The minimum duct opening required for introducing outdoor combustion air into a confined space is based on the need to ensure adequate airflow for combustion appliances. The correct choice indicates that you need one square inch of opening for every 4,000 BTUs per hour of total input, with a minimum requirement of 100 square inches.
This is crucial for ensuring that there is enough oxygen available for the combustion process, which is necessary for safe and efficient operation. Insufficient combustion air can lead to incomplete combustion, increasing the risk of carbon monoxide buildup and reducing the efficiency of the heating system.
The stipulation of not less than 100 square inches reinforces the importance of providing a reasonable amount of airflow, regardless of the total BTU input being less than what would otherwise dictate a higher requirement. This consideration helps to address potential scenarios where small heating appliances may still need a sufficient volume of air to operate safely.