For a room containing a gas furnace and water heater using a two opening method for combustion ventilation, what is the minimum size opening required?

The minimum size opening required for combustion ventilation in a room containing a gas furnace and water heater using the two opening method is determined by considering the appliances' BTU (British Thermal Units) input and local codes or standards that apply, like the National Fuel Gas Code (NFPA 54) or International Fuel Gas Code (IFGC).

In this case, the calculation involves the total BTU input of the appliances, which dictates the volume of combustion air needed. For gas appliances, a common rule is that 1 square inch of free area is needed for every 1,000 BTUs of input for vented appliances, divided between two openings.

The calculation would generally indicate a need for larger openings when combining both the furnace and the water heater's input. Based on the codes, the sizes mentioned apply with free area considerations, where certain factors might define the required openings further.

Choosing 41.25 square inches as the size aligns with typical requirements for providing sufficient combustion air volume for appliances with larger BTU input, ensuring proper combustion and reducing potential safety hazards from insufficient ventilation. Overall, this ensures that there are adequate airflows available for the gas appliances to function safely and effectively.