What is the requirement for indoor combustion air volume if the air infiltration rate is unknown?

The requirement for indoor combustion air volume when the air infiltration rate is unknown is calculated based on the need to ensure a proper air supply for combustion appliances. This requirement is vital to maintain adequate combustion efficiency and safety by preventing the buildup of harmful gases, such as carbon monoxide.

The standard guideline often referenced for determining the volume of indoor air needed for combustion is that for every 1,000 BTU of appliance input, there should typically be a minimum of 50 cubic feet of combustion air supplied. This number comes from various codes and standards designed to ensure safety and efficiency in heating appliances and equipment.

Choosing 50 cubic feet aligns with these established standards, making it the correct answer for ensuring that the appliances operate safely and effectively, especially when the rate of air infiltration is not specified. The other options provide higher volume requirements that are unnecessary in this scenario, given that the baseline standard is for 50 cubic feet.