Burning wood to heat a home is an excellent budget reducer. Burning wood that is not dry enough can be inefficient and be a fire hazard. I have burned wood for almost 40 years and understand the need for dry firewood. I researched the subject of the correct moisture content in firewood, and this is what I found.
What is the ideal moisture content for firewood? The ideal moisture content for firewood is 15% – 20% moisture. There are many reasons why the moisture in wood affects the wood burning system. This article takes a look at those systems and the reasons to burn dry firewood.
How Wood Burning Stoves Work
Wood burning stoves started to appear in the 1980s shortly after the “Energy Crisis” and the desire for alternative sources of heat. In the 1970’s we were told that due to the “crisis” we all needed to turn down our thermostats and conserve energy.
In 1980, after living through setting my thermostat at 64 degrees F, I decided to look into alternative heat sources. I had access to a reasonable supply of trees, so wood heat seemed a good option.
In 1981, I saw the first homemade air-tight wood stove. It was crude and had pipes and tubes out the sides of it, but it worked very well. There were very few commercial options so building a stove from scratch seemed like the best chance. So, I did!
It was way too big and heated our 1300 sq. ft. home to 80 degrees most days. To cut the heat back, I had to reduce the air input to the stove. What I quickly found out was by doing this, the creosote built up in the chimney quickly.
The stove was connected to an existing masonry chimney. I had to clean the chimney every two weeks to make sure there was not a chimney fire.
This was a common theme for early wood burning stoves. Before air-tight wood burning stoves, a fireplace was the only real option to heat with wood. Fireplaces were not a good option since most of the heat in the room and the house, as a whole, was sucked up and out the chimney. The natural draft of a hot fire and a chimney caused any heat from a fire to be quickly drawn up the chimney, and away from the areas, it was meant to heat.
The fireplace doors (metal and glass) helped reduce the draft, but little heat was introduced into the home the fire was meant to heat. Then the wood burning stove was introduced.
Once the steel and the firebrick inside the stove were heated up, this new device (in the 1980s) could easily heat a room. Fans were added, and they would now heat an entire house.
More importantly, you could load it and have the wood last 6-10 hours depending on the size of the stove. Less wood, more heat! The only downside was by cutting the air supply to the stove; creosote would develop on the walls of the stove and the chimney.
Inspecting and cleaning the chimney became a required activity in the winter burning month’s the loud roar of a chimney fire was not something I or anyone else wants to hear. It can be loud, hot, and scary.
Chimney fires were much rarer with fireplaces, but the first one I experienced was at my parents’ home on a Christmas Eve. My Dad thought putting the wrapping paper in the fireplace to dispose of it was a great idea. It was not! The masonry chimney, which was never cleaned before, got a thorough cleansing by fire. The house rumbled for 10 to 15 minutes, and then it was over.
I did not want that to happen at my house, so I cleaned the chimney regularly. The creosote would build up, but the monthly cleanings would remove it and reduce the risk of fire in the chimney.
Next Generation of Wood Burners
In the mid-1980s, burning wood changed from a novelty to a more mainstream heat source for a home. Names like Buck stove and Sierra Wood stoves were the early manufactures that drove the sales of wood stoves to the market.
With burning wood becoming popular, the government was not far behind. The reduced air in wood burning stoves only increased the creosote that coated the stove and chimney interior. Wood stoves also sent more particulates into the air thus causing additional air pollution.
By 1988 the first regulations were in place. Manufacturers were required to build stoves that reduced the emissions from wood burning stoves.
The regulations were increased in 1998. Stove manufacturers had to develop a process to limit the PM or Particulate Matter that a wood burning stove emitted. A typical 1980 stove would emit 30-40 grams/hour of PM. The 1989 standard was 8.8 grams/hour. They reached this goal by two methods, catalytic stoves, and non-catalytic stoves. Each had a different process, but both were able to reduce the PM’s emitted from the stove in the smoke.
Catalytic Stoves
Catalytic stoves were designed to operate similarly to catalytic converters in vehicles operate. The stove has a by-pass damper that is opened when the wood burner is first lit or cools down. When the stove reaches the desired temperature, the damper is closed, and the smoke is passed through the catalytic converter. The smoke will re-burn at this point which reduces the additional PM’s, making the remaining smoke “cleaner.”
This re-burn does make the new burning stoves more efficient. The smoke that exits the stove is much cooler and also has much lower (70% – 80% lower) emissions that are harmful to humans. Getting more heat out of the wood that is burnt is a positive aspect of the new stoves, but there are some negative results from this.
The new wood stoves do burn much hotter. This makes early fall and late spring home heating an issue. If you only need to raise the temperature of your home 5 – 10 degrees F that may be an issue when the stove requires a minimum temperature of 350 degrees for the catalytic converter to work properly. It will work at a lower temperature but not efficiently and will require cleaning or eventually replacing the catalytic converter. Replacement catalytic converters for wood burners run from $95.00 – $350.00.
Non-Catalytic Stoves
Not a very descriptive name but since it is used, here is how they work. A non-catalytic stove has a baffle installed before the chimney. Pre-heated air is introduced to the smoke as it passes over the baffle. This process re-burns the smoke and causes the PM to be reduced in this process.
Again, 70% – 80% of the PM’s are eliminated in the non-catalytic process. The smoke that exits the chimney is cleaner and cooler.
This makes the stove more efficient and also burn hotter. The non-catalytic stove, while generally not quite as efficient as the catalytic stove, does have an advantage in not having to burn as hot in the low demand times of late spring and early fall.
Non-catalytic stoves also do not require an expensive replacement of the catalytic device, so maintenance costs are reduced.
Pellet Burners
A new wood stove has entered the market place in the past few years. The pellet burning stove. Pellet stoves are typically smaller with a very small firebox. The stove has a large hopper that holds the pellets before burning. The pellets are fed into the fire by gravity or with an auger. An electronic igniter and fan system control the fires burn, and the home is heated to the desired temperature. This is similar to a gas or propane furnace except the wood pellets need to be added to the hopper daily or as required.
The chimney stays much cleaner for several reasons. First, the pellets are dried to the precise moisture content for a “clean” burn. Second, the burn is at the correct (high) temperature to burn clearly. An additional secondary burn in some pellet stoves removes most remaining PM’s to comply with even the new 2020 standards for wood stoves.
Dry Firewood
With an understanding of wood burning stoves, how dry should firewood be? When wood is cut, it can have a moisture content as high as 90%. Even wood will burn if enough heat is applied to it. The problem is more heat is used to dry the moisture out of the “wet” wood than is gained from the heat of the burn.
It is very difficult to consistently heat a wood stove to 400+ degrees F with high moisture content wood. The stove will burn in the 250 – 300 degree F range for hours, burning off the moisture in the wood. Then you will have a short time of good 400 – 500 degree F heat and have to start over again with wet wood on the next load of wood put into the stove.
At the same time, the moisture released as steam will cling to the walls of the stove and chimney. The shiny tar-like substance – creosote – is a fire hazard waiting to happen. If the fire does get too hot while trying to dry out the wood, the walls can ignite turning the tar into a powder-like substance in minutes. If the fire spreads to the chimney, it becomes dangerous.
The wood should be dried or seasoned for a minimum of nine months after cutting and splitting. This allows the maximum amount of air exposure on the wood surface and the quickest drying time. The wood should be stacked in rows and be off the ground on a rack or a pallet. This allows the sun and the wind to do its job and dry the wood naturally.
For wood ready to use, the moisture content should be between 15% – 20%. There are several ways to verify the moisture content. A moisture meter is the most accurate. An inexpensive moisture meter is around $30.00.
Another check is the weight of the wood. A dry piece of wood weighs considerably less than fresh, wet wood.
Some types of wood will develop check marks on the ends when they are dry. This is a split mark starting at the edge of the wood, and sometimes in the center.
You can also use the sound method to check for dry wood. A wet piece of wood sounds solid, making a dull thud when two pieces are hit against one another. Dry wood sounds hollower, the sound is not as deep and is less “solid” in dry wood. This sound is something you hear clearer over time.
Drying Firewood
How do we get firewood to the 15% – 20% moisture content? Drying wood depends on where we are starting; percentage of moisture when cut, how long we have to season the wood, and what the climate is for drying the wood?
The wood should be cut and split as soon as possible to begin the drying process. The wood should be stacked in single rows. Stacking the wood off the ground keeps the wood from absorbing additional moisture from the ground and let’s air pass under the wood drying it even more. A wood rack or pallets will help this process.
Direct sunlight usually works best, and then cover or tarp the wood to remain dry once it reaches the desired dryness.
If you have the room, a woodshed is the ideal drying setting. A wood shed has a roof with an open front and three open walls. The walls are typically 1″ x 6″ with 6″ spaces between for air to move.
Keeping the wood out of the rain is not as important as keeping the wood in warm moving air. The wind will dry the wood in its own time and efficiently.
Wood that is Too Dry
Yes, wood can be too dry. Firewood that is below 15% moisture content is too low. Wood that is dead and has dried beyond 15% moisture content will not burn properly. It burns too fast and is difficult to control. The low amount of moisture that remains in the wood is necessary to control the burn process.
Wood that has been lying in a woods on the ground will become too dry. Trees that have fallen over are usually dead to start with and have stopped drawing moisture up from the ground. Depending on the type of wood they will be too dry to burn properly in two to five years of lying on the ground.
A dead log is also very hard to cut. The moisture in a log is also beneficial to the chain of a chainsaw. The sawdust is pulled away from the wood as it sticks together better when moist. The dry dust tends to clog up and not move away while cutting. This dulls the chain quicker and slows the cutting process.
Dead wood also draws bugs. These bugs such as ants and termites nest in the dry, dead wood. When used for firewood these bugs, which are dormant in the cold, come back to life in a warm home. We have found them crawling around in our family room after falling from a dead log.
Conclusion
Drying wood to the correct moisture content is critical to having a consistent hot wood burner. It makes the process of burning wood more efficient, safe, and environmentally friendly.
Additional Questions
What are the best places to find local firewood? Try tree trimming companies and excavating companies. For additional ideas see my article on, The Best Places to Find Local Firewood!
Are wood burning stoves safe? Yes, the new versions of wood burning stoves are safe. The dangers of wood burning stoves are created when safety items, such as chimney maintenance and cleaning are not followed. Have a look at my article on, Wood Burner Safety!