Sheet Metal Journeyman Practice Test

A furnace rated at 7,500 BTUH requires how many square inches of free air?

15 6 12 18

Correct Answer:

Explanation:

Providing enough combustion air is essential for safe and efficient operation of a gas furnace. The amount of free air is determined from the appliance’s input rating, using a standard rule: about 1 square inch of free opening per 500 BTU per hour. This rule helps ensure there’s enough oxygen for complete combustion and proper venting of the flue gases. For a furnace rated at 7,500 BTUH, you divide 7,500 by 500, giving 15 square inches of free air required. If you only had six square inches, that opening would support only about 3,000 BTUH; twelve square inches would handle about 6,000 BTUH; eighteen square inches would handle about 9,000 BTUH. Since 7,500 falls in between, fifteen square inches is the appropriate free-air requirement under this rule.

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