Outside air for vertical combustion is defined as an opening size of 1 square inch per 4000 BTU, not less than 3 inches.

Outside air for vertical combustion is defined as an opening size of 1 square inch per 4000 BTU, not less than 3 inches.

• A. 1 square inch for every 4000 BTU opening not less then 3inch
• B. 1 square inch per 2000 BTU
• C. 2 square inches per 4000 BTU
• D. 4 square inches per 8000 BTU

Answer: (A)

Outdoors air for vertical combustion is sized by a simple rule: for every 4,000 Btu per hour of the appliance input, you need 1 square inch of outside air opening. In addition, there is a practical minimum dimension requirement, not to be smaller than 3 inches, so the opening isn’t too narrow to flow air effectively. The combination means the correct description is that the opening area should be 1 in² per 4,000 Btu/h, with openings not less than 3 inches in their smallest dimension. For example, a 12,000 Btu/h appliance would require 3 in² of outside air opening, and the opening configuration must include at least one opening that respects the 3-inch minimum dimension rule. The other options don’t match the established ratio (1 in² per 4,000 Btu/h) used for vertical combustion air, so they’re not correct.

Outdoors air for vertical combustion is sized by a simple rule: for every 4,000 Btu per hour of the appliance input, you need 1 square inch of outside air opening. In addition, there is a practical minimum dimension requirement, not to be smaller than 3 inches, so the opening isn’t too narrow to flow air effectively. The combination means the correct description is that the opening area should be 1 in² per 4,000 Btu/h, with openings not less than 3 inches in their smallest dimension.

For example, a 12,000 Btu/h appliance would require 3 in² of outside air opening, and the opening configuration must include at least one opening that respects the 3-inch minimum dimension rule.

The other options don’t match the established ratio (1 in² per 4,000 Btu/h) used for vertical combustion air, so they’re not correct.