When a single permanent opening, from the outdoors, is provided for combustion air a vertical or horizontal duct shall have a minimum free area of 1 square inch per ______ Btu/h of the total input of all equipment in the enclosure.

When a single permanent opening, from the outdoors, is provided for combustion air a vertical or horizontal duct shall have a minimum free area of 1 square inch per ______ Btu/h of the total input of all equipment in the enclosure.

• A. 6000
• B. 5000
• C. 4000
• D. 3000

Answer: (D)

Providing enough combustion air is about ensuring a safe, unobstructed flow of outdoor air into the space that houses fuel-burning equipment. When there’s a single permanent opening from outdoors, the required minimum free area of that opening is based on the total input rating of all appliances in the enclosure. The code uses a simple ratio: for every 3,000 Btu/h of total input, you need 1 square inch of unobstructed (free) opening. That’s why the blank is filled with 3,000 Btu/h. Think of it this way: if all the appliances in the room total 9,000 Btu/h, you’d need at least 9,000 / 3,000 = 3 square inches of free opening. The actual free area should account for any obstructions (screens, louvers) so you may need a larger opening to meet the required free area.

Providing enough combustion air is about ensuring a safe, unobstructed flow of outdoor air into the space that houses fuel-burning equipment. When there’s a single permanent opening from outdoors, the required minimum free area of that opening is based on the total input rating of all appliances in the enclosure. The code uses a simple ratio: for every 3,000 Btu/h of total input, you need 1 square inch of unobstructed (free) opening. That’s why the blank is filled with 3,000 Btu/h.

Think of it this way: if all the appliances in the room total 9,000 Btu/h, you’d need at least 9,000 / 3,000 = 3 square inches of free opening. The actual free area should account for any obstructions (screens, louvers) so you may need a larger opening to meet the required free area.