Sunday, September 16, 2012

Heat by Wood: How Much Wood/Work/Time per Year

Many of us, particularly in cities, I think, rely on “black boxes” of magic for the conveniences we enjoy. We click a switch “over here” so that lights or heat or air conditioning or TVs turn on “over there”. When they fail to work, as they do in an era of “planned obsolescence” or a power outage, we either buy something new or enlist shaman-like experts called electricians or plumbers or auto mechanics or computer technicians to wave their magic wands.

Living off-grid in Alaska has made me more aware of all that goes into those things I used to take for granted. The article below outlines how much labor we expend to generate heat for ourselves, and outlines comparisons to alternative heat sources enjoyed by people who don’t live in the middle of a forest.

Since passing chemistry (barely), I hadn’t given a thought to British Thermal Units. In Alaska though, as you can imagine, that concept means a lot. We produce the heat for our cabin by chopping down birch trees that we then cut, age, and stack to stoke a small but efficient wood stove. How much wood do we need? How long does that take to acquire? How long does it last? How much heat does it produce?

The measurement for wood is a cord (4 x 4 x 8 feet). For dry birch, which is the available wood of choice for heating our cabin, this volume weighs about 3000 lbs (“green” wood weighs much more, about 4500 lbs). We store about 8 cords, or 32 x 4 x 8 feet and 24,000 lbs in our “wood corral.”

Since the property we bought had not been previously inhabited, we have been fortunate to find many of our BTUs very close to home, after a windstorm or after excessive rain saturates the soil and rips the shallow roots (because of perma-frost) out of the ground by a pendulum- like swaying top weight. About half the wood derives from trees that were so close to the cabin as to pose a potential fire or falling danger. Another component derives from standing dead trees in the woods.

A forester’s rule of thumb is that one tree, 22 inches in diameter at chest height, or 4-6 saplings (or comparable long boughs) of 10 inch diameter, will produce about a cord of wood. My husband estimates the labor component to fell one tree, buck (chop into logs) and limb it in place (with a chainsaw), transport the rounds of wood to a nearby storage yard by backhoe or snow machine sled or wheelbarrow and then split it up (by axe) into firewood as 80 hours per cord. This work takes about 8 days, spread over a year.

Felling a tree is dangerous and exhausting. We try to mitigate the dangers in a number of ways. The first day I usually go out and weed whack the brush around the tree we have targeted in order to survey the surface ground for holes, stumps, rotted logs hiding beneath the ferns and devil’s club leaves that would impede a steady foothold for my husband. We examine the tree and try to anticipate its natural falling trajectory, given a hill or top weight or indications of rot. Birches are notorious for rotting from the inside out, and on one part of a trunk but not another, so we look for clues, like vertical cracks in the bark of the trunk, or high branches broken off or with no leaves. Because of interior weakness, the tree’s falling direction may be determined less by the angle of the trunk than by the weight and angle of various boughs up above. Either way, we clear an area for an escape route away from the direction where we think the tree will fall.

Next, my husband, who is much more comfortable applying a chainsaw to standing trees than I am, approaches the future BTUs. He wears a hard hat, safety glasses, ear protection, gloves, Kevlar chaps, and hard boots. He carries a chainsaw, a wedge, and a crow bar. I stand further away than the height of the tree, also wearing hearing protection and a hard hat. My job is to watch the tree for any signs that it will fall sooner than expected or in a different direction. I wear a whistle that I hope he can hear through the cacophony of the chainsaw engine. First he cuts a low, horizontal notch about 1/3 the way through tree, at a 30-45 degree angle, on the side of the tree in which direction he believes the tree is naturally inclined to fall (or away from a building!). Next, he retreats to the back of the tree. Here he cuts another notch, higher and deeper than the first. This one is almost 2/3 of the circumference, leaving a “hinge” of untouched trunk between the two cuts (about 1/10 the depth of the tree). If all goes well, this second cut causes the tree to fall forward, as though on a hinge. If the saw cuts into a rotted section, the tree “slumps” onto the saw, pinching it. That’s where the crow bar and some sweating come in. As soon as the tree starts to fall, Bryan retreats from the stump area, since dangers exist the whole time the tree is moving. For example, a branch could get hung up in an adjacent tree and break and fall like the sword of Damocles onto the logger below. Or the trunk can pivot. We had one do this and the very heavy trunk just grazed the corner of our food shed! Sometimes the angle and weight of the upper boughs can cause the “foot” of the tree to kick back several feet behind the trunk, where the logger should no longer be standing.

Once down, we look up to see if anything dangerous is trapped in the tree canopy. If all is clear, we figure out the best way to limb the downed tree. Some branches are obvious, but others are like legs for an enormous centipede. They support the trunk’s position above the ground and have to be worked around until last. As he cuts a branch, I haul it out of the way to clear his cutting zone. The gas tank on our chain saws lasts for about two hours, and by that time, Bryan is tuckered out from the exertion, stress, vibrations, weight, and noise. It generally takes two tanks to limb a large tree, and a third to buck the trunk and large limbs into rounds, done over two or three days. The fourth day involves hauling the heavy rounds back to our utility yard. Some trees are in locations where we can use a back hoe or wheelbarrow. But for others, we can only transport in the winter, by snow machine trailer, so these trees we flag with orange or pink flagging tape so we can identify the standing dead trees.

Then we wait a year for the wood to shed water weight.

The following year, my husband chops the rounds into firewood with an axe. This is so exhausting that, although he is a strong man in good shape, he can only work for about an hour at a time before a rest.  This yields about 10 rounds cut into 40-60 logs. Some of these logs he sets aside for me, and every once in a while I chop them up into kindling (small pieces less than 2 inches thick). For the first few years, I heard how much he loved this activity - it was so meditative and such good core exercise. Now I hear more about sales on log splitters. But until those drop below $1600, I don't see one in his Christmas stocking.

Our volume (8 cords) of nice, dry, aged wood can generate 20.8 million BTUs. If we didn’t leave our cabin during the coldest months of winter, we would probably use 5 cords per year (15,000 lbs taking up 20 x 4 x 8 feet) to heat our little two room cabin. Fortunately, we have lots of trees.

By contrast, to produce close to 20.8 million BTUs by other means (enough to warm a 2 room cabin for a year or more), one would need 136 gallons of fuel oil or 23 therms (2300 cubic feet) of natural gas or 2000 lbs of coal.

Remote Northern Alaska villages pay about $10/gallon for fuel oil, because they live in a tundra environment with no trees. There is current talk about the potential for natural gas and coal in those regions, but tiny and scattered communities will still pay a lot more than a larger population centers. Other methods, such as gas and coal, cannot be bought or produced by a single family for personal use because they are only cost effective if the capitally intensive infrastructure can be spread across hundreds and thousands of homes. Anchorage has just built a “wind farm” on an island in Cook Inlet. It will produce electricity sold to the utility provider for about 6000 homes. Good diversification.

At least wood heat is something we can do ourselves, albeit with a lot of time and effort. So please! Respect what your thermostat and your half cord of fireplace logs represent! Somebody, somewhere, is doing a lot of work so you don't have to.


  1. We heat our house with a wood stove, with "back-up" heat provided by a propane furnace that only kicks on if we are gone for a weekend or more. We also live out in the woods (although in southern Ohio), and I cut all our wood from the surrounding forest. I split about 90% of it by hand with an ax, even though we have a hydraulic splitter inherited from my dad, because I enjoy the work involved (at least for now, going on 7 years that we've heated with wood). My wife is hooked on the kind of warmth that only a wood-burning stove provides, and we love not being dependent on the electric company for heat (our power goes out often, sometimes for days at a time, due to the lines in our area being vulnerable to falling trees and branches). Even though most of the activities you described are familiar, I greatly enjoyed reading this. --Joe in Ohio

  2. Fruits of our common forest trees, though sometimes as inconspicuous and nearly as fleeting as the flowers that produced them, Important link

  3. Thanks for letting us know about the wide range of firewoods
    for various purposes.I have bookmarked this article for future reference.

  4. In preparing to go off-grid in a year or thereabouts, I wanted to know how much Birch would be needed and how long it might take to obtain the amount needed for winters. I am glad I found your article. Thanks.