For a fuel oil burner installed in an enclosure, the burner is rated at 1.25 gph. What is the free area needed for each of the two required combustion air openings?

• A. 100 sq inches
• B. 175 sq inches
• C. 225 sq inches
• D. 300 sq inches

Answer: (B)

Air for combustion must be sized to match the burner’s energy input. The rule is that each opening provides 1 square inch of free area for every 1,000 Btu/h of input. Convert the burner rating to Btu/h: 1.25 gallons per hour × about 140,000 Btu/gal ≈ 175,000 Btu/h. Divide by 1,000 to get the required free area per opening: 175,000 / 1,000 = 175 in². Since there are two openings, each needs 175 square inches of free area.