Core TradesQuestion 194 of 700
Using the all-outdoor two-permanent-opening method at a rate of 1 square inch per 4,000 Btu/hr per opening, what is the MINIMUM net free area of EACH opening for a 100,000 Btu/hr furnace?
a.10 square inches
b.25 square inches
c.50 square inches
d.100 square inches
Explanation
100,000 Btu/hr divided by 4,000 = 25 square inches for each of the two openings. 10 and 50 misapply the ratio, and 100 square inches would result from using 1 square inch per 1,000 Btu/hr, which is the indoor-air, not all-outdoor, rate.
Law Reference: 2025 CA Mechanical Code, Chapter 7 (Combustion Air)Practice all 700 questions free — no signup required.
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