Which outdoor air opening size is required for a non-draft-control appliance with an input of 150,000 Btu/h installed in an airtight enclosure?

• A. 7 sq in
• B. 11 sq in
• C. 36 sq in
• D. 5 sq in

Answer: (B)

Outdoor air must be supplied to combustion equipment inside airtight enclosures to prevent negative pressure and backdrafting when the appliance relies on natural draft. For a non-draft-control appliance, the minimum outdoor air opening is based on the appliance’s input, using a rule that provides 1 square inch of opening per 15,000 Btu/h. So you divide the input by 15,000 to get the required opening in square inches.

With 150,000 Btu/h, that calculation gives 10 square inches as the minimum. Since openings are chosen from practical sizes, you pick the smallest option that is not less than the calculated minimum. That is 11 square inches.

The smaller options would be insufficient, and the larger option is more than needed. So the required outdoor air opening size is 11 square inches.