In calculating free area of combustion air openings, what percentage of the opening is obstructed by metal louvers?

The calculation of free area for combustion air openings is vital in ensuring proper ventilation for gas-burning appliances. When metal louvers are present in these openings, they typically obstruct airflow to some degree due to their design, which can partially block the passage of air.

In practice, the industry standard recognizes that metal louvers can obstruct airflow significantly, and thus, it is commonly accepted that about 25 percent of the total opening is obstructed. This means that only 75 percent of the area is effectively usable for air intake. By acknowledging this percentage during the calculation of free area, inspectors can ensure compliance with safety standards and adequate supply of combustion air, avoiding risks related to insufficient ventilation, such as backdrafting or carbon monoxide buildup.

Other values, such as 10 percent or 50 percent, might underestimate or overestimate the obstruction caused by louvers, leading to potential issues with combustion safety and efficiency in gas appliances. The choice of 75 percent effective area aligns with practical experience and standards used in the field, which is why recognizing a 25 percent obstruction is the accepted figure in the calculation.