Abstract: A retrofit assembly for adapting two replacement rooftop air handling units to a multizone installation that originally had only one air handling unit. The two replacement units include one heating unit and one cooling unit, while the original unit typically had provided both heating and cooling to several zones in the building. The retrofit assembly includes a valve manifold and a curb adaptor that make it possible to use the building's existing roof curb and air ducts. The curb adaptor supports the cooling unit on top of the existing roof curb and includes an air duct for conveying air from the existing return air ducts in the building to the inlet of the cooling unit. The heating unit is mounted on its own separate roof curb and the valve manifold receives the air discharged from both replacement units and directs it into the existing plurality of supply air ducts which further convey the air to the plurality of zones in the building.