How should combustion air openings be sized for appliances?

The correct answer is that combustion air openings should be sized at one square inch per 4000 BTUs. This guideline is essential in ensuring that appliances receive adequate airflow to support proper combustion, which is necessary for efficient operation and to prevent the risk of incomplete combustion, leading to dangerous carbon monoxide accumulation.

Sizing combustion air openings appropriately is crucial because insufficient combustion air can result in decreased efficiency, higher emissions, and can create hazardous conditions. Each BTU rating represents the amount of heat energy an appliance produces, and calculating the required air intake using this ratio ensures that the appliance operates safely and effectively.

The other options for sizing combustion air openings do not provide the correct amount of airflow necessary for the given appliance BTU output, which can hamper the safe operation of the appliance and increase the risk of combustion-related issues. Sizing by one square inch per 4000 BTUs strikes a balance that accommodates the needs of most standard appliances while adhering to safety regulations.