CSLB C-20 HVAC ContractorVentilation and Air DistributionHard
A 100,000 BTU/hr gas furnace is installed in a confined space that obtains all combustion air from outdoors through two permanent openings, one near the top and one near the bottom of the enclosure. Per CMC combustion air sizing rules (1 sq in per 4,000 BTU/hr for outdoor air openings), what is the minimum required free area for EACH opening?
- A10 sq in
- B100 sq in
- C50 sq in
- D25 sq in
Show answer & explanationAnswer & explanation
Correct answer: D. 25 sq in
For outdoor air supplied through two openings, each opening must have 1 sq in of free area per 4,000 BTU/hr of total input. 100,000 ÷ 4,000 = 25 sq in required for each opening (top and bottom).
Why the other options are wrong
- A. Uses too large a divisor for the calculation.
- B. Uses the indoor-air single-opening method instead of the two-opening outdoor method.
- C. Doubles the requirement incorrectly.
Combustion Air Sizing (Two Openings, Outdoor Air)
When combustion air comes from outdoors via two permanent openings, each opening requires 1 sq in of free area per 4,000 BTU/hr of appliance input.
- Openings placed within 12 in. of top and bottom of enclosure.
- Indoor air two-opening method instead uses 1 sq in per 1,000 BTU/hr.
- Confined spaces need adequate combustion, ventilation, and dilution air.
Memory trick: Outdoor air: four-thousand per inch.