What is the required opening free area for combustion air supplied from outside for appliances located in confined spaces?

The requirement for combustion air supplied from outside for appliances located in confined spaces is critical for ensuring adequate airflow for the safe operation of heating appliances. The correct choice indicates that for every 1,000 BTUs of the appliance's input rating, there should be at least 1 square inch of free area available for combustion air intake when it's sourced from outside the confined space.

In this case, the ratio of 1 square inch per 4,000 BTU accommodates the necessary air volume while considering the combustion process's efficiency and safety. This ensures that the appliance can operate without creating negative pressure in the confined area, which could lead to the potential backdrafts of combustion gases, thereby improving safety for occupants.

Other ratios would either provide insufficient air for optimal combustion or may exceed the necessary requirements, leading to inefficiencies or complications in appliance operation. Selecting the appropriate ratio is essential for maintaining code compliance and safe operation in confined spaces.