A room having a volume equal to at least 12 times the total volume of a furnace and at least 16 times the total volume of a boiler. The total volume of the furnace or boiler is determined from the exterior dimensions and is to include a fan compartment and burner vestibule, where used. Where the actual ceiling height of a room is greater than 8 ft (2. 44 m), the volume of the room is to be figured on the basis of a ceiling height of 8 ft (2. 44 m).