As a wood stove owner I have heated my home with a wood stove almost exclusively for over 40 years. In doing so, I have learned some lessons regarding the process involved as a fire burns in a stove. That fire is a chemical reaction. How a stove is burned can create a different result depending on what is put into the stove. We will explore the fire triangle to help us understand this, but the simple answer is next.
What is that sticky black substance that form on the inside of a wood stove and chimney? It is Creosote. It is the material left behind caused by the lack of air and particles that are not fully consumed in a “hot” wood stove fire. This substance which is left on a wood stove or chimney wall and has a tar-like consistency. It can build up to a half inch thick or even close the chimney off completely if not removed.
Creosote can be dangerous when excessive amounts of it build up in a wood stove or chimney. This build up can cause a chimney fire if not properly removed from the stove and chimney system. Just a few minutes of an uncontrolled fire can ignite the tar-like substance and cause the stove and chimney to burn out of control.
To understand creosote, a description of what makes a fire burn will help.
Composition of a Fire
There are three elements which need to be present for fire to occur. Fuel, heat, and air. If any one of these are removed, the fire will eventually be extinguished. The fuel for a wood stove is obviously wood. The air for a stove is introduced by opening an input air vent in the stove. Air may also be controlled to a lesser degree by opening and closing a damper in the chimney. The heat is created when the stove is lit, usually with a torch or internal gas lighting system.
What Wood to Burn
The wood used in a fire has a large effect on the amount of creosote buildup in a stove. The type of wood is critical and has a dramatic impact on several issues when heating with wood. The denser the wood, such as hardwoods like oak, ash and hickory, the longer they burn and typically the hotter they burn.
No one wants to stoke (add wood to) a stove every three or four hours all day long. That is why hardwoods are typically used when a wood stove is the primary source of heat in a home.
Softwoods like pine and maple burn fast and typically do not heat as well. These soft types of wood are generally full of saps that can contribute to creosote buildup. There are wood stoves that will tolerate softwood better than others but generally, a soft wood burns faster and will build creosote faster than hardwood.
For a more thorough discussion on wood types see my article on What are the Best Types of Wood to Burn?
Moisture Content of Wood
The wood you burn should be properly cured or dried before burning. When wood is burnt that has too much moisture, the moisture has to go somewhere. Just as boiling water in a pan on the kitchen stove creates a steam cloud, the moisture in a log that is too wet is released from the wood and collects on the wood stove walls and chimney walls.
This moisture collects along with the other particles that would typically be burnt up in a very hot fire and builds up on the walls of the wood stove and chimney and becomes creosote.
Air In – What Comes Out?
Once a stove is filled with wood, every wood stove that I am aware of is controlled by the amount of air that is sent to the fire box. There are many different ways that this can be accomplished.
The simplest method is air valves on the door of the stove. These can be a simple spinning gear that covers a slot into the stove. Opening and closing the gear allows more or less air into the firebox. More air creates more fire and less air dampens the fire. These valves are normally on the doors but can be located anywhere (usually on the front of the stove) so that air is introduced to the fire above the ashes that build up on the bottom of the stove.
In many stoves (especially pellet burners) the air is forced into the fire with a small fan. This method heats the fire much quicker and makes for a hotter fire. Using a fan to quickly heat the fire may also be necessary in stoves with a catalytic device that require a hotter fire. This is due to the need to heat up the catalytic device to temperature. The catalytic device will then burn up the particles that would escape the stove in “cooler” burning situations.
Less Air?
To make the wood last longer, most wood stove users cut the air into the stove. This was the basic premise of a wood stove vs. an open fireplace. The fireplace sent all the air in the room, and pulled the air from anywhere it could (which meant every other room in the house) up the chimney. The room where the fireplace was located may be warm, but every other room in the house was cold. The “airtight” wood stove solved some of this issue by reducing the airflow up the chimney.
What Particles
What we soon found out was that reducing the air flow into the stove had a negative effect on the smoke that left the stove. By reducing the air input to the stove, the smoke that now drifted slowly up in the stove and out the chimney would carry additional Particulate Matter (known as PM). At the lower temperatures, the smoke left a residue on the walls of the stove and especially on the chimney wall called creosote.
The tar-like substance would eventually collect and build up to a point where it would clog the chimney. It could reduce the inside diameter of the chimney to a point where the smoke would back up in the wood burning stove and enter the home.
In addition to gumming up chimneys, the PM was viewed by the EPA as a pollutant to the air. Wood stove manufacturers are now required to produce stoves that limit the amount of PM that leaves a stove. Every stove manufactured in the US and Europe must meet standards that continue to reduce this amount of PM.
The Solution
To accomplish the reduced PM the EPA required, manufacturers came up with two solutions to the problem. The first solution is called a catalytic stove the second type is ironically called, a non-catalytic stove. A catalytic stove has a device installed in the burn chamber before the exhaust that is similar to a vehicle catalytic converter. The catalytic device when heated to the required temperature will burn up the PM that would normally be sent up the chimney.
The good side of adding these to a wood stove is, the PM is disintegrated in the catalytic device. The down side is the stove must be operated at a higher temperature. This burns up more wood and also requires a higher heat in the spring and fall when lower temperatures are desired.
The second type, the non-catalytic stove, introduces pre-heated air which causes the smoke that is passed over a baffle to re-ignite and burn off the PM. This method also requires the stove to burn hotter than a standard (pre 1988 models) wood burner.
Both types of stove (catalytic and non-catalytic) are able to remove the PM to achieve the EPA standards required. These standards have been increasing over the years (lower amounts of PM allowed) and the technology has changed to meet these standards.
As the amount of PM has been reduced over the years, the deposits of creosote in chimneys has decreased. With hotter burning stoves and the reduction of small particles being exhausted from wood burners, creosote buildup in chimneys has also been reduced.
No More Creosote?
This does not mean there is NO creosote buildup in your chimney because you have a catalytic (or non-catalytic) wood stove. These stoves reduce the amount of PM in chimneys but cannot eliminate it completely. Chimneys will still have particles buildup on the inside surface. These particles need to be removed manually to provide for a safe burning experience.
I recommend you have a professional inspect and clean the chimney if necessary. These buildups of creosote when overheated can quickly turn into a raging chimney fire. Most chimneys can sustain a chimney fire if it is quickly extinguished (by shutting off the air flow) but should always be inspected after this happens.
Conclusion
Creosote buildup in chimneys has been reduced by the new technology introduced to wood stoves. This makes wood burning cleaner for the environment and safer for the homeowner as less PM (Particulate Matter) is sent up the chimney or building up in chimneys.
Wood stoves are still a simple mechanism. They burn wood for heat. The buildup of creosote should be monitored in any wood stove system and checked by an expert to provide for a safe, efficient system.
Additional Related Articles
If you are looking for a wood stove and not sure which one is for you, check out this article.
The 25 Best Wood Burning Stoves. It describes the features and pitfalls of 25 of the best stoves on the market today.
A new wood stove requires some accessories to operate it correctly. An accurate thermometer is essential for the proper operation of a stove. Check out: The 10 Best Wood Stove Thermometers, to find the best on the market today.