503.8 Section 2, A mechanical draft venting system excluding direct vent appliances, shall terminate at least 4' below, 4 ' horizontal from, or 1' above any door, operable window, or gravity air inlet onto the building(This actually applies to both commercial or residential).
503.8 Section 3, gives clearance based on BTU input. For direct vent appliances.
10,000 BTU or less, 6"
Over 10,000 BTU but less then 50,000 BTU, 9"
Over 50,000BTU input, needs a 12"
Both the bottom of the intake and vent shall have at least 12" clearance above grade.
Sorry, I didn't type it word for word, Its longer then section 3.