These notes have been distilled from over twenty years’ practical experience of woodburning stoves for those who are considering making the transition to this new way of heating.
We hope you will find the following information informative and helpful: installed and operated correctly, there are very few other purchases you could make which will give you and your family so much pleasure and comfort over so many years. And don’t just believe us -- ask anyone who has lived with a woodstove. Over the years, scores of happy customers have told us that it has been the best improvement they have ever made to their home, and that their only regret is that they had never heard about them earlier!
In order to get the best out of burning wood, a little knowledge about its value as a fuel is necessary.
During its life, a tree takes up moisture from the ground, together with various mineral salts essential for its growth, and this is generally known as sap. During the summer months an oak will lose about 99% of its daily uptake by evaporation through its leaves, and may take up more than 100 gallons of water a day through its roots. Contrary to popular thought however, the bulk of the solid material of a tree is not formed from substances derived from the ground: hard as it is to believe, all those tons of wood are formed primarily “out of thin air” -- or, to be more precise, from carbon dioxide in the air.
Using energy derived from sunlight via the process of photosynthesis, carbon dioxide combines in the presence of chlorophyll with water to form sugars. This food from the leaves then flows in solution through the inner bark, or phloem, and is used for the generation of new growth. New wood or “sapwood” grows in annual rings around the perimeter of the tree, whilst the older wood nearer the centre dies and forms the stronger heartwood.
The amount of sap present very much depends on the time of year. During the spring and summer, the sap rises and the moisture content of the wood is considerably higher than during the dormant winter months. After felling, this moisture within the wood finds it difficult to evaporate away unless the bark is removed. During the life of the tree, the bark acts as a very efficient barrier, not only keeping disease organisms and rainwater out, but conserving moisture by preventing evaporation: and unless the bark is removed after felling, it will continue to slow down the now-desirable loss of moisture.
Different species of tree naturally have different levels of moisture content -- Ash, for instance, has one of the lowest natural levels, whilst species such as Willow and Poplar, both of which thrive in wet ground next to streams and springs, have a high moisture content.
Most firewood offered for sale originates as a by-product of some other operation -- forestry thinnings and “lop and top”, road widening schemes, building site clearance, and some from private gardens. Because of the state of the ground in winter and also because of nesting birds in the spring, most felling takes place during the summer and autumn periods, ie. after the sap has risen.
Typically, such newly-felled timber will have 50%-70% moisture content by weight. The aim is to reduce that level of moisture content down to the level found in the atmosphere, about 15%-20% during the summer months. Without putting the timber in a drying kiln, it is not possible to maintain a lower content than this since if the wood is drier than the atmosphere, it will simply absorb moisture until an equilibrium is reached.
This process of reducing the moisture content of wood is termed “seasoning”: and the best method to accomplish this is to facillitate as much air circulation and warmth to reach the woodstack as possible. For this reason, wood to be seasoned should not be stacked or thrown into the garage or a closed shed: all that will happen is that large growths of mould and fungi will thrive on the damp, decaying wood in the cool, still conditions, and after several months the wood will still be too damp to burn. Similarly, wood stacked or thrown in a heap next to the shaded and cool north side of a building, or hidden out of sight under the rhoderdendrons where the sun rarely reaches, the rain drips and the humidity levels are high after every downpour, is very unlikely to produce the desired result.
The best place for your woodstack is, so far is is possible, right out in the open. A southerly aspect is best, perhaps along a paddock fenceline, or stacked against an outside wall. A simple lean-to type of roof structure could be put up with little effort or expense to keep the worst of the rain from soaking down through the top of the stack, or a tarpaulin sheet could be stretched and weighted down along the top. But remember that rain water is not the main problem, at least during the summer, since that will dry off with a day or two of hot weather. Rather, it is the sap level within the wood itself which is the problem and which must be reduced to an acceptable level.
Thus is becomes evident that successful woodburning depends to a large extent on a regular cycle being established. Wood needs to be felled, or bought in, by June at the very latest for use the following winter: and this should really be regarded as an absolute minimum. Many species, such as oak, beech and elm have a dense grain, and unless cut and split into small pieces, will have scarcely seasoned sufficiently by October / November when the cold weather sets in. A better cycle to aim at -- and it may take a year or two before you can get organised enough to store the extra volume of timber -- is to allow two summers for seasoning, laying in a fresh stock of wood for use 18 months later during the following winter. You will certainly notice the difference both in ease of lighting and in the amount of useful heat which is produced by your stove.
Wood should ideally be stacked with a lot of air spaces between the logs. If you have acquired cordwood (long lengths of timber such as branches and thinner trunks), it should be cut up into stove-length pieces before being stacked for seasoning. Any logs more than about 4 inches (100mm) in diameter should be split in half lengthwise to expose the grain and allow the wood to dry off over a large surface area. Larger “rounds” of wood should similarly be split into a number of smaller chunks. As explained above, the bark is very efficient at preventing moisture loss both from the living tree and the felled timber: an unsplit log can therefore only dry out from its exposed end grain, and will consequently take much longer to lose its moisture if stacked unsplit.
For those who feel at home with percentages, it may be helpful to consider that a given piece of wood, once properly seasoned, will yield approximately 300% (ie. three times) more useful heat into your home compared to when the tree was just felled. With such a “return” available on your “investment” in your woodpile, it certainly makes economic sense to try to get into an 18-month cycle as soon as possible. Conversely, it is really very uneconomic to struggle with unseasoned wood, quite apart from all the possible complications which can result from the formation of tar and creosote which will probably result from burning wood with a high moisture content.
Let us suppose for a moment that you try to burn some logs which are either improperly seasoned, or are perhaps soaked with rainwater. Before you can get much useful heat from burning such fuel, all the moisture in the wood must be literally boiled out and turned into steam. And, just like the kettle or cooking pot boiling on the hob, that requires a huge input of energy. Each pound or kilogram of wood only has a finite quantity of latent energy which is released during the combustion process: and a large proportion of that energy is now being literally wasted in converting water (sap) into steam. So if you ever hear a fire hissing and spitting away, and can see the whitish sap boiling out of the ends of the logs, you will now know what is going on!
Because of this tremendous variability in the potential heat available from different species and samples of timber, it might appear logical that output ratings which are claimed for different stoves and boilers by their respective manufacturers should all be based on the same agreed species, moisture content and rate of wood consumption in kg/hour. Unfortunately no such agreed standard exists, and different manufacturers use their own criteria as a basis for their claimed outputs. We would thus recommend that the maxim “let the buyer beware” (or perhaps better rephrased, “let the buyer educate himself and think logically about claims which he may read”) should be applied.
A well-seasoned log has certain characteristics to look out for. First, it should be noticeably lighter in weight than freshly cut timber. There should be no visible moisture oozing out of the ends of the log, and indeed there should be deep cracks radiating out from the centre caused by the loss of moisture tearing the wood fibres apart. And when two seasoned logs are struck together, a sharp ring should be heard rather than a dull thud which is typical of wood with a high moisture content.
The surface area of a log also comes into the equation. A large log can only ignite on the outside, so it will take longer to burn up but will release heat at a slower rate. Thus larger logs are best when you wnat a long but slower burn, but are not much use if you want to get a lot of heat quickly! For that you need smaller pieces of wood with a lot of surface area exposed to the flames. Thus smaller, thinner pieces are best to get the stove up to temperature quickly from cold, whereas larger, denser logs are fine once the room is warm enough and you just need to maintain the temperature. You will also find that in general, softwoods such as pine are better for releasing lots of heat quickly (ie. they act as a “sprint fuel”), whereas dense hardwoods such as oak are best for long but rather slow and unenthusiastic burns.
The surface area of a log also comes into the equation. A large log can only ignite on the outside, so it will take longer to burn up but will release heat at a slower rate. Thus larger logs are best when you wnat a long but slower burn, but are not much use if you want to get a lot of heat quickly! For that you need smaller pieces of wood with a lot of surface area exposed to the flames. Thus smaller, thinner pieces are best to get the stove up to temperature quickly from cold, whereas larger, denser logs are fine once the room is warm enough and you just need to maintain the temperature. You will also find that in general, softwoods such as pine are better for releasing lots of heat quickly (ie. they act as a “sprint fuel”), whereas dense hardwoods such as oak are best for long but rather slow and unenthusiastic burns.
If you are at all dubious about the condition of your logs, or if you would like to have some point of reference to know what your stove is capable of, try burning a couple of dry used pallets, some old floorboards, or a bag of reconstituted wood-waste fuel which is obtainable from our showrooms. You will then have a yardstick by which you can gauge your stove’s performance using your normal wood.
A woodburning stove is an extremely efficient means of converting firewood into useful heat. And a highly efficient stove should really only be connected to a well-insulated and lined chimney. The problem however is that most older properties built prior to 1966 have only unlined brick flues (usually one brick square inside, ie. 9” x 9”, although sometimes larger as with large inglenook fireplaces). Such old chimneys are often only one brick thick, have already been exposed to the weather and had substantial use over a period of perhaps 60 years or more, and were in any case built using soft lime mortar rather than modern cement mortar. All of this poses a problem.
But some may understandably enquire, “People have been lighting an open fire in this fireplace every winter’s day for the past 50 years, and there’s never been any problem yet: so why should I consider spending a lot of money just because someone claims its a desirable thing to do?” A reasonable question, and certainly one which deserves an explanation. And the explanation is as follows.
Two things are basically very different when comparing a modern stove with an open fire. The first major difference, already alluded to above, is one of efficiency.
Efficiency: Typically, only about 5%-15% of the total heat produced by a fire in an open fireplace benefits the house: the other 85%-95% goes straight up the chimney. This is of course very wasteful and expensive in fuel, but at least the flue gases (the “smoke”) remain hot, and relatively little condensation of steam or unburnt volatiles takes place. The loss of heat up the chimney means that the flue gases remain above the “dew point” (you may remember the term from GSE chemistry?!), and are thus mostly emitted safely into the atmosphere.
With a woodburning stove however, between 60%-75% or more of the heat released by the burning wood will be radiated and convected into the room as useful heat. This means that, depending on the size of the original open fireplace and its flue cross-section, a stove is typically 4 to 10 times more efficient in turning fuel into useful heat. Large inglenooks are the worst culprits, having huge draught requirements and providing relatively little benefit except in the immediate vicinity of the fire: but then it should be remembered that their original purpose was as much to preserve meat and fish by smoking as it was to heat the room.
But when so much heat is retained within the building with a stove, the “downside” is that the flue gases are consequently much cooler and are more likely to condense onto the sides of the flue. Now, for those who can recall their chemistry lessions, one simple experiment involved heating scraps of wood in a glass retort over a bunsen burner. After a short while, the surface of the wood begins to char, and if a lighted taper is held close to the neck of the retort, the wood gases which are being driven off by the heat of the bunsen burner ignite and burn with a yellow flame. This in fact mimmicks what is happening when you burn wood in a fire.
The second part of this simple experiment illustrates the problem with unlined chimneys. Using a rubber bung, a glass “condensation tube” is fitted onto the neck of the retort -- a length of glass tube surrounded by a glass coil through which cold water can be circulated. This apparatus now presents a cooled surface to the passing wood gases, and the result is an almost immediate film of sticky black condensate. (If you were never shown this experiment, now you know what you missed!)
The same thing happening up a chimney will result in a shiny, and often sticky black layer of tar and creosote (general terms used in the stove industry to describe what is in fact a complex mix of distillates) forming on the sides of the flue. This condensate can then do several unwelcome things. For a start, tar is highly inflammable, so the risk of having a potentially dangerous chimney fire at some time in the future is considerable. Tar is also highly corrosive, attacking and eating into old brickwork and the soft lime mortar. Old bricks are porous and tend to absorb moisture, so one result (and often the first symptom to be noticed) is that brown stains penetrate through the chimney breast and begin to appear through the plaster or wallpaper in an upstairs bedroom. (An examination of the chimney stack if it passes through the loft will often reveal the tell-tale evidence of brown staining before it reaches that stage lower down.)
Stains are not only unsightly, but the semi-liquid tar is also highly pungent and almost impossible to get rid of without stripping the plaster, inserting an impermeable barrier of some sort, and then replastering and redecorating. Arguably this is rather an expensive remedy for what is only the avoidable result of poor stove management.
The second major difference between a stove and an open fire is its ability to be operated in “slowburning mode”.
Slowburning: It is really not possible to burn an open fire slowly in the same way as a modern airtight stove, ie. by closing off its combustion air supply. To slow an open fire down, two methods have traditionally been used. The first approach has been to place one or two “green” (ie. unseasoned) logs on the fire: because of the high moisture content, it would take some time before sufficient sap had been driven off as steam to allow the logs to burst into flame. As you will probably appreciate better by now, this approach is not really recommended.
A second way to obtain a longer burn with an open fire is simply to use very large logs which have a minimum surface area to mass ratio. As mentioned above when dealing with stoves, the larger the surface area, the quicker the burning time for a given mass of timber. Exactly the same principle applies with an open fire, and perhaps even moreso due to the lack of a stove’s forced draught.
But with a closed airtight stove, it is physically possible to close down the air controls to such a degree that almost no combustion air can get to the fire. By thus starving the fuel of oxygen (air), the wood gases released by the heating of the unburnt wood are unable to ignite, and the fuel then smoulders at a much lower temperature. It is possible to burn a stove so slowly by this means that it can be made to “stay in” for perhaps 6 to 10 hours at a stretch without refuelling.
But since the wood gases (or “volatiles”) then escape up the chimney unburnt, and since almost half the available energy in wood is obtained by properly burning those gases, it therefore follows that the efficiency of a stove can drop markedly during any slowburning period. The only exception to this is where all the volatiles have already been burnt off, and all that remains in the stove is charcoal: since there are then no wood gases remaining unburnt, under these conditions slowburning can be caried out safely and efficiently.
Thus, whilst a woodstove can on the one hand be a highly efficient and safe means of converting firewood into useful heat, and on the other hand can offer the means of burning (or more accurately, smouldering) wood, using it in slowburning mode is relatively inefficient. When the air supply is shut right down before the wood has been reduced to charcoal, the efficiency can drop from over 70% to perhaps only 30%.
More importantly, slowburning for long periods can be unsafe, especially when the stove is fitted in an unlined chimney. As a rule, in order to be able to burn a stove slowly and safely on a regular basis, it must be connected to a lined and insulated flue of a high standard.
If this is your first experience with a woodburning stove, this section will help you to understand some of the why’s and wherefore’s of using wood as a fuel.
First of all -- and it bears repeating because of its importance -- do remember that a stove burning in slowburning mode will only provide about half as much useful heat from a given load of wood compared to when it is being run generously.
In Britain, we do not have the same historical association with woodburning stoves as is found over on the Continent, in Scandinavia, and in the United States and Canada. We therefore lack a popular “folklore” of how, and how not, to operate a stove. As a consequence, it is not uncommon in this country to find the new or prospective woodstove owner believing that smouldering his stove for almost 24 hours a day, from early autumn to late spring, is an acceptable practice. Of course, our temperate climate does not help in this respect, and nor does the British tendency to divide the internal space up into small rooms. Most other countries where woodstoves have a long-established history experience much colder, longer and drier winters, and it is frequently the case that houses are designed on a more open-plan basis. As a result of these factors, it is common for stoves to be run generously for much of the day, whereas our generally smaller rooms, combined with higher outside average winter temperatures, encourage stove owners to slowburn for much longer periods of time.
But on the Continent they know better. With generations having used wood as their primary source of space heating and cooking, they have accumulated a “stove wisdom” which we would do well to learn from. And one thing that you will not find them doing often is “stewing” their fuel for hour after hour, day in and day out. They understand the inherent risk of tar formation in the chimney caused by burning wood too slowly for too much of the time, and they avoid the potential problem through proper stove and fuel management.
This understanding of the basics has led several countries to tackle potential problems at source. On the basis that an ounce of prevention is worth more than a pound of cure, they have come up with a local official known as the “master sweep”. This gentleman is invested with fairly sweeping powers (forgive the pun) in his local area of jurisdication. For instance, it is one of his responsibilities to inspect every new chimney that is built and, if in his opinion it does not conform to rigorous standards, no fire may be lit until the required alterations have been carried out and rechecked.
But his powers go further still. They also extend into the standard of the woodpile itself! He and his associates have the right to inspect the householder’s wood store and, it the timber is insufficiently seasoned, an order may be issued preventing that wood from being burnt until its moisture content has been reduced to the maximum permissible level.
Now all this rather goes against the grain for the average Englishman. There is arguably already too much red tape around not to have yet more paid officials poking around our gardens and homes. However, someone needs to act as watchdog to ensure that minimum standards are met: and arguably the best person to shoulder that responsibility is the householder -- which is you! It is, after all, your home, and the safety of your family, which is at stake. And if you know what the potential problems are, then you can take the necessary steps to avoid them.
So, the “Golden Rule of Happy Woodburning” is: use your stove most of the time for doing what it’s best at -- producing lots of inexpensive warmth! If you know you’re going to be out of the house all day, it is much better stove management to light it and run it hard when you get home, rather than smoulder it like a cigarette all day and all night long.
In the past, some well-meaning people have given the advice that, providing a stove is allowed to burn hard for perhaps an hour a day, it can then be safely left to smoulder for the remaining 23 hours. However, this practice is really not to be recommended: slowburning for such long periods is very likely to result in the formation of dangerous tarry deposits.
So what should you do? Does this mean that all slowburning is out of the question and to be avoided at all times? Does it mean that every time you want to pop out to the shops for a couple of hours you mustn’t turn the stove down to keep the fire going until you return? Or that when the weather turns freezing cold, you must not attempt to keep the fire going overnight?
The answer is really that the proof of the pudding is in the eating. Get to know your stove and chimney, and always be aware of what the results are of how you are running your stove in your particular circumstances -- your stove installation is unique, and no two stoves will work quite the same in two georaphical locations or in two different chimneys. Even two stoves at opposite ends of the same house may have quite individual operating characteristics.
If you want to be really safe, ideally slowburning of wood should only commence once the wood gases have been allowed to burn off completely, leaving a red-hot bed of charcoal. The stove can then be turned right down and allowed to 'slumber' on charcoal without any risk of accumulating wood tar deposits up the chimney.
In practice, a fair compromise is always to run the stove hot for about fifteen to twenty minutes after adding a fresh load of wood before shutting the stove down. This will allow the remaining moisture to be driven off quickly and at a high temperature, and at least the outside of the logs will be well charred before slowburning commences.
Now that your stove is installed and working, use a torch to regularly check the state of the inside of the fluepipe and chimney. The ideal state should be a deposit of fine soot, grey / black in colour. Less desirable are cornflake-like crispy deposits hanging off the walls of the flue. Anything in the way of a black shiny glaze or worse still, brown or black sticky or liquid deposits, indicate that the flue gases are falling below the dew point and condensing into tar. Deposits should come off easily when a tight-fitting sweeping brush is used to rod the flue: if they don't, it is a clear warning signal that you need to ask for help to find out what you are doing wrong. You are probably either running the stove too slowly, using wood which is insufficiently seasoned -- or both. You must then quickly change what you are doing so that such symptoms disappear.
Once you have established a safe pattern of use, you should be able to space out your chimney sweeping to perhaps just once a year. But twice is generally advisable, and in practice it must be swept 'as often as necessary to keep it clean.' Remember the adage, 'A clean chimney never caught fire!'
Never be tempted to let the chimney go from one year to the next without being swept. Apart from the sooty deposits, fly-ash (fine ash carried up with the hot rising gases) may have settled out to a considerable depth at any bends in the flue. You may also have unknowingly acquired a bird's nest or two during the spring, and end up with a room full of smoke and / or a chimney fire come the autumn when you light up for the first time.
And chimneys do not like catching fire under any circumstances. If you have clay liners installed, they will be likely to crack during a fire as they are unable to stand what is called 'thermal shock', and once they have cracked any condensation or rain water will be able to soak into the surrounding brickwork resulting in stains and smells. If a stainless steel liner is fitted, you may succeed in buckling or even smelting it -- but in any event it will be ruined as taking stainless steel above red heat changes its metallurgical properties resulting in accelerated corrosion. And if no liner is fitted, then old brick chimneys -- often only half a brick thick -- may get hot enough to ignite adjacent timbers.
Points to remember
Exterior
The normal finish for woodburning stoves is a matt or metallic heat resistant paint. Matt black paint is available in a can and can be applied with a paintbrush; but all the other paints are supplied only in aerosol spray cans.
The paint should be applied to a clean, dry surface, and the stove should be cold. Two or three thin coats is better than one thick coat to prevent it from running; as it becomes touch-dry in only a few minutes, it does not take too long to do. Although dust and ash can be removed from day to day with a soft brush or damp cloth, many owners touch up at least the front and top of their stove once a year to keep it looking like new.
After painting, an acrid smell will be given off the next time the stove is fired up; this is quite normal and happens while the new paint is curing (hardening). Leave the window open and the door of the room shut, and wait until the fumes have disappeared.
Enamelled stove finishes should be cleaned with a damp cloth when cool. The enamelling may chip if subjected to a knock by, for instance, a log or kettle. Crazing of the enamel sometimes occurs, and there is unfortunately not much one can do about it -- if this would upset you, much better to buy a stove with a painted finish which can be resprayed and kept in 'as new' appearance.
Bright cooking surfaces may be rejuvenated by applying a little Zebracier or similar proprietary metal polish.
Stove Doors
The 'glass' fitted to modern stoves is likely to be in one piece and made of a ceramic material, whereas in older stoves narrow strips of borosilicate glass were used. The strips are quite fragile and easily broken with a knock or, for instance, when the door is closed against a misplaced log. The strips also tend to distort and finally break after a few years' use. Expect to replace them about every 3 years.
Ceramic panels are much tougher and have comparatively low coefficients of expansion; they are however more expensive to replace when they do break. Again it tends to be knocks rather than heat which does the damage.
Because of the fragile nature of glass and the obvious possibility of accidents, it is never covered by stove guarantees.
Be careful when replacing glass strips or panels not to overtighten retaining screws / clips. Old worn gasket material around the glass should be replaced at the same time.
Other sealing gaskets (made today from ceramic fibre / fibreglass rope in various diameters, rather than asbestos as in the past) will need replacing about every four or five years. You will usually find them around the edges of the doors and the ashpan cover (if your stove has one.) Scrape out the remains of the old gasket and adhesive with an old screwdriver tip, and use either high temperature adhesive or a special high temperature silicone (we now stock a silicone cartridge which will stand 1200 degrees C) to glue in the new gaskets. It is best to allow about 24 hours for the adhesive to set cold before lighting the stove.
The Baffle Plate
The baffle plate, which is normally fitted immediately above the flames, and the firebricks (where fitted), are 'sacrificial' in the sense that part of their purpose is to bear the brunt of the flames and protect the stove body. In time both will need replacement. How long they last will depend on a number of factors, but mainly on how often the stove is used and especially on whether it has been 'over-fired' at any time.
The Grate
Important! If you have a 'multifuel' stove or cooker, you will have some sort of fixed or removable grate fitted. It is very important not to allow the ash level to build up under the grate such that it touches its underside, otherwise the metal will almost certainly overheat and the grate can then burn through in a matter of days or weeks.
The grate in a stove where the coal ash is always removed daily should last for years; however, because of the possibility of abuse, grates -- like glass -- are always specifically excluded from the manufacturers' guarantees.
Central heating systems
If your stove runs a central heating system, here are a few tips:
This information has been written and made available over the internet in the interests of safer and more rewarding woodburning by the New Forest Woodburning Centre, The Old School, Church Lane, Sway, Nr Lymington, Hampshire SO41 6AD.
Tel: 01590-683585 (2 lines)
Fax: 01590-683587
E-mail: newforestwoodburning@woodburners.com