CSLB C-20 HVAC ContractorHeating SystemsHard
During a load calculation, a technician must size two permanent combustion air openings using horizontal ducts connecting a mechanical closet directly to outdoors. The closet houses a furnace and water heater with a combined input of 240,000 Btu/h. Per the California Mechanical Code, what is the minimum free area required for EACH opening?
- A120 square inches
- B100 square inches
- C80 square inches
- D60 square inches
Show answer & explanationAnswer & explanation
Correct answer: A. 120 square inches
For horizontal ducts connecting directly to outdoors, the CMC requires 1 square inch of free area per 2,000 Btu/h of total input, applied to EACH of the two openings. 240,000 ÷ 2,000 = 120 square inches per opening.
Why the other options are wrong
- B. This matches the vertical duct rule (1 in²/2,000... actually vertical uses 4,000) — miscalculated ratio.
- C. Undersized based on the 1 in²/2,000 Btu/h horizontal duct rule.
- D. Would correspond to a much smaller input rating.
Combustion Air Sizing — Horizontal Ducts
When combustion air is supplied via horizontal ducts to outdoors, each of two required openings must provide 1 sq in of free area per 2,000 Btu/h of total appliance input.
- Vertical ducts/openings direct to outdoors: 1 sq in per 4,000 Btu/h
- Horizontal ducts: 1 sq in per 2,000 Btu/h (double the vertical requirement)
- Applies to EACH of the two openings, not combined
Memory trick: Horizontal ducts work harder — half the ratio, double the size.