Carbon monoxide (CO) is a colorless, odorless, and tasteless gas that is a byproduct of burning any fuel source, including wood. Burning wood absolutely gives off this gas. CO is sometimes called the “silent killer” because it is undetectable by human senses but can cause serious illness or death by replacing oxygen in the bloodstream. Understanding the chemical processes that create CO, the conditions that increase its production, and the proper safety measures is necessary for safe wood burning.
The Chemistry of Incomplete Combustion
The production of carbon monoxide during wood burning is governed by the principles of combustion chemistry. When wood burns in the presence of sufficient heat and a generous supply of oxygen, complete combustion occurs. This ideal reaction converts the fuel’s carbon content primarily into carbon dioxide (CO2) and water vapor, releasing the maximum amount of heat. This is the most efficient and cleanest type of burn, resulting in minimal smoke.
However, wood burning appliances rarely achieve perfect conditions, leading to incomplete combustion. Incomplete combustion happens when there is a limited oxygen supply or when the burning temperature is too low. Instead of fully converting carbon to carbon dioxide, the process yields carbon monoxide (CO), a partially oxidized carbon molecule. This less efficient reaction also produces unburned carbon, seen as soot or smoke, and releases less heat than a complete burn.
In a real-world scenario, some degree of incomplete combustion is unavoidable, meaning CO is always produced. When wood is heated, it first undergoes pyrolysis, releasing organic gases that then burn. If these gases do not have enough time, temperature, or oxygen to fully combust, they escape as smoke, carrying high concentrations of carbon monoxide and other harmful particulates.
Environmental Factors That Increase CO Risk
Several operational and environmental factors can shift the burning process toward incomplete combustion, significantly increasing the hazard of CO production. One common issue is restricted airflow, often caused by improperly setting the appliance’s dampers. Reducing the air intake starves the fire of the oxygen needed for a clean burn, causing the fire to smolder and release high levels of carbon monoxide. A low-temperature, smoldering fire produces more CO than a hot, actively flaming fire, which is why actively managing the air supply is important.
The type of fuel used also plays a major role in CO output and overall safety. Burning wood that is wet or “green,” meaning unseasoned, requires a large amount of the fire’s energy to boil off the trapped moisture. This moisture-removal process lowers the overall temperature of the fire, promoting the inefficient incomplete combustion that generates more CO. Wood should be seasoned for at least six months and ideally have a moisture content below 20 percent for safest use.
Blockages within the chimney or flue system present a risk by preventing the exhaust gases from properly exiting the home. Creosote, a tar-like residue that accumulates from smoke, can build up over time and restrict the flow of combustion byproducts. Other obstructions, such as bird nests or debris, can also impede ventilation, causing CO to back up into the living space. Using materials like treated wood or household trash in the fire can introduce additional toxic chemicals and increase CO output.
Essential Safety Measures and Detection
Mitigating the risk of carbon monoxide exposure begins with implementing a multi-layered safety strategy, starting with certified CO detectors. These devices provide the only reliable warning for the presence of the odorless gas and are a necessary safeguard in any home with a combustion appliance. Detectors should be installed on every level of the home, especially near sleeping areas, to ensure occupants are alerted even while asleep.
For the room containing the wood-burning appliance, the CO alarm should be placed on a wall or ceiling between 1 and 3 meters horizontally from the appliance. Placement on a wall should be high, about 5 feet off the floor, or near the ceiling, following the manufacturer’s specific instructions. Testing the detectors regularly and replacing them according to the manufacturer’s recommended lifespan is necessary to maintain functionality.
Regular maintenance of the entire heating system is essential for prevention. A professional chimney sweep should perform an annual inspection and cleaning of the flue and chimney. This service removes accumulated creosote and checks for any structural damage, leaks, or blockages that could compromise the exhaust system.
Beyond maintenance, adopting safe burning practices further reduces the hazard. Always burn dry, properly seasoned wood to encourage a hot, clean burn. Ensure that the fire has an adequate supply of air, avoiding the practice of shutting down the stove too early or letting the fire slumber overnight, which promotes CO-producing incomplete combustion. Keeping the appliance doors closed except when loading or stoking the fire also prevents the release of gases directly into the room.