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Wood Heating

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Wood drying  stove

Here in the foothills of the Adirondacks where we live there exists among woodsmen a certain peevish pickiness about the kinds of wood that qualify as firewood.  Hickory, oak and beech top everyone's list along with maple, birch and ash.  Softwoods such as hemlock, white pine, red pine, the firs and spruces are not considered worthy of the woodpile.  Aspen (or popple), technically a hardwood, in its various forms as quaking aspen, big tooth aspen, balsam poplar, cottonwood, etc. receives the blanket categorization of "garbage wood."  Willow and, to a lesser extent, basswood are also considered unworthy of burning.  Pioneer species like gray birch, alder, pin cherry, fire cherry and sumac, as well as understory trees like hop hornbeam, shad bush and blue beech are already labeled by foresters as weed trees and few will take the time to harvest them for burning.  There is a common preference for wood split from the bole of the tree and a rejection of small diameter rounds and "limb wood."  A friend once said to me as he helped me to work on my woodpile, "What are you burning here, twigs?"  As a result, I often see around our neighborhood piles of logs every bit as nice as the ones we milled into the structural timbers of our house and barn, these logs slated for the splitter and woodpile. 

In the face of these many prejudices, for the past twelve years or so Laurie and I have heated our home almost exclusively with the residues of our clearing and building processes.  Every species I have mentioned so far (and some that I've missed) has at one time or another been a part of our fuel supply.  We have learned a great deal in that time which leads to the conclusion that, while these fuel prejudices may have some basis, they result in tremendous resource waste.  For example, the belief that pine is not good firewood stems in part from the myth that burning pine will cause creosote buildup and lead to chimney fires.  In fact red oak contains nearly twice the creosote forming volatiles that pine has by volume and oak's tendency to hold moisture makes it a far greater culprit in creosote buildup.

It helps to understand how creosote formation takes place.  About half the combustible substance of wood is in the form of volatile compounds.  The rest is in the carbonized residue which is left when the volatiles are evaporated or driven off. (Charcoal is manufactured by exposing wood to high heat in the absence of oxygen.  Volatiles are driven off leaving just carbon.)  As a piece of wood heats up in a fire, volatile compounds in the wood leave the wood as gasses.  Any moisture in the wood is emitted as steam.  If the fire is hot enough and there is a supply of oxygen, the volatiles will ignite.  Otherwise they are carried up the chimney and mostly exit as smoke.  However, if the chimney is cool or cold then some of the volatiles condense on the chimney walls like water drops on a beer glass.  These solidify and, if circumstances favoring their formation remain unchanged, over time a layer of creosote builds up in the chimney.  This creosote is highly flammable and, because it consists of volatiles with high combustion temperatures, once it begins to burn it produces intense heat.  The point here is that creosote buildup can occur regardless of the kind of wood you burn and there are easy ways to prevent it from happening.

1.) Make sure your wood is dry.  Water boils at roughly 212 degrees F. to produce steam.  This same steam is what fills the firebox when you try to burn wet wood.  The steam actually cools the firebox making combustion of gasses practically impossible. 

2.) Make sure your stove is sized properly for the space you expect to heat.  Next to wet wood, the most common cause of creosote buildup is too large a stove.  An oversized stove will be consistently underfired either by being damped down or by being fed with insufficient wood for the size of the combustion chamber.  Wood burns efficiently when firebox temperatures reach 1200 to 1500 degrees F.  Some volatiles in wood need temperatures approaching 2000 degrees F. in order to ignite.  To reach these temperatures you need an ample supply of air and the firebox needs to be well stacked with wood.  This is a good reason to have a healthy mix of smaller and larger pieces in your wood supply.  Limb wood and small pieces ignite quickly and quickly raise the fire box temperature to the point where volatiles from the larger pieces will ignite as they are given off.  These high firebox temperatures generally translate into a hot chimney, which is the other component to avoiding creosote.  The hotter the chimney the less condensation will occur.

3.) Make sure the firebox of the stove is properly insulated with firebrick.  Firebrick should line the bottom of the box and the sides excepting usually the door.  (I have seen stoves with firebrick on their doors as part of the design.)  This insulating brick contributes to proper firebox temperatures.  Look inside the stove.  If you can see black sooty buildup inside the firebox this is a sure sign that the stove is burning too cool.  Also look at what comes out of the chimney.  If all you see is heat waves then everything is being consumed.  If the smoke is so thick you can't see through it you can be sure that creosote is condensing in the inside of the chimney.  This is a normal condition when a fire first starts but should disappear quickly.  In any case all chimneys should be inspected frequently for the presence of creosote and cleaned as necessary.

If you have a large stove that blasts you out of the room, understand that it is not necessary to burn it hot all the time.  However it should be fired vigorously for at least a half-hour every day.  Pick a time when the room temperature is lowest (usually morning) and give the stove a good firing.  This will burn off the accumulations of the previous day and, ideally, evaporate any minor buildup in the chimney. 

Getting back to the question of the different species of wood, here again it helps to understand the nature of the beast.  Of the woods I mentioned all of them have roughly the same energy content by weight.  (It is possible that wood grown in radically different circumstances like the tropical rain forest might be different, but in the forests of northeast North America this holds consistently.)  Thus a pound of white pine and a pound of hickory will both yield about 7000 Btu’s (British Thermal Unit - a measure of heat energy) when burned.  White pine actually gives a slightly higher yield at about 7200 Btu’s.  Hickory however weighs twice as much by volume as does white pine.  Since firewood is measured and sold by volume, usually by the cord (128 cubic feet or a stack 4 feet by 4 feet by 8 feet), a cord of hickory weighing approximately 4400 pounds will contain nearly twice the energy of a cord of white pine weighing approximately 2200 pounds.  Likewise a stove full of hickory will last twice as long as a stove full of pine.  None of this diminishes the value of the lighter woods as fuel.  Personally I have come to greatly appreciate our "variety pack" woodpile.  I often find myself rummaging through the wood box looking for just the right piece.  In the morning it might be a couple of chunks of popple or white pine that will leap to flame at the first touch of last night's coals.  Before bed it will be oak or beech in nice big chunks or rounds that I know will hold all night.

Several years ago on a particularly cold January day a friend and neighbor called.  She wondered if we could come over and look at her stove, which she was sure was broken.  It would not burn at all, she said, and smoke billowed into the room when she opened the stove door.  The stove was an Ashley, one of those that looks like a modern appliance with the tan enameled, pebble finish shroud covering the cast iron fire box; a Cadillac of its class.  It sat near an outside wall in the biggest room of her tiny three-room cabin.  Ignoring the stove for a moment we went straight to the chimney, which was concrete block lined with 8" by 8" flue tile.  When I looked inside it I was amazed.  The interior was packed solidly with creosote.  There was no air passage left at all.  Using a twelve-foot length of two-inch cast iron pipe we managed to ram a hole through the mess.  It was the work of an entire day to get the chimney cleaned.  Even the metal pipe from the stove through the wall was clogged.  The neighbor's situation was an illustration of just about all that can be wrong in a wood fuel system.  First, the stove was much too large for the space.  It worked great, she said.  In fact she had to keep it turned down practically all the time.  Her woodpile in the back yard was covered with snow.  She shoveled out what she needed.  Of course, the wood was very wet.  Each day before going to work she would stack the stove full and turn the damper down.  When she came home she would add some wood and open the draft for a little while.  She couldn't leave it open long, she said.  It got too hot.  She'd fill it again before bed and damp it off.  Most of the time she didn't have a fire, she had a smolder.  It's a miracle she didn't have a fatal chimney fire.

Here are some tips to help avoid such a dangerous situation.  First, and to my mind, most important, build a wood shed.  Wood exposed to weather is never dry no matter how long it sits.  Some woods because of their structure are particularly susceptible to moisture.  Red oak and aspen both act like sponges, taking up moisture very quickly when exposed to wet.  The prejudice against aspen may arise from the fact that it won't dry unless it's under cover.  The woodshed should have substantial overhangs and a roof as low as possible while allowing comfortable access. (Wood should not be stacked higher than your head.  A man in Gloversville was killed a few years back when his too high woodpile collapsed on top of him.)  Depending on your best solar exposure, your wood shed should be oriented with its long side facing southeast or southwest.  These orientations allow morning (or afternoon) sunlight to penetrate the shed and provide additional drying, even in the dead of winter.  Wood is bulky fuel.  Make a shed with plenty of room.  A shed that will hold two years of wood, once full and maintained, will be a joy to behold and a source of great pleasure when the snows of winter are swirling about.  As wood is removed from the shed over the course of winter you'll also discover it's a handy place to store other things.  Ours is currently home to three wheel barrows, a bird feeder, a stack of five-gallon buckets filled with compost, sand and other amendments for the spring potting soil mix, bags of sawdust and bean hulls as well as splitting wedges, mauls, etc.  In the summer and fall it can be a great place for hanging herbs or other vegetable matter for drying.

If you're planning on getting a woodstove, take the time to calculate your heating needs.  Modern woodstoves come with information about average Btu output, which makes it possible to get the right size to fit your needs.  If you're considering making wood your primary heating source then you should consider installing a masonry stove or so-called Russian fireplace.  These are typically located in the center of the house and work best with open floor plans although they will serve in any style of house.  The simplest masonry stove consists of a fire box attached to a serpentine flue which winds its way through anywhere from twenty-five to fifty tons of masonry (brick, stone, concrete, etc.)  The firebox is filled with wood and fired at high heat.  As the flue gasses circulate, their heat is absorbed by the masonry.  Slowly, over a period of many hours, this heat is radiated into the living space.  Because the stove is burned at high heat, combustion is complete and very clean.  Practically all the heat from the fire is transmitted to the masonry making this the most efficient form of wood heat available.  Typically the stove is only fired once daily or less.  Only in the severest cold would it be fired more often.  Rob Roy has written books on everything from underground houses and cordwood homes to stone circles (like Stonehenge) and building your home mortgage free.  Somewhere amongst his writings is an excellent treatise on masonry stoves.  Off the top of my head I can't say where but if you're interested in masonry stoves it is worth looking for. 

For more information on the properties of wood and heating with wood I recommend the following books:
Understanding Wood by Bruce Hoadley.  This book is written primarily for the woodworker but is an encyclopedic source of information on wood in general.  In particular the section on moisture and wood is of great interest to anyone heating with wood.
Heating With Wood by Larry Gay.  This book is somewhat dated in terms of stove designs but contains a wealth of information about wood heating.  It also contains compelling demonstrations of both the economic and ecological soundness of heating with wood.
A Reverence For Wood by Eric Sloane.  This book is a mix of history, how-to and technical information.  It is a treasure for anyone who has ever marveled at the smell of wood, thrilled at its feel or wondered at its beauty.

Jim and his partner Laurie take turns shoveling a path to their woodshed in beautiful Meco, NY.

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