Wood Burning Stoves

As our population has become ever denser, pollution has become an increasing problem for all of us. During the winter months in many parts of the United States, fine particles and noxious gases from wood stoves were making up to 30% of the pollution problem. Because of this pollution almost all new wood stoves that are sold now must be certified before the dealers are allowed to sell them.

Wood fuel will go through three stages of combustion as the wood breaks down into heat. First stage; heat from the flame has to dry out any moisture that remains in the wood. If the wood is not adequately seasoned there will be a hissing sound and stream will rise off the wood. Because the heat produced at this stage is being used to burn off the moisture, there is very little residual heat left for other purposes.

As wood burns there are three stages that the wood fuel goes through as the fuel breaks down into heat. The first stage begins when the heat from the flame has to dry out any moisture that remains in the wood. If the wood is not thoroughly seasoned you will hear a hissing sound and see steam rise off of the wood. There is very little heat being given during this stage of combustion as all the energy of the frame goes to drying out the wood. Please review our article on preparing your wood for the heating season.

After the first stage has dried out the moisture, the second stage of combustion starts as the gases begin to rise off the wood. The heat is still inadequate to completely burn all the gases that are produced by the heat from the wood, instead the vapors escape up the chimney as unstable gases composed of VOCs (volatile organic compound), tars and charcoal or carbon which can affect the environment and human health. As the VOCs escape up the chimney the gases tend to cool and become very concentrated with moisture that is laden with creosote that clings to the walls of the chimney, and other pollution gases that are carried with the smoke particles.

Stage Three is the charcoal stage, where the wood actually produces heat. As the temperature goes above 600 degrees Fahrenheit, the gases start to burn; the gases are ignited by nearby flame. As the temperature reaches 1000 degrees and more the logs become charcoal that then emits the most heat.

A few examples to illustrate the different stages: If a homeowner uses freshly cut wood or unseasoned wood as the industry calls it, this wood has a moisture content of 40% to 60%. This means that as the wood starts to burn and break down into heat, the available energy (heat) that is produced just goes to dry out the moisture that is in the wood. This will extends stage one burn time so that when you finally get to stage two there is very little fuel left to heat the home.

For our second example: Let’s assume that you have seasoned wood, but you are using an open fireplace or a stove that is old and outdated. The heat from the fuel is not concentrated in the stove, but allowed to exhaust straight up the chimney. The fire never get a chance to become hot enough to ignite the VOCs, consequently efficiencies drop, reducing the amount of heat that can be radiated into the room

Next example lets assume you have great wood, a new certified stove, but you bought a big stove to heat your small living room. Problem now is the room over heats and of course you damper down the stove to cool off the stove. Using stage three for this example. The fire is kept to cool to burn efficiently. Therefore creosote clings to your chimney and smoke once again carries particles and noxious gas that pollute the environment, even though you did everything else correctly.

Loss of efficiency is not the only problem associated with the VOCs that are going up the chimney. The tars and volatile gases are either sticking to your chimney walls or going out of the chimney into the atmosphere, will can create a significant health and environmental pollution. Many studies have documented that children that are subjected to wood smoke in their environment are more likely to develop asthma and other repertory diseases. The particles that are trapped in wood smoke and carried out into the atmosphere can be inhaled in the lungs of people. Some of these particles are so small that they are smaller then the diameter of a human hair. Like asbestos these particle tend to stay in the lungs and over time can buildup creating a carcinogen in the lungs, which may lead to lung diseases.

A typical wood stoves prior to 1988 had fine particle emitting of 244 lbs per year, The new standard implemented by the EPA in 1988 required wood stoves and pellet stoves to have less then 7.5 grams per hour or less, non catalytic stoves required 4.1 grams per hour or less. Most states accepted these standard for any solid fuel appliance sold within their state. WA and CA where two exception that required their own standard of 4.1 grams per hour or less for non catalytic stoves and 2.5 grams per hour or less for catalytic stoves.

New technology was produced by the different manufactures to meet and exceed these standards into what we now call a certified stove. These stoves improved the air-flows and baffling systems to create a very near total combustion of all the smoke and gases, the result not only help cleaned up the environment, but reduced creosote that other wise adhered to the chimney walls creating a potential chimney fire. Efficiencies virtually doubled form the old non-certified stoves, reducing the amount of wood that was consumed during the winter month by about half. There are still a some areas that do not require the new certified stoves, but the stoves that are not certified stoves are not a great bargain even if they cost less as they consume more wood, therefore more expensive to operate then the new certified stoves.

It is always a good idea to consult with your local municipality or talk to one of the local hearth dealer for any restrictions and codes that you may be subject to.