is at this early stage of seasoning,   particularly with freshly felled logs, you ideally need to leave the logs in loose piles exsposed to the sun and wind preferably cut and split. Wood that is cut and split exposes more surface area to the elements and will dry much quicker than larger pieces left in the round. There a number of ways to split wood. The more physicals methods requite the use of an axe, maul or wedges. A maul is wider and heavier than an axe a little like a 6lb sledge hammer honed to a stubby wedge shape. A wedge can be literally wedge shaped and made of hard woods such oak or preferably in my opinion metal and need to be struck by a sledge hammer to be driven into the head of the log.  Alternative wedges are available often trading under explosive log pseudonyms and are usually contorted conical shapes with many faces cut into them culminating in a very sharp point to drive into the log. The key to splitting wood is to remember it is not the sharpness of the tool, as you are not cutting or even chopping the log, you are literally splitting it along the grain.   Alternatively, there is a number of mechanical log splitters available either powered manually, hydraulically, electrically or petrol driven.

It is recommended that you store wood in open structures such as carports, lean-tos or purpose built log stores with open sides to allow sufficient ventilation and exposure to the weather for the logs to dry and stay in premium condition. If you do not have a suitable open building to store your logs there are many ready made log stores available to purchase. Logs stores can range from very simple structures easily built by your self using a few discarded pallets or timber to very elaborate buildings that would require a master craftsman to construct. Remember a simply designed log store made from waste timber will dry and store your logs just as satisfactorily as a purpose built building costing many hundreds of pounds. Alternatively, stack wood on level ground protected by a tarpaulin, concrete or on a pallet to reduce contact with the ground moisture and cover the top with a tarpaulin to protect from the falling rain. Ideally stacks of logs should be approximately four foot in height and positioned away, if only by a few inches, from walls, structures and other log piles to ensure air can pass round the wood.   It is a good idea, if at all possible to place your log pile facing away from the prevailing weather, and face your log store, or stack, south to catch the drying benefits of the full sun.

An aesthetically pleasing way to store logs in the absence of a log store is the traditional Scandinavian round, looking like a haystack.   Start by placing a vertical pole or even better a piece of holed leached field drainage pipe cut to the desired height of your stack in the ground and mark out a rough circle. Lay out spokes of logs bark side down from the centre pole before concentrating on building up the outside wall. As the walls climb fill the centre with more logs again, bark side down. The orientation of the bark will help keep the wood dry allowing it to fulfil one of its functions of keeping the tree dry before it was felled.  As you build the wall incline it in slightly so you are in effect placing concentric circles of logs on top of each other.  Continue filling the middle keeping your log pile even until you start to near the top when you need to slope your log pile to form a cone shape to allow the rain to run off. It is also necessary as you near the top to place your logs bark side up, some even suggest placing a plastic sheet or tarpaulin under the last few layers of logs to help keep your logs protected from rain.  It is good practice to bring logs into the house and store them in a basket a day or two before using as fuel. (for further information on log storage click here)

 

Scandinavian Log Pile using wood as a fuel source

There is no reason why you can not burn salvaged or waste wood which can greatly reduce if not eliminate your heating costs but caution must be the by word.  Just because it is flammable doesn’t mean you should be burning it. Burning many wood products should be avoided on health and safety grounds as they produce hazardous fumes inside and out. Burning the wrong fuels is dangerous to your health, environment and damaging to your stove and chimney. It can increase the potential of a fire hazard through the gumming up of the chimney with highly flammable tars and creosotes. This list of what not to burn is not definitive but offers the reader the opportunity to think about what it is they are burning. Green wood, pressure treated timber due to the chemical compounds used to treat the timber, any painted or varnished woods. Do not be tempted to burn salt water drift wood as the salt in the wood will accelerate the burn rate and give off corrosive and toxic fumes. You can burn fresh water drift wood but be aware it may contain silt and gravel. Steer clear of manufactured boarding such as ply wood, mdf and particle board etc  due to the compounds contained in the glues emitting toxic and corrosive fumes.  Remember burning inappropriate fuels potentially damages the chimney, stove and more importantly your health and defiantly the environment.
 
Finally to end on a more positive note there is much truth in the old adage good wood warms you twice, once in the preparation and once in the burning.  I would go even further to contest good wood warms you more than twice but there is something ultimately satisfying in cutting, splitting, and stacking logs in the spring, summer and early autumn that is immensely satisfying, relaxing and helps combat the condition of the modern lifestyle, that of alienation . To cut, split, and stack a pile of logs before Sunday lunch is not only good exercise but a great stress buster, which is revisited again when refuelling the fire, or just sitting in the warm watching the dancing flames, and embers, of a real log fire.  I have come across plenty of anecdotal evidence that you very quickly, and with a little experience, build up a wealth of knowledge and quite an intense relationship with your fuel, what species of tree the log originates from, what are its burning qualities, where to cut the log, where to strike it with the axe or maul, where a particular piece fits on the log pile to maximise ventilation and stability, how seasoned a log is. I do wonder on dark cold winter evenings, in warm snug sitting rooms bathed in the light of the fire resonate to the all knowing words of no, not that one, the other one and it needs to go there, as somebody stretches out to put another log or two on the fire or log burner.

By Phil Cleaver

 

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