What is the standard unit for the free area of combustion air openings?

• A. Square inches
• B. Square feet
• C. Cubic inches
• D. Square meters

Answer: (A)

Free area refers to the unobstructed cross-sectional area through which air can flow into the combustion chamber. Because it’s an area, not a volume, it’s measured in two dimensions. The standard unit used for this opening area is square inches, which provides a precise, manageable scale for the typically small openings involved. Using square feet or square meters would describe much larger openings, and cubic inches would measure volume, not area. So the appropriate unit is square inches.