CSLB C-20 HVAC ContractorVentilation and Air DistributionHard
A natural draft water heater is installed in a utility closet using the alternate single-opening combustion air method per the California Mechanical Code. The single opening is located within 12 inches of the ceiling and communicates with a space having adequate infiltration. What is the minimum required free area for this opening, expressed per BTU/hr of appliance input?
- A1 sq in per 5,000 BTU/hr
- B1 sq in per 4,000 BTU/hr
- C1 sq in per 3,000 BTU/hr
- D1 sq in per 1,000 BTU/hr
Show answer & explanationAnswer & explanation
Correct answer: C. 1 sq in per 3,000 BTU/hr
Under the single-opening combustion air method, the opening must be sized at a minimum of 1 square inch of free area per 3,000 BTU/hr of total input rating of all appliances in the space, provided it is located within 12 inches of the ceiling and the space has adequate infiltration. This differs from the two-opening method, which typically uses 1 sq in per 4,000 BTU/hr per opening when connecting to outdoors.
Why the other options are wrong
- A. This ratio is less restrictive than code allows and would undersize the opening.
- B. 1 sq in per 4,000 BTU/hr applies to the two-opening method connecting directly to outdoors, not the single-opening method.
- D. This ratio is more restrictive than code requires and does not match the single-opening method sizing factor.
Single-Opening Combustion Air Method
An alternate method allowing one opening near the ceiling to supply combustion air, sized at 1 sq in per 3,000 BTU/hr, provided the space has adequate infiltration or an additional outdoor air connection.
- Opening must be within 12 inches of ceiling
- Sizing factor: 1 sq in per 3,000 BTU/hr
- Two-opening method uses 1 sq in per 4,000 BTU/hr (outdoor communicating) per opening
Memory trick: 'Single opening near the ceiling: three-thousand per inch is the dealing.'