Final answer:
The volumes of spaces in different stories in indoor combustion air should be considered interconnected when connected by openings in doors or floors with a specific total free area requirement.
Step-by-step explanation:
In the context of indoor combustion air, the volumes of spaces in different stories should be considered interconnected when connected by openings in doors or floors. These openings should have a total free area of 2 square inches per 1,000 Btu/h of the total input rating of all appliances. This requirement ensures proper ventilation and air circulation for combustion purposes.