Which gas is most commonly vented through a chimney?

The gas most commonly vented through a chimney is carbon monoxide (CO). This is primarily a byproduct of combustion processes, especially in appliances that burn fossil fuels, such as gas furnaces, water heaters, and fireplaces. When these fuels burn, they produce carbon monoxide, which is a colorless, odorless gas that can be dangerous if accumulated indoors.

Chimneys are specifically designed to safely exhaust combustion gases, including carbon monoxide, from living spaces to the outside environment. Proper venting is critical for safety to prevent the risk of carbon monoxide poisoning, which can occur if gas is inadequately vented.

While carbon dioxide is another byproduct of combustion, it is not typically vented through the chimney in significant amounts compared to carbon monoxide. Methane, although it is a combustible gas, is primarily associated with unburned fuel rather than combustion byproducts, and nitrous oxide is produced in smaller quantities from high-temperature combustion processes, making carbon monoxide the predominant gas that chimney systems are designed to expel.