What is the minimum total free area required for a 120,000 Btu furnace supplied with combustion air from inside a building?

The minimum total free area required for a furnace burning fuel to operate efficiently is determined by the amount of combustion air needed for safe operation. For a 120,000 BTU furnace, the National Fuel Gas Code, as well as other HVAC guidelines, stipulates a specific formula to determine the necessary free area.

The general rule of thumb is to provide at least 1 square inch of free area for every 1,000 BTU of input for the combustion air intake. Therefore, for a 120,000 BTU furnace, the calculation would be as follows:

120,000 BTU ÷ 1,000 BTU/square inch = 120 square inches of free area needed.

This requirement ensures that enough oxygen is supplied for combustion and helps to prevent hazardous conditions within the building.

Hence, the correct choice reflects the calculated minimum needed for the furnace to operate safely and efficiently while being ventilated appropriately from within the building.