Vertical outside air opening requirements specify 1 square inch per 4000 BTU, not less than how many inches?

Vertical outside air opening requirements specify 1 square inch per 4000 BTU, not less than how many inches?

• A. Not less than 3 inches
• B. Not less than 6 inches
• C. Not less than 1 inch
• D. Not less than 12 inches

Answer: (A)

Vertical outside air openings are sized to ensure enough combustion air is available, using the rule of 1 square inch of opening per 4,000 BTU of appliance input. That area requirement guarantees the flow capacity to support the appliance. In addition, there is a minimum physical size so the opening isn't too narrow to actually allow air to pass. The vertical opening must be at least 3 inches in height, regardless of the calculated area. So, even if the BTU-based calculation yields a small opening, you still must meet this 3-inch minimum height. This is why the best choice is a minimum of 3 inches. The other options would imply a larger minimum that isn’t required by the standard, or a smaller one that would not ensure adequate airflow.

Vertical outside air openings are sized to ensure enough combustion air is available, using the rule of 1 square inch of opening per 4,000 BTU of appliance input. That area requirement guarantees the flow capacity to support the appliance. In addition, there is a minimum physical size so the opening isn't too narrow to actually allow air to pass. The vertical opening must be at least 3 inches in height, regardless of the calculated area. So, even if the BTU-based calculation yields a small opening, you still must meet this 3-inch minimum height. This is why the best choice is a minimum of 3 inches. The other options would imply a larger minimum that isn’t required by the standard, or a smaller one that would not ensure adequate airflow.