For an appliance without a draft hood and an input of 235,000 BTUH, the combustion/dilution air inlet size is which of the following?

For an appliance without a draft hood and an input of 235,000 BTUH, the combustion/dilution air inlet size is which of the following?

• A. 7 sq inches
• B. 18 sq inches
• C. 29 sq inches
• D. 42 sq inches

Answer: (A)

When an appliance doesn’t have a draft hood, the combustion/dilution air inlet is sized based on the appliance’s input rating so there’s enough air for safe combustion. The standard rule is 1 square inch of free opening area for every 33,000 BTU per hour. For 235,000 BTUH, you divide 235,000 by 33,000, which gives about 7.12. The practical result is 7 square inches of inlet area. So the inlet should provide roughly 7 square inches of free area. The other larger options would oversize the opening for this input, which isn’t required by the sizing rule. If you’re combining multiple openings, you’d add their free areas to equal the total of 7 in^2, and ensure openings remain unobstructed.

When an appliance doesn’t have a draft hood, the combustion/dilution air inlet is sized based on the appliance’s input rating so there’s enough air for safe combustion. The standard rule is 1 square inch of free opening area for every 33,000 BTU per hour. For 235,000 BTUH, you divide 235,000 by 33,000, which gives about 7.12. The practical result is 7 square inches of inlet area.

So the inlet should provide roughly 7 square inches of free area. The other larger options would oversize the opening for this input, which isn’t required by the sizing rule. If you’re combining multiple openings, you’d add their free areas to equal the total of 7 in^2, and ensure openings remain unobstructed.