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Both models present a low profile and do not distract from the architecture of the building. MODEL 585E is designed for horizontal side-by-side duct connections and can be installed at ground level or on a roof. The 585E is ideal for the replacement market. MODEL 579D has the supply- and return-air openings on the bottom of the unit. The 579D is designed for rooftop installation and mounts on a factory-supplied, roof-mounting curb. The ductwork connects to the curb so that the air ducts and cu