Final answer:
The minimum allowable size for each combustion air opening for a 90,000 Btu/h hot water boiler is 22.5 square inches.
Step-by-step explanation:
The minimum allowable size for each combustion air opening from adjacent spaces for a 90,000 Btu/h hot water boiler can be determined using the National Fuel Gas Code (NFPA 54). According to the code, the minimum net free area of each opening should be at least 1 square inch per 4,000 Btu/h input rating of the fuel-burning appliances served. In this case, the input rating of the boiler is 90,000 Btu/h, so the minimum net free area should be:
90,000 Btu/h ÷ 4,000 Btu/h per square inch = 22.5 square inches.