A common percentage of Carbon Dioxide (CO₂) in the flue gases of a natural gas furnace ranges from:

In a natural gas furnace, the typical concentration of carbon dioxide (CO₂) in the flue gases is an important indicator of combustion efficiency. A range of 7-9% CO₂ reflects effective combustion, where the natural gas is being burned cleanly and efficiently.

When the CO₂ level is within this range, it typically indicates that there is sufficient oxygen in the combustion process, allowing for more complete reaction of the fuel. This completeness is essential for maximizing energy efficiency and minimizing harmful emissions. It also suggests that the furnace is properly adjusted to maintain an optimum air-to-fuel ratio during operation.

If the CO₂ levels are too low, it may signify that excess air is being used, leading to wasted energy and less efficient heating. Conversely, if the CO₂ levels are too high, it may indicate incomplete combustion, where carbon monoxide (CO) could be produced, presenting a safety hazard and reducing overall efficiency.

In summary, the range of 7-9% CO₂ confirms that the furnace operates efficiently while minimizing waste and ensuring safety.