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Thursday, April 23, 2015

Wood Heating Primer

A question came up about wood heat and the pros/cons of the different types, so I thought it was time for a refresher. Wood heat is mostly radiant heat rather than convective heat, so it will feel different than what most people are used to feeling from a furnace.

Wood heat has two components: the wood and the heater.

The Wood

As a fuel, wood is probably our oldest source of stored energy. Almost any wood will burn, but some types are better suited to use for heating than others.

Wood is measured in old, odd terms. A rick of wood is a stack* 16” wide, 4 feet tall and 8 feet long. Three ricks equals one cord, or a stack 4'x4'x8'.
*Notice I said “stack” and not “pile”. Wood is kept stacked for reasons I'll cover later, but piles are hard to judge for volume. Rectangular stacks are easier to measure to ensure you're getting what you pay for. Unless you have access to a scale and are paying by the ton, avoid buying wood in piles- there won't be as much there as you think.
A cord of wood is the standard unit of measure, and that's what most references will use. For those of you who have never worked with firewood, a full-sized (8 foot box) pickup will hold 2 ricks (2/3 of a cord) if filled to the top of the box. A common (6 foot box) pickup will hold about ½ of a cord.

Hard vs. soft
Hard woods are more dense and contain more “heat” than the softer woods, but they are more difficult to cut and weigh more per cord. For a comparison of some common species, here's a chart of the density and heat content. For more specifics like fragrance and ease of splitting, this chart has more species listed.

If you look at the charts, you'll see that there is a large difference in the heat content between a hardwood like Oak (~30 million BTU/cord) and softwoods like Pine (~15-20 million BTU/cord). Put simply, it will take you 1.5 to 2 times as much Pine to get the same amount of heat as you will from Oak.

Cutting and curing
Cutting wood is a chore, no two ways about it. The standard saying is that wood heat will warm you at least three times; once when you cut it, again when you split and stack it, and finally when you burn it.

Felling a tree is not something I can teach with words; it has to be shown. There is a bit of an art to being able to read a tree as it stands and putting it on the ground where you want it, so I'll assume that most of us will be dealing with trees or branches that are already down.

Once a tree is on the ground and has lost its leaves, it becomes a bit harder to determine what kind of tree it was. Looking at the bark, structure (how the branches grew), and the rings inside the wood will give you clues and if you have a good field guide you should be able to narrow it down. For use as firewood, all you really need to worry about is the density since that is a good indicator of the heat content. Dense, tough wood is better but you may need to burn whatever you can find. Avoid wood that is rotted through or spongy, as it will have very little heat content left in it.

Firewood is normally cut into 16” lengths. That's why a rick is 16” wide: it's one piece stacked on another with none going end-to-end. 16” is a good length for carrying and it will fit into most any wood stove or fireplace. If your stove or fireplace has a larger opening you can cut your wood to size and save a few trip when carrying wood.

Firewood is split into chunks that are manageable. This will vary from person to person, but most people can't comfortably carry a piece of wood larger than about 8” diameter and 16” long. Thicker pieces will need to be split into wedge-shaped chunks, and the splitting will actually help the wood cure and dry faster. Splitting round logs also makes them easier to stack, since they are less likely to roll if they have at least one flat side. Split wood also increases the surface area available for flame propagation, which is a fancy way of saying it makes it burn faster. Splitting is done with a hydraulic ram if you're lucky, or an axe/maul if your' not so lucky. If you're being punished for something you did in a past life, you'll get stuck with a sledgehammer and some steel wedges. Different types of wood split differently, and variables like how dry the wood is, the temperature of the wood, knots and crotches in the piece of wood can make this either a breeze or a nightmare. I've learned that the best time to split wood is after is has been cut, but before it has dried or cured, and then waiting for a day where the temperature drops well below freezing. The residual moisture in the wood will freeze and when you hit it with an axe or maul, it just “pops” apart.

All firewood should be cured for at least two years before being burned. If you're cutting your own wood, look for dead trees that have already lost their bark for fuel for the first year or two and cut enough to store greener wood for the third and fourth years. You'll need to plan ahead for your fuel needs and it is always better to have some wood left over in the Spring than to have to go out and cut wood in the snow. The smaller the pieces you split the wood into, the faster it will dry, but it will also burn faster once it's on the fire.

Wood stores best outside in stacks with space for air to flow through. A tarp or roof over the stack to keep the rain and snow off is important if you're going to store it for more than a few months. Rain will accelerate the decay processes and snow will cement the wood together. Trust me, it's no fun having to break firewood out of a block of ice, just so you can carry it into a nice warm house where the ice melts before you can burn the wood.

Most people who burn wood will keep a day's worth inside near the stove and restock as the weather allows. Old houses that relied on wood heat often had a wood bin that penetrated the outside wall, with doors on both the inside and outside. You could fill the bin from the outside and empty it from the inside.

Stacked wood will attract vermin. The air passages make good burrows for mice and rats and I've heard of badgers and woodchucks taking up residence in wood sheds. For this reason you don't want to stack wood directly against your house, leave a good break between the wood and the house to discourage vermin from moving in with you. Cats seem to like wood piles since they provide them with a good place to get above their prey and it also gives them a good place to lie in the sun.

A lot of trees have insects living in them. Ants, termites, and roaches are the most common and when winter hits they tend to go dormant right up until the time you bring the piece of wood into the house to have on hand to feed the fire. That's when they thaw out and start looking around for a nicer place to live, namely with you. Check your firewood before you bring it in and if it's full of crawly things, chuck into the fire before they wake up.

Pellets and other types of wood-like fuel
  • Pellet stoves are a fairly new type of wood stove that uses extruded pellets of sawdust (a waste material) held together with a binder (usually starch or some other natural glue) as a fuel source.
  • Compressed sawdust logs are a novelty item, for when you want a pretty fire in the fireplace and aren't worried about the heat output. They burn for a long time, but don't put out much heat.
  • There are machines that can recycle newspaper and other scrap paper into a burnable form, but I have no experience with them. Most newspaper should be safe to burn since the inks are now predominantly made from soybean oil instead of petroleum.

Emergency Supplies
  • Pallets are generally made of cheap wood, but can be found anywhere. Beware of “fatwood”, a greasy, fatty wood found in the South that is often sold as a fire starter. I have seen pallets made of this stuff and it is messy. Fatwood burns very hot and very fast, and can overheat a stove.
  • Furniture. Real furniture is made of hardwoods like oak, walnut, teak, and hard maple. Even cheaper sofas and chairs will have a real wood frame buried inside them, and while not as good in quality, they're easy to find.
  • Corn. Believe it or not, we grow enough corn in the USA to be able to burn food for heat. Many of the pellet stoves come with an attachment or adapter plate to allow you to switch over to corn (maize) if it is cheaper than pellets. I have also seen corn mixed into the coal being fed to industrial boilers. When the price of corn is low (like it is right now) or if you find yourself with several thousand bushels of corn and nowhere to sell it, this may be an option for those who live in fly-over country.
  • In the Plains, where trees were scarce, early settlers and Native Americans burned well-dried buffalo dung. Other areas of the world with similar lack of firewood have done the same.

What Not to Burn
  • Pressure treated. Creosote pressure treating isn't very common any more with the EPA stomping on petroleum products that can leach into soil and water, but there's still a lot of black, tarry wood out there. Railroad ties and bridge planks are common in a lot of places and should never be burned where you breathe. One of the newer methods of treating wood (CCA or green-treated) to prevent insects and microbes from destroying it is based on arsenic (not good to breathe). The other method (CQA or brown-treated) is based on copper oxide and a fungicide -- not as bad as arsenic, but still not good to breathe.
  • Adhesives and particle board. The crap furniture that you get from Wally-world is made of sawdust held together with adhesives. The sawdust will burn just fine, but the adhesives can produce toxic fumes when burnt. Watch the flames when you light a piece of suspect wood- any color other than the normal yellow means you're burning something other than the wood.

The Heater

Air-tight Wood Stoves
All new wood stoves are built as “air-tight” stoves and most have an option for drawing outside air in for the firebox. Air-tight is a bit of a misnomer, since they do leak a bit, but they are a lot tighter around the seams and joints than an old cast iron stove. Every airtight stove I've seen is either of welded construction or made of a mass of stone, making the stove a large, heavy piece of gear. The major advantage to having a stove airtight is to allow better control of the air that enters the combustion chamber, allowing you to better control the burn rate and heat output. Airtight stoves are also more efficient at getting the most heat out of a quantity of wood.

Non-airtight Wood Stoves
A lot of old stoves were made of cast iron pieces that are bolted together. There is usually an asbestos rope gasket placed at the joints to fill the gaps and slow down the air leaking into the firebox. Advantages include the ability to break the stove down into pieces for transport (important if you're setting up in a remote location) and generally lower prices. Disadvantages include much lower efficiency, they're “dirtier” to run due to the fact that they will leak around any seam and smoke tends to get out,

External (HAHSU)
Some wood-burners are actually set up with the fire box placed outside, near the house, with either hot air or heated liquid piped into the house. These Heat and Heat Storage Units (HAHSU) have the advantage of larger fireboxes, capable of taking larger pieces of wood and burning for longer and thus requiring fewer feedings. Since the firebox is outside, it doesn't draw air for combustion from the living area of the house, making them more efficient than most fireplaces and regular wood stoves.

Pellet Stoves
Most pellet stoves have a hopper that you'll fill with fuel and an auger that delivers it to the combustion chamber at a steady rate. Depending on the size of the hopper, a pellet stove can run unattended for up to a day. Pellets can be purchased in bags or by the ton and provide a “cleaner” form of heat. Pellets don't attract insects or provide shelter for vermin the way a stack of firewood can, and they're easier to handle. Availability and price of the pellets can vary greatly by the season and vendor, with some vendors only keeping them in stock during the heating season or not carrying them at all if there isn't enough demand.

Wood Burning Furnaces
Often sold as multi-fuel furnaces, they will have more than one combustion chamber with each set up for a different fuel. Wood/oil and wood/gas models are available and they install and operate much like a standard oil or gas furnace. Having flexibility in your fuel is nice, but they will require electricity to run the blower and move the air through the normal duct work. One disadvantage of a wood-burning furnace is the fact that most of them are placed in a basement and that can make it hard to get wood to it and remove the ashes from it without making a mess.

A fireplace is common enough that I shouldn't have to describe it. The main problems with a standard fireplace are:
  • They use the air that they are heating for combustion. 
  • Since heat rises and the fire is at the base of a vertical chimney, fireplaces can actually suck more air out of a room than they can heat. This is known as negative efficiency.
  • Without a proper screen, burning knotty wood (which pops a lot), can put you at risk of setting your floor on fire.

Campfire/ Open fire
For cooking, a well-tended open fire is actually more efficient than either gas or electricity. Since wood heat is radiant heat, an open fire heats in all directions. Sitting around a campfire everyone gets warmed (even if the smoke follows you where ever you move). The Native Americans had figured it out with a tipi and a fire in the center: Everybody got a fair share of the heat produced, and since there is a hole at the peak for a chimney, the door controlled the temperature by letting in fresh air.

Final Thoughts
Trying to distill over 40 years of experience burning wood as primary and secondary heat hasn't been easy. I've had to delete or leave out a lot of information in the interest of keeping this article to a reasonable length and it still grew to twice my normal. Given the time to develop a lesson plan and a classroom, I could easily turn this into a three-day class and still wouldn't be able to cover everything.

If you're thinking about installing wood heat or have “inherited” a wood-burner with the purchase of a house, look around for a local contractor to get recommendation. Local types of wood, normal heating seasons, and proper cleaning of chimneys are best dealt with using experience from your 

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