Final answer:
When combustion air for gas-fired appliances is provided by two horizontal ducts to the outdoors, each opening should have a minimum free area of 1 square inch per Btu/h.
Step-by-step explanation:
When combustion air for gas-fired appliances is provided by two horizontal ducts to the outdoors, each opening shall have a minimum free area of not less than 1 square inch per Btu/h.
This requirement ensures that there is enough air supply for the combustion process of the gas-fired appliances.
For example, if an appliance has a rating of 10,000 Btu/h, it should have at least two openings with a total free area of 20 square inches to ensure adequate air supply.