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The minimum free area requirement for an opening used for air supply based on British Thermal Units per Hour (BTUH) is established to ensure adequate ventilation and air supply for fuel-burning appliances. The correct answer specifies that for effective air supply, a minimum of 2 square inches of free area is required for every 1000 BTUH of input for those appliances.

This requirement is important because proper air supply is crucial for the safe and efficient operation of heating systems. You need enough airflow to maintain combustion processes, prevent backdrafts, and ensure that appliances function correctly without producing harmful exhaust gases.

In situations with insufficient air available, the performance of HVAC systems can be compromised, potentially leading to dangerous situations, such as carbon monoxide buildup. Therefore, the 2 square inches per 1000 BTUH standard aims to balance system efficiency with safety needs in various installations.