CSLB C-20 HVAC ContractorHeating SystemsHard
A confined utility room contains a 100,000 Btu/h furnace and a 40,000 Btu/h water heater. The installer plans to use the single permanent opening method (1 square inch per 3,000 Btu/h) with the opening located within 12 inches of the ceiling, connecting directly to the outdoors. What minimum free area is required for this opening?
- A23.3 square inches
- B46.7 square inches
- C33.3 square inches
- D70.0 square inches
Show answer & explanationAnswer & explanation
Correct answer: B. 46.7 square inches
Total input = 100,000 + 40,000 = 140,000 Btu/h. Using the single-opening outdoor-air method requires 1 sq. in. per 3,000 Btu/h: 140,000 ÷ 3,000 = 46.67 sq. in., which rounds up to 47 sq. in. of free area for the single opening.
Why the other options are wrong
- A. 23.3 sq in would result from using the two-opening 1-in-per-4,000-Btu/h method incorrectly on total input, not the single-opening method.
- C. 33.3 sq in reflects using only the water heater load, an incomplete calculation.
- D. 70 sq in would apply if using a 1:2,000 ratio, which is not the standard single-opening rule.
Single Permanent Opening Method
A combustion air method requiring one opening within 12 inches of the ceiling, sized at 1 square inch of free area per 3,000 Btu/h of total appliance input, connecting directly or via duct to outdoor air.
- Total input of all appliances in confined space must be combined
- Opening must be within 12 in. of ceiling
- Differs from the two-opening method (1 sq in per 4,000 Btu/h from outdoors)
Memory trick: Single opening: divide total Btu by 3,000.