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Constitution
EPA Certified Fireplace
13. Wood Fuel
Hardwood vs. Softwood
Your fireplace’s performance depends a great deal on
the quality of the firewood you use. Contrary to popular
belief, one species of wood varies very little to the
other in terms of energy content. All seasoned wood,
regardless of species, contains about 8,000 BTU’s per
pound. The important factor is that hardwoods have
a greater density than softwoods. therefore, a piece
of hardwood will contain about 60% more BTU’s than
an equal size piece of softwood. Since firewood
is commonly sold by the cord (128 cu. ft) a volume
measurement, a cord of seasoned oak (hardwood)
would contain about 60% more potential energy than a
cord of seasoned pine (softwood).
There are many definitions of hardwood and softwood.
Although not true in every case, one of the most reliable
is to classify them as coniferous or deciduous. Softwoods
are considered coniferous. These are trees with needle-
like leaves that stay green all year and carry their seeds
exposed in a cone. Examples of softwood trees are
Douglas fir, pine, spruce and cedar. Softwoods, being
more porous, require less time to dry, burn faster and
are easier to ignite than hardwoods. Deciduous trees
are broadleaf trees that lose their leaves in the fall.
Their seeds are usually found within a protective pod
or enclosure. Hardwoods fall into this category. Some
examples of deciduous trees are oak, maple, apple,
and birch. However, it should be noted that there are
some deciduous trees that are definitely not considered
hardwoods such as poplar, aspen and alder. Hardwoods
require more time to season, burn slower and are usually
harder to ignite than softwoods. Obviously, you will use
the type of wood that is most readily available in your
area. However, if at all possible the best arrangement is
to have a mix of softwood and hardwood. This way
you can use the softwood for starting the fire giving
off quick heat to bring the appliance up to operating
temperature. Then add the hardwood for slow, even heat
and longer burn time.
Moisture content
Regardless of which species of wood you burn, the
single most important factor that affects the way your
fireplace operates is the amount of moisture in the
wood. The majority of the problems fireplace owners
experience are caused by trying to burn wet, unseasoned
wood. Freshly cut wood can be as much water as it
is wood, having a moisture content of around 50%.
Imagine a wooden bucket that weighs about 8 pounds.
Fill it with a gallon of water, put it in the firebox and try
to burn it. This sounds ridiculous but that is exactly what
you are doing if you burn unseasoned wood. Dead
wood lying on the forest floor should be considered wet,
and requires full seasoning time. Standing dead wood
can be considered to be about two-thirds seasoned.
The problems with burning wet, unseasoned wood are
two fold. First, you will receive less heat output from wet
wood because it requires energy in the form of heat
to evaporate the water trapped inside. This is wasted
energy that should be used for heating your home.
Secondly, this moisture evaporates in the form of steam
which has a cooling effect in your firebox and chimney
system. When combined with tar and other organic
vapors from burning wood it will form creosote which
condenses in the relatively cool firebox and chimney.
See the maintenance section of this manual for more
information regarding creosote formation and need
for removal.
Even dry wood contains at least 15% moisture by
weight, and should be burned hot enough to keep the
chimney hot for as long as it takes to dry the wood out
- about one hour. To tell if wood is dry enough to burn,
check the ends of the logs. If there are cracks radiating
in all directions from the center, it is dry. If your wood
sizzles in the fire, even though the surface is dry, it
may not be fully cured.
Seasoning
Seasoned firewood is nothing more than wood that is
cut to size, split and air dried to a moisture content of
around 20%. The time it takes to season wood varies
from around nine months for soft woods to as long as
eighteen months for hardwoods. The key to seasoning
wood is to be sure it has been split, exposing the wet
interior and increasing the surface area of each piece.
A tree that was cut down a year ago and not split is
likely to have almost as high a moisture content now
as it did when it was cut.
