Journeyman Plumber ExamGas PipingHard
A plumber is installing a gas-fired commercial water heater in a mechanical room. The manufacturer's instructions specify that the appliance requires 100 cubic feet per minute (CFM) of combustion air. The mechanical room has two permanent openings to the outdoors: a high opening measuring 12 inches x 12 inches and a low opening measuring 18 inches x 6 inches. Are these combustion air openings adequate for the water heater?
- ANo, because the high opening is too small to provide sufficient airflow.
- BYes, because the total free area exceeds the minimum required for 100 CFM.
- CNo, because the low opening is too small to provide sufficient airflow.
- DYes, because the combined free area of both openings meets the 1 sq inch per 1,000 BTU/hr rule.
Show answer & explanationAnswer & explanation
Correct answer: B. Yes, because the total free area exceeds the minimum required for 100 CFM.
For direct outdoor air, common code is 1 sq inch of free area per 4,000 BTU/hr, or 1 sq inch per 10 CFM. 100 CFM would require 10 sq inches of free area. The high opening is 144 sq inches, and the low opening is 108 sq inches. Both are significantly larger than 10 sq inches, making the openings adequate.
Why the other options are wrong
- A. The high opening (144 sq in) is more than sufficient for 10 CFM (10 sq in required).
- C. The low opening (108 sq in) is more than sufficient for 10 CFM (10 sq in required).
- D. The 1 sq inch per 1,000 BTU/hr rule applies to interior combustion air from adjacent spaces, not direct outdoor air based on CFM requirements.
Combustion Air Sizing (Outdoor)
The calculation of required free area for openings that supply combustion, dilution, and ventilation air directly from the outdoors for gas appliances.
- Often based on 1 sq inch per 10 CFM.
- Alternatively, 1 sq inch per 4,000 BTU/hr.
- Requires high and low openings for proper ventilation.
Memory trick: Combustion air needs BIG HOLES, high and low, to FEED the FLAME.