Outdoor air supply opening required for appliances without a draft hood and input of 125,000 Btu/h.

Outdoor air supply opening required for appliances without a draft hood and input of 125,000 Btu/h.

• A. 3 inches
• B. 4 inches
• C. 5 inches
• D. 6 inches

Answer: (B)

Outdoor air supply openings provide combustion air from outside for appliances that do not have a draft hood. The size of that opening is set by a code table that connects the appliance’s input rating (BTU/h) to the minimum outdoor opening diameter, ensuring enough air is available to support safe, complete combustion and to prevent negative pressure or backdrafting. For an appliance without a draft hood with an input of 125,000 Btu/h, the table specifies a 4-inch diameter opening. This size yields about 12.6 square inches of air passage, which is enough to supply the needed combustion air for that level of input without being excessively large. A smaller opening (like 3 inches) would typically not provide sufficient air for this input, while a larger opening (5 or 6 inches) would exceed the minimum requirement but is not necessary for this rating.

Outdoor air supply openings provide combustion air from outside for appliances that do not have a draft hood. The size of that opening is set by a code table that connects the appliance’s input rating (BTU/h) to the minimum outdoor opening diameter, ensuring enough air is available to support safe, complete combustion and to prevent negative pressure or backdrafting.

For an appliance without a draft hood with an input of 125,000 Btu/h, the table specifies a 4-inch diameter opening. This size yields about 12.6 square inches of air passage, which is enough to supply the needed combustion air for that level of input without being excessively large. A smaller opening (like 3 inches) would typically not provide sufficient air for this input, while a larger opening (5 or 6 inches) would exceed the minimum requirement but is not necessary for this rating.