For a 400,000 BTU gas furnace in a confined space, what is the minimum allowable free area of each permanent opening for ventilation?

To determine the minimum allowable free area of each permanent opening for ventilation for a gas furnace rated at 400,000 BTUs in a confined space, we rely on the guidelines outlined in the National Fuel Gas Code. The code specifies ventilation requirements for gas-burning appliances to ensure that there is sufficient air for combustion and to prevent dangerous conditions, such as backdrafting or insufficient oxygen levels.

For appliances with a BTU input of 400,000, the code dictates a specific calculation for determining the necessary combustion air. Typically, the formula states that for every 1,000 BTUs of input, a certain amount of free area is required for proper ventilation. A common rule of thumb is that for each 1,000 BTUs, there should be at least 1 square inch of free opening. Therefore, with a furnace rated at 400,000 BTUs, this translates to approximately 400 square inches of required free area for ventilation.

This understanding aligns with the answer that states 400 square inches as the required free area, ensuring that the furnace will function safely and efficiently without risking combustion-related issues or hazards in a confined space.