Mechanical Code…
A room having a volume of at least twelve (12) times the total volume of a furnace or air-conditioning appliance and at least sixteen (16) times the total volume of a boiler. Total volume of the appliance is determined from exterior dimensions and includes fan compartments and burner vestibules when used. When the actual ceiling height of a room is greater than eight (8) feet (2,438 mm), the volume of the room is figured on the basis of a ceiling height of eight (8) feet (2,438 mm).
(Uniform Mechanical Code® 220.0)