Given: Combustion air vents for a gas fired 25,000 btu/per hour appliance will terminate in the attic. The attic space communicates directly with the outdoors. What minimum diameter vertical vents are required? (If between sizes, go to larger size.)

Given: Combustion air vents for a gas fired 25,000 btu/per hour appliance will terminate in the attic. The attic space communicates directly with the outdoors. What minimum diameter vertical vents are required? (If between sizes, go to larger size.)