All wood burns, but not all of it burns the same. Some give off more heat, burn slower and cleaner, and emit a wonderful aroma. Others burn quickly, are smoky, and their resin residues rapidly clog the chimney. However, smoke is beneficial in smokers; delicious shish kebabs are made with aromatic fruit woods, and the essential oils of coniferous species have a positive effect on us in the sauna. Thus, the best trees for firewood are that which optimally performs a specific task.
To know which wood to take for a home wood, sauna or masonry stove, fireplace, grill, or boiler, one must understand their characteristics. The main characteristics of wood (and bark) needed to evaluate them as firewood are:
- Moisture content;
- Ash content;
- Density;
- Resin content;
- Caloric value (heat output);
- Splittability;
- Storage life.
Moisture content
Moisture content of wood Wood that is not sufficiently dry burns poorly, uses energy to evaporate moisture rather than heating the air, and produces a lot of soot. For wood used as firewood, the relative (not absolute) moisture content is used – the amount of moisture currently contained in the fibers relative to their own weight.
The amount of water in dried wood depends on the species, the time of its felling, and the degree and method of drying. It is recommended to chop a tree for firewood in winter when it contains the least amount of water. Such wood will dry to the required condition in about 6 months; if chopped in July, it will only dry out after a year.
Moreover, it should not be forgotten that the bark hinders the evaporation of water from the wood, so thin round trunks also need to be chopped or sawed into parts.
Logs are dried both in the open air and in special chamber ovens. In chambers, at lower temperatures, wood can be dried to a lower moisture content and much faster. The latter method is more expensive, but it ensures high thermal power of the wood. Green wood weighs 70-100% more than seasoned wood.
Dry wood usually has a moisture content of 20% and below, which can be checked with a moisture meter. If such a device is not available, an ancient method can be used: when dry logs are struck against each other, they should not make a dull sound but a ringing one.
Read The Ultimate Guide to Choosing a Firewood Moisture Meter
Pin-Type Moisture Meters
Pinless Moisture Meters
Density (Specific Weight)
This is a common physical quantity that indicates how much substance weight corresponds to a standard volume (usually per cubic meter). The density depends on the wood’s moisture content, and it is the primary reason for its calorific value. The denser the wood, the “hotter” the firewood produced from it. Less dense species burn faster, and therefore the furnace needs to be reloaded frequently to heat it to the required temperature.
Wood species are conventionally divided into three groups based on density.
The assessment is conducted at 12% moisture content:
- Low-density (up to 540 kg/m3): coniferous – juniper, pine, fir, spruce; among deciduous – aspen, poplar, willow, linden, alder;
- Medium-density (550-740 kg/m3): weeping birch, beech, elm, maple, apple, common ash, larch, etc.;
- High-density (750 kg/m3 and above): hornbeam, oak, iron birch, sandy and white acacia.
Calorific Value of Firewood
To assess the amount of heat the firewood will give, one should use the specific heat value, which is a quantity derived from density and calorific performance. Based on the calorific value, firewood from tree species growing in US is divided into three groups:
- The most heat (200-350 kcal) is given by properly harvested and dried firewood from birch, beech, hornbeam, ash, elm, larch, maple, elm, and oak.
- The second group in terms of heat output includes firewood from alder and pine.
- And the least calorific are aspen, willow, poplar, linden, spruce, and cedar.
Despite the fact that the mass calorific value of coniferous wood is higher than that of deciduous wood, due to its lower density, the resulting calorific value of coniferous firewood is less.
Ash Content of Firewood
Ash is the mineral substances that remain after the complete combustion of the organic part of the firewood. The more minerals the tree contains, the more ash it leaves. The ash content of firewood is measured in percentages of the total mass of the firewood fuel. It is divided into internal (ranging from 0.2 to 2.16%), i.e., that which is present in the tree under natural conditions, and external (up to 20%), brought in during harvesting, storage, or transportation (for example, soil particles left after skidding). The latter, as we see, significantly exceeds the former.
The most ash-producing firewood species are:
- Pine – 2.19…1.39%;
- Spruce – 2.23…2.31%;
- Birch – 2.43…0.52%;
- Aspen – 2.73%.
Resin Content
Resin is not only present in coniferous species; they just have it in significantly larger amounts. For example, birch tar is also a form of wood resin. The resin content in wood depends on the species of tree, the location of its growth, the timing of felling, and the duration of drying. Deciduous species release fewer unburned volatile resins.
Conifer logs, especially when burned in conditions of insufficient oxygen, greatly pollute the chimney and furnace. Birch wood is also not recommended for pyrolysis boilers due to the high amount of resinous substances they emit when there is not enough oxygen.
Splitability of Firewood
The splitability of wood depends on its elasticity and density; the higher the elasticity and the lower the density, the easier it is to split. In general, damp fresh wood is easier to split than overly dried wood, but if it’s too wet, it becomes sticky and difficult to split. (Read How to Choose Splitting Wood Axe? Useful Tips for Splitting Firewood)
However, frozen deciduous wood, even when dry, is easy to split, which is why many advise dealing with firewood in winter, especially for hard species of trees. For coniferous species, the time of year does not matter; they can be split in summer or winter. The easiest species to split are chestnut, aspen, beech, and linden. However, defects and knots will make splitting any wood more challenging.
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Shelf Life of Firewood
Contrary to popular belief, firewood has a shelf life. Coniferous species last longer due to the large amount of resin in their fibers, while deciduous species, including birch, have a limited shelf life. Only alder and aspen are well-preserved for more than 2 to 3 years. After this period, the wood begins to rot, and burning rotten wood is dangerous for health as it releases toxic substances. Poplar wood has a very short time before it begins to decay, potentially lasting only a few months.
Physico-mechanical Properties of Wood Used for Firewood
| Species | Density, kg/m³ (average at 12% moisture) | Volumetric specific calorific value, kcal/dm³ |
|---|---|---|
| Birch | 640; hard; tenacious on impact. | 1389…2240 |
| Beech | Hard, durable – 680, withstands high impact loads. | 1258…2133 |
| Hornbeam | Highly durable – 795 | 1654…2148 |
| Ash | Durable – 730, with high impact toughness. | 1403…2194 |
| Elm | 650 – withstands high impact loads. | 1282…2341 |
| Maple | 690 – hard, withstands high impact loads. | 1503…2277 |
| Oak | 750 – hard, durable, calorific value – kcal/dm³. | 1538…2429 |
| Alder | 525 – has average indicators. | 1122…1744 |
| Aspen | ρ12=496, soft, not durable. | 1002…1729 |
| Willow | Extremely soft | 1128…1840 |
| Poplar | 455 – soft | 839…1370 |
| Linden | 495 – soft | 1046…1775 |
| Larch | 660 – durable | 1084…2207 |
| Pine | 500 | 1282…2130 |
| Spruce | 445 | 1068…1974 |
Note: The variation in the specific heat released is explained by factors such as wood moisture, storage conditions and duration, age of trees, and others.
Best Firewood by Application
What kind of firewood is suitable for a heating stove? Firewood for a heating stove should produce a lot of heat, burn long and steadily, leaving ash in the stove rather than on the walls of the chimney. Hard deciduous species possess these properties: oak, birch, ash, maple, and most fruit trees. They contain little sap and resin, which means they don’t produce soot. Deciduous species, in most cases, leave behind not only ash but also glowing coals after burning. This helps to retain heat for longer.
Not all deciduous trees provide good firewood. Aspen, linden, and willow have very soft wood with low heat productivity. However, their performance is slightly better than most conifers.
Conifer wood burns so well that it almost instantly burns through, so you need much more of it than you would of the hard deciduous species. At the same time, it can smoke heavily and soot up.
Firewood for a Fireplace
Fuel for open home systems should not smoke or spark, should give off heat well, and have a pleasant scent. The best species for fireplace fuel are: alder, oak, maple, ash, hornbeam, and beech. You can also use aspen, acacia, and fruit trees for this purpose. Conifer firewood throws sparks and soot, although it has a pleasant scent. They, like birch, soil the fireplace chimney with soot.
Firewood for a Sauna
For a sauna, high temperature, scent, and antiseptic properties are important, which some wood species release when burning.
Good for a sauna are:
• conifers. Their advantage is not in caloric content, but in the phytoncidal properties of their essential oils and pleasant scent;
• birch. It also sanitizes the air, although in a sauna it is sterile anyway, quickly heats the stove and retains heat for a long time thanks to smoldering coals. It also has a pleasant scent;
• alder – heats up the space quickly, doesn’t smoke;
• linden. It aromatizes the air and gives it antiseptic properties, heats the stove well.
What kind of firewood to use for cooking pizza?
For making pizza, as with any other food, you need firewood that provides enough heat, gives it a pleasant, not repelling scent, and doesn’t smoke. A good pizza comes out on oak, ash, maple, beech, cedar, linden, and fruit firewood. Do not use spruce, fir, pine, and birch logs for cooking pizza. The substances present in their composition spoil the taste of dishes.”
Which firewood to choose?
Firewood from alder
Alder wood and bark can be called a universal fuel, it combines all the positive qualities. For harvesting alder as firewood, trees are taken that grow away from water, on uplands. Therefore, they easily dry out and are suitable for heating home stoves. They are stored for a long time, but after 3 years they lose their aromatic properties, so well used in smokehouses.
Alder does not smoke much, that’s why it has been used for heating baths “black” for a long time. It is also good for burning in fireplaces. And if you add birch logs to it, it will be even warmer and burn longer. Alder also cleans soot from chimneys.
Oak firewood
Oak firewood is expensive and rare. They are hard to burn, but give a lot of heat. Even a few sticks, lined with other tree species, will provide a long burning time. Oak leaves long smoldering embers when burning, keeping the stove heated. Oak wood is flavorful and is good for fireplaces, pizza and other dishes. As we know real Italian pizza is cooked on such logs. However, in Italy oak wood is very rare and expensive, so there they are replaced by … grapevine.
But only young oak wood has the qualities described above. The older the tree is, the less heat is given by firewood made from its trunks. There is another disadvantage of oak firewood – they leave a lot of ash after combustion and heavily pollute chimneys.
Birch firewood
Although this is a fast-growing tree, it is suitable for firewood at the age of 40-60 years. Only then does it give more heat than pine and aspen.
Birch wood is easy to burn even if it is not dried or damp, and its bark contains resin, so it is often used for other firewood (e.g. aspen or oak). It burns long and hot, but emits a lot of soot that settles in chimneys. Therefore, it is recommended to use birch wood together with aspen wood, which is characterized by its ability to clean chimneys.
Since ancient times birch wood has been used for heating baths. When burning, they not only well and quickly heat the room, but also disinfect its air. Birch burns completely, leaving no hard coals at the end. This type of firewood is not suitable for cooking kebabs and other dishes – tar gives food bitterness. Birch firewood is not stored for a long time, they rot after 2 years, especially if they lie in the open air. Therefore, you should not stock up on birch wood for a long time.
Ash firewood
Ash firewood burns more intensely and hotter than oak firewood. This is due to the structure of their wood. Inside their trunk there are fiber tubes filled with air, so the wood is heated from the middle outward.
Ash wood has little moisture, dries quickly, and burns with an even flame. This wood does not smoke or contaminate the parts of the stove. Suitable for any purpose except chimney cleaning.
Firewood made of linden
Linden is difficult to ignite, but burns intensely and gives a lot of heat. It is well suited for baths. Since ancient times, honey burned with logs has been used along with linden to cure baths. It was said that this aromatic combination has a beneficial effect on the respiratory system and helps to heal wounds on the skin.
Unfortunately, the health-improving properties of linden wood last only for 2 years. If stored for a longer period of time, firewood from linden will be suitable only for heating the house.
Aspen firewood
Aspen firewood is used to stoke the stove before laying coal, although the aspen itself needs to be ignited with birch bark or paper, this process is difficult for aspen.
It is suitable for cleaning chimneys from soot, does not smoke, does not leave a lot of ashes after combustion, but there is no heat from it.
Because of the cheapness of wood from aspen they can be used for heating bathhouses and fireplaces, but you have to burn a lot of them, they will not spoil and kebab, although the flavor will not add to it.
Aspen is stored longer than birch, well spiked in a fresh state.
Firewood from willow and poplar
This is a cheap, and sometimes free (during the annual planned felling in city parks), but unprofitable type of fuel. Firewood from these woods burns quickly and therefore does not give much heat. To heat a room with their help, it is necessary to put a large number of piles into the stove.
But it is possible to heat houses and bathhouses with them, they do not smoke and do not smoke, although they emit a characteristic aroma and spark strongly. Poplar is not easy to cut, and it can be stored for a very limited time – only a few months.
