Heating with Wood: An Introduction

Wood has been a source of heat for thousands of years; in recent decades, it has been largely replaced by fossil fuels and electricity. With rising energy prices and a desire to get back to nature, heating with wood is making a comeback. While many think of heating with firewood as burning logs in fireplaces, modern heating stoves have greatly increased the efficiency of burning wood, thus making wood-burning stoves safer and more fuel-efficient while producing more heat and less pollution. In many ways, burning wood is comparable to other heating sources. For those who are new to or interested in switching over to using wood as a heat source, there is some additional effort and knowledge required to maximize efficiency when using this relatively cheap, rewarding, and renewable heat source. This article will touch on wood pellets and explain how one can obtain, split, and store firewood for heating.

Renewable Nature of Wood

In 2020, Pennsylvania grew 368 million feet³ of wood a year after accounting for all mortality. This means that wood in Pennsylvania is a highly renewable resource that grows back after use (FIA, 2022). This is true for the United States as a whole, which has an average annual growth of 10 billion feet³ (FIA, 2022). However, one of the largest threats to the forests of Pennsylvania, and the Northeast more generally, is high-grading (cutting only high-value wood), which damages the short and long-term health of the forest, reduces the health of the ecosystem, and greatly damages the economic value of the forest. Cutting, selling, or using firewood is a great way for forest owners and loggers to harvest low-value wood that cannot be sold through other markets, helps prevent high-grading, and conserves the forests for the future. Firewood harvesting can also take place after harvests to reduce fuel for potentially dangerous fires in wildfire-prone areas, thus helping to conserve the forest.

Pellets

Pellet stoves were invented in the 1930s as a method for sawmills to sell sawdust byproducts. Modern pellets are made by pressing sawdust, wood chips, or other biomass (e.g., rice hulls, alfalfa, and switchgrass) into a pellet. The biomass is dried by the heat from the pressure and friction from pressing it to a very low moisture level. Pellet plants control the mix of wood and moisture in the pellet to maximize heating efficiency, so that a modern pellet stove can burn with 90% efficiency (Hank Stelzer, 2017).

Homeowners can purchase pellets relatively cheaply at most hardware and farm stores in 50-lb bags, while large operations can buy them by the ton directly from manufacturers. Not only are pellet stoves efficient and easy to use, but the cost of buying and installing a modern pellet stove often qualifies as a tax credit.

To use a pellet stove, pellets are loaded into the hopper or loading area. The stove will slowly feed itself to maintain heat. Little pollution is created due to the high heat, controlled air mixture, and low moisture. Pellet stoves require occasional maintenance and may have issues if heated beyond the manufacturer's guidelines. It is important to follow the manufacturer’s guidelines for specific pellet stoves.

Using Wood

What to Burn

While every species of tree can be burned, it is important for firewood burners to know what they are burning, as some woods are more preferred than others. Ideally, conifers like pine, fir, and spruce should be limited in their heating use, as they have a number of secondary compounds that can coat a chimney with substances like tar, soot, and creosote, which may lead to a chimney fire (if the chimney is not cleaned regularly). While many wood burners prefer to burn oak, most hardwood species have similar heat value, which is measured in British thermal units ([BTU], [Table 1]). However, it is important that wood burners check the mix of wood that is burned because burning large amounts of high BTU-producing wood like Osage orange or ironwood can burn through the bottom of some stoves.

Be sure to never burn toxic species like poison ivy, poison oak, poison sumac, or any wood that has been treated or stained, as doing so can release dangerous compounds. Rotting or decomposing wood (Figure 1) should not be burned as they have a high internal moisture, which reduces the heat and produces large amounts of ash and smoke. Additionally, not all fungal smoke is safe.

How to Burn Wood

While heating with firewood conjures the idea of a large fire blazing away in a fireplace, that is not the best method for heating. Modern wood stoves may lack some of the charm of a fireplace, but they are much more efficient. A fireplace is only 10-20% efficient at creating heat (if the flue is left open with no fire, it may have a negative impact on heating), whereas modern wood stoves are 80% or more efficient. Stoves produce less pollution by burning wood at a higher temperature, having more control of airflow, and they can have filters that remove pollutants. The purchase of a modern wood stove may also be covered as a tax write-off.

Where to Get Wood

Finding firewood can be an eternal struggle for those who heat with wood. Many wood burners harvest their own firewood, all for the cost of a chainsaw, an ax, several wedges, personal protective equipment, and a few gallons of gas. When using a chainsaw, be sure to wear personal protective equipment (PPE) and always practice safe chainsaw handling. When harvesting firewood, do not harvest standing dead trees, known as snags, as they can be very difficult to fell and are an important home to many species of animals. One should also avoid harvesting large high-value trees (12 inches or larger in DBH), as they can be worth more as sawlogs than firewood.

Those who own a few acres of land can harvest firewood can be used as a method of improving the forest by harvesting undesirable trees and creating desired habitat. While every forest is different, firewood harvesting is an excellent way to remove low-value stems, invasive species, or trees that cannot be harvested in commercial operations.

Heating with firewood is not just for forest landowners—anyone can buy a permit to harvest firewood from most state and national forests relatively cheaply. Firewood permits may come with some guidance, like harvesting only dead and down wood or slash (waste logging material) from timber harvests, or seasons for cutting. People can also work with local logging companies, landscapers, wood shops, and arborists to harvest slash or other wood byproducts that cannot be sold in large markets. Depending on the local market and company, this material can be free or bought for a few dollars. In municipalities with communal dumps, wood may be picked up or bought from the municipality. One can also find trees or wood that people would like removed by searching local marketplaces or social media. Always remember never to burn treated wood of any kind of plywood, treated wood, or painted wood, as they can release dangerous fumes.

For those who do not want to harvest their own wood, firewood can be bought kiln-dried or air-dried from local firewood cutters, logging companies, or arborists. Depending on the location, wood may be sold kiln-dried or undried, though more communities and states are requiring wood to be dried to prevent the spread of forest pests like emerald ash borer (Agrilus planipennis), oak wilt (Bretziella fagacearum), and Asian long-horned beetle (Anoplophora glabripennis). Kiln-dried wood is dried in a kiln—a large oven—so it tends to have a much lower moisture content than air-dried wood, but it can be more expensive. Wood can be bought unsplit or split and delivered or picked up. Generally, the more work that the buyer asks the firewood seller to do, the more they will pay for it. It is not uncommon for kiln-dried delivered wood to cost $80-100 a cord. While this seems expensive, a cord of seasoned northern red oak (Quercus rubra) can produce the same amount of heat as 159 gallons of fuel oil.

Measuring Firewood

Firewood is generally not sold by the ton, but by the cord—a stack of wood that is four feet high, eight feet long, and four feet in width. It may also be sold by the face cord, which is generally only 16 inches in width (though it may be between 12 and 32 inches). Due to the thinner width, a face cord can contain ¼  to ½ the wood of a cord. Outside of the US, wood is generally sold by the steres or square meter. It takes roughly 3.5 meters² to make a cord. A standard truck bed can generally hold 1.5 cords or 5.6 steres of cut wood (Figure 2).

To determine how many cords of wood one has, measure the stack of wood (in feet); multiply the height, width, and length of the pile, and then divide by 128.

Formula Determining for Cords: (Height x Width x Length) / 128 = x cords

For example, a stack of wood that is four feet high, eight feet wide, and eight feet long is two cords of wood.

Example: (4 x 8 x 8) / 128 = 2 cords

When to Cut Firewood

Most homeowners and firewood cutters do not consider when to cut wood; instead, they harvest firewood when necessary (e.g., after a tree falls and needs to be removed). This is a mistake that can greatly impact the quality of wood and increase the drying time of firewood. The best time to harvest a tree for firewood is in February or March, when the air is crisp, and trees have not begun to bud. The dry winter air helps to dry wood faster than moist springs, and the cool temperatures freeze cells, making the logs more brittle and easier to split. Winter felling works well for all species and sizes of trees, though it may be difficult to access some areas in deep snow.

Fresh-cut wood has a high moisture content—freshly-cut northern red oak can have a moisture content of 60-80%. A log with a high moisture content will greatly lower the efficiency of the stove and increase chances of fungal damage to stored wood, all of which means more wood is needed to heat. While it has been said that harvests should be conducted in the winter when trees have their water in their root system to reduce moisture, that rumor is not backed by evidence. Wood moisture levels remain relatively similar across seasons.

Some firewood cutters prefer the practice of “wilting” cutting—a style of harvesting that is common in Scandinavia. A small- to medium-sized tree is felled when its buds are large and just about to burst. Some cutters will not completely sever the tree—they leave a strip of vascular tissue that connects the tree to the stump to ensure the tree produces leaves or “leafs out.” After leaf out, the strip of tissue needs to be severed, and the tree is left to dry for several weeks. The leaves photosynthesize, which draws moisture from the trunk. Since the trunk is not connected to the roots, the lost water cannot be replaced. When done correctly, this can greatly speed the drying process and reduce the amount of time logs need to dry before burning. This style can also be used when harvesting firewood from trees that have leafed out. Harvested trees may be left unprocessed for a week, but not much longer or rot may set in.

Limbing and Log Cutting

After felling or when cutting slash, firewood cutters need to remove any limbs (“slash”); this process is called “limbing.” While there is no one way to limb a tree, a good way to approach this task can be to start at the base of the tree near the stump, cut two-thirds of the way through the tree in the desired log length, working up towards the canopy. Through this process, the cutter can remove any small or non-weight limbs that may be taken off with a chainsaw. Large limbs that are weight-bearing or under tension can be dangerous to cut, so approach them with caution. When working with wood that is under pressure or bearing weight, be sure to remove as much weight as possible on them prior to cutting, make several cuts to reduce the tension, put wedges in the cuts to hold them open and keep the chainsaws from getting bound. An axe can be very useful when cutting wood under pressure; the cut they make is large enough not to be impacted by tension or pressure. On large limbs, it is very common to use an axe, wedge, and chainsaw to remove them—a few taps to a wedge or chops with an axe are much faster and safer than removing a bound-up saw.

The length of log will vary by the size of the stove or fireplace; generally, the ideal firewood log size is 14-16 inches long. These logs are long enough to make a stable stack and light enough to handle, while reducing the number of cuts needed. Wood smaller than an inch or two in diameter does not produce meaningful amounts of heat and can be left on site to decompose, although some kindling and small wood may be collected to start a fire.

Splitting

Most logs need to be reduced in width by being cut with the grain—wood fiber—or “split” to ensure that it fits into the stove. The width of the log depends on the stove, but wood should generally be split to 2-6 inches wide.

This can be done with a maul ([a heavy ax designed for splitting wood], [Figure 4]), splitting wedge, and sledgehammer (Figure 5), or wood splitter. Splitting wood with a maul or wedge is best done when the wood is frozen, as the cells are frozen and the wood is more brittle than usual. Taking advantage of the weather can make the toughest ironwood crack open with a single swing. Using a splitting maul is generally faster than a hammer and wedge, as a maul can split a clear piece of wood in a single stroke, where wedges need to be set into the log with several taps. Wedges are best for logs with many knots or other defects.

Motorized or pneumatic wood splitters also work well when splitting knotty or tough wood. Due to their mechanical nature, weather is not a factor in their use. Safety glasses, pants, and boots should always be worn when splitting to protect the operator from any shards of wood or metal.

For those who are able, splitting wood by hand is great exercise for one’s back, shoulders, core, and training fine motor control. People splitting wood should always wear eye protection and boots, take care not to overwork themselves and be sure to always lift with their legs when splitting. While mauls and sledgehammers that weigh up to 16lbs are widely sold, there is no need to buy the heaviest tool possible; a 4-8lbs (1.8 or 3.6kg) maul or hammer is able to generate the same amount of force as a heavy tool because the user is able to generate a higher speed with the lighter tool. Selecting a lighter tool allows a user to get less tired when splitting, have better control of their tool, and reduce the likelihood of injury.

After selecting a tool, pick a splitting or chopping block—a large piece of log that the firewood will be split on. The splitting block should be carefully selected, as it needs to handle all sizes of log and catch any missed strokes. When selecting a splitting block, look for a particularly tough species like locust, black walnut, or oak. The block should be between 14-18 inches tall. A taller block will reduce the speed at which the maul or sledge hits the wood and make splitting more difficult. A block that is too short may force the splitter to overextend themselves, which may lead to injuries. A good splitting block can be tough to find, so be sure not to split it after splitting a season’s worth of firewood—there is always more wood to chop and a good splitting block can be hard to find.

Stacking and Drying

Wood should be dried to as low a moisture content (~20%) as possible to ensure it burns cleanly and heats well. This can be done in a kiln or by exposing it to open air, a process called “seasoning.” Stacked wood needs to be allowed to dry rapidly to prevent the wood from becoming a home for fungi or insects, which will lower the heating value of the wood and may make it unsafe to burn. Poorly stored wood will gain or maintain moisture, which makes it either unusable or heat poorly (Figure 7). Taking these few simple steps can easily avoid the waste of time, effort, and money that results from a rotting wood stack.

Drying Site Selection

Selecting a good wood stack site is the first step to fast drying. While many people’s first instinct is to stack freshly split wood against a building to reduce the travel distance, this should be avoided. Instead, wood should be initially dried in an area that is dry, sunny, and gets plenty of wind. This will ensure that wood dries as rapidly as possible. After the wood is dried, it can be moved near or into the home.

Stacking

Contrary to some people’s initial thoughts, a good stack of wood should not be a tight, impenetrable wall—instead, the wood should be stacked just loose enough to not fall over (Figure 6). This maximizes sunlight and airflow, which helps keep moisture low and pests out. The stack of wood should be kept off the ground on pallets or metal tubes to reduce moisture from the ground from seeping into the bottom logs. The top of a stack of wood should be covered with a sheet of corrugated aluminum or tin to reduce precipitation. Where aluminum cannot be used, the stack can be topped with tightly packed split wood that runs perpendicular to the stack or canvas or sheeting that is weighted down can be used as a roof. The stack should never be fully covered in tarp, plastic, or canvas, as this creates a tent that holds moisture and will cause wood to rapidly decompose. A skilled wood stacker can create shapes and designs that are both artistic and functional.

Seasoning

Wood generally takes 6-12 months to dry down to a point that is optimal for burning (~20% moisture). The ideal schedule for firewood is as follows: Wood should be cut by April, split after felling, stacked by May, and allowed to dry or “season” for at least six months (Table 2). Well-seasoned wood should have small splits or “checks” at the end of the log, which shows that most of the moisture has left the wood rapidly (Figure 7). Remember that wood was a (once) living material, so even well-seasoned wood has a shelf-life of a few years (Figure 8). To make sure that wood is used before it rots, it is best not to mix wood from various years but to separate by year and use the oldest wood first.

Wood can be cut, split, and stacked in the summer/fall (e.g., when a tree falls in the yard), but it will take longer to dry as it may have more moisture or miss the summer heat. So, be sure to separate split wood “out of season” from well-seasoned wood to avoid confusing the two and to not introduce moisture to a seasoned pile. While it may have a higher moisture content, do not be afraid to tap into unseasoned wood in a cold winter if stocks are running low.

Safety

While burning wood is generally safe, there are a few safety protocols that one should follow, like always wearing the appropriate protective equipment when using a chainsaw or splitting wood. Moving firewood has spread many forest pests like the emerald ash borer, Asian longhorned beetle, and oak wilt. It is important for the health of the forests that firewood is not moved more than a few miles from where it is cut. Ideally, firewood is cut and burned on the same property. When burning wood, be sure to never overpack the stove or fireplace and clean them regularly to avoid ash and soot buildup. Fireplaces should have a curtain or shielding between the wood and the room to prevent embers from getting out. Finally, stoves should never be heated beyond the manufacturer's specifications.

Conclusion

Wood can be an excellent, low-cost, renewable source of heat that puts users in touch with nature, all while providing a good workout. At first, it can be a bit difficult to find wood, but there tends to be a relatively large supply of cheap wood that can be obtained with some effort. When harvesting wood, try to cut it in the late winter/early spring and split it relatively quickly after felling (Table 2). It should then be stacked in a dry sunny area on a raised platform and allowed to season for at least six months prior to use. If trees present themselves for harvesting outside of the ideal season, do not be afraid to use them, but be aware that they may take additional time to season. For those unable to procure their own wood, it can be purchased cheaper than fossil fuels (depending on the area).

Table 2. A recommended activity calendar for firewood heating. Some species may require more than six months of seasoning.

Acknowledgements

This article would not be possible without the help of Brent Harding, Kevin Cook, Michael Skvarla, and Stone Valley Forest.

Citations

USDA Forest Service. 2020. Forests of Pennsylvania, 2022. Resource Update FS-251. Madison, WI: U.S. Department of Agriculture, Forest Service.

Hank Stelzer. 2017. How to Buy and Sell Firewood. University of Missouri Extension

Hank Stelzer. 2017. Preparing Wood for Your Wood Stove. University of Missouri Extension