Arrived on site to perform two system furnace diagnostic. Upon arrival Client Stated that another company had came out and said that the units had to be replaced due to inefficiencies. At this time client states that both units are operational. Begin diagnostic at right wing of house unit And found that unit was installed backwards not giving an easy access for service. Return intakes are under size and not supplying the proper CFM for Airflow. System was retrofitted incorrectly. Indoor blower Aaron mixing cabinet is heavily covered in dust and debris. Static pressure on unit is two times higher than what it is designed for due to undersize ducting. Temperature rise was in range. Inspected left side unit and found that unit is from 1994 with the original AC. Unit has no secondary drain pan on evaporator coil. Static pressure is extremely high due to undersized returning take What’s a result in higher energy bills. Indoor blower cabinet is also covered in dust and debris significantly