A case for an HVAC system has at least two molded shells joined together along a parting line to enclose a heat exchanger chamber, a blower chamber, and a diffuser section. The diffuser section includes a floor, a ceiling, an outer wall, and an inner wall around a longitudinal axis. The walls provide an airflow path through the diffuser between the blower and heat exchanger chamber, and the airflow path makes a substantially right angle turn into the heat exchanger chamber which results in a tendency to create a high flow region at the outer wall because of centrifugal effects. The outer wall is shaped to form a wall guide partially projecting into the airflow path in the diffuser, wherein the wall guide has an upstream encroaching surface and a downstream retreating surface so that a portion of the guided air is directed from the outer wall toward the inner wall. At least one of the floor or the ceiling includes a vane projecting into and deflecting the guided air in the airflow path, wherein the vane has an upstream end proximate to the wall guide and a downstream end extending toward the heat exchanger chamber for initiating a portion of the substantially right angle turn for a portion of the airflow.