What is the minimum cross sectional dimension of a combustion air duct?

What is the minimum cross sectional dimension of a combustion air duct?

• A. 4
• B. 5
• C. 6
• D. 3

Answer: (D)

Airflow for combustion appliances must not be restricted by a duct that is too small. Codes set a minimum cross-sectional dimension to ensure enough air can reach the appliance and to keep pressure losses low, preventing backdrafting and incomplete combustion. Because the minimum dimension is 3 inches, a round duct must have a diameter of at least 3 inches, and a rectangular duct must have its narrowest side at least 3 inches. Larger ducts will also meet the requirement, but the question asks for the minimum, which is 3 inches.

Airflow for combustion appliances must not be restricted by a duct that is too small. Codes set a minimum cross-sectional dimension to ensure enough air can reach the appliance and to keep pressure losses low, preventing backdrafting and incomplete combustion.

Because the minimum dimension is 3 inches, a round duct must have a diameter of at least 3 inches, and a rectangular duct must have its narrowest side at least 3 inches. Larger ducts will also meet the requirement, but the question asks for the minimum, which is 3 inches.