This paper reports on the use of CFD for predicting and improving the performance of a rooftop AC unit. The current work considered the hydrodynamic and thermal fields on the air flow side of the unit with exact modeling of fans and heat exchangers. This is in addition to predicting condensation on cooling coils. Because only the air flow side is considered, the evaporator and condenser compartments are decoupled and the solution in each section is established separately. In the evaporator compartment the flow is solved as a two-phase flow (gas and liquid) with the gas phase being composed of two species (dry air and water vapor). In the condenser section however, the flow is treated as a single phase flow. The exact modeling of heat exchangers and fans increased the grid size and computational cost, but resulted in realistic results and reliable model. A total of 31 million control volumes are used to model the evaporator and condenser sections. Results indicate the presence of several recirculation zones in the evaporator compartment. Sensible and latent cooling capacities for several design conditions predicted by the model are in close agreement with available experimental data. The differences between the total capacities predicted by the model in the evaporator section and those reported experimentally are within 2.7% for all cases considered. Predictions in the condenser section resulted in a load that is only 0.00136% different than the one calculated using experimental data. To improve the performance of the unit, six different modified designs of the evaporator coil are developed and tested. The newly modified designs are based on changing the coil inclination angle and/or number of fins per unit length for the same coil height and surface area. One of the designs resulted in 6.18% decrease in the cooling capacity, while the remaining modifications increased the cooling capacity by values ranging between 2.17% and 8.6%. (C) 2010 Elsevier Ltd. All rights reserved.