This study demonstrates the performance improvement of an air-to-air roof top unit (RTU) achieved by optimizing an evaporator s refrigerant circuitry using evolutionary algorithms. The subject of this study is a unit with a cooling capacity of 7.5 Tons (26.4 kW). The RTU employs two separate refrigerant cycles having separate compressors, condensers, and thermostatic expansion valves (TXV) but using a single evaporator slab in which two separate refrigerant circuits are implemented. We modified the RTU by replacing the refrigerant-to-air condensers with water cooled brazed plate heat exchangers in order to facilitate testing. Performance tests were conducted in a conditioned environmental chamber in line with AHRI standard 340/360; in order to accomplish this, we maintained the liquid line saturation pressure and subcooling from the manufacturer s test data by adjusting the condenser water flow rate and temperature. We also measured the in-situ air velocity profile using Particle Image Velocimetry (PIV), a non-intrusive, laser-based technique. The measurements showed that the range of air velocities passing through the heat exchanger varied from 0.5 ms-1 to 3.0 ms-1, with the integrated average of the measurements being 1.75 ms-1. The PIV data was used to generate a map of the air flow distribution through the heat exchanger, which served as the basis for refrigerant circuitry optimization."