Jon the best thing to do is always try to find the Data plate on the appliance. This will give you the manufacturer, serial number etc. You can then get on line and find Installation manual with proper clearances etc.
Sometimes all the info will be right on the data plate.
Direct vent appliances generally have less clearance requirements than mechanical vents. Under 10,000 BTU's must be at least 6 inch's from any opening into building. 10-50,000 9 inches and over 50,000 12 inch's from any opening into building. Bottom of vent shall be at least 12 inches above grade.
Where there any openings in the breezeway?

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Dave