What is the min area of a duct supplying combustion air to a 152,000 BTU commercial fuel oil burner.

• A. 24 sq. in.
• B. 38 sq. in.
• C. 50 sq. in.
• D. 64 sq. in.

Answer: (B)

Combustion air duct size is based on the burner’s input and a standard sizing rule: use 1 square inch of duct cross-section for every 4,000 BTU per hour. This ensures enough oxygen for proper combustion. For a burner rated at 152,000 BTU/h, the minimum duct area is 152,000 ÷ 4,000 = 38 square inches. If you picture a round duct, that area corresponds to a diameter of about 7 inches (D ≈ sqrt(4A/π) ≈ 6.95 in). This sizing helps prevent backdrafts and incomplete combustion by delivering adequate air.