In this study, by cooling coil load calculation in under floor air distribution systems, the effect of separate location of the return and exhaust vents and return vent height on energy consumption, thermal comfort conditions and indoor air quality have been investigated. Based on the results obtained from this study, when the height of return vent is equal to 2.0, 1.3, 0.65 and 0.3 m, the amount of energy usage reduction compared to no return vent is equal to 10.9, 15.3, 18.9 and 25.7 percent respectively. Limiting factors in the amount of this reduction are thermal comfort of occupants and indoor air quality. To this end, thermal comfort indices (Predicted Mean Vote and Predicted Percentage of Dissatisfied), local thermal discomfort index (Temperature gradient in vertical direction), and indoor air quality index (Mean Local Air Age) have been probed with changing return vent height by CFD methods (AirPak software with SIMPLE algorithm by using Indoor Zero Equation turbulence model). Based on the results, by reducing the height of return vent from ceiling to floor, the exhaust air temperature increased, which causes to temperature gradient increase in vertical direction. The survey was conducted that choosing the location of 1.3 m(upper boundary of occupied space in seated mode) for return vent, causes to 15.3 percent reduction in the amount of energy consumption while maintaining the states of thermal comfort conditions and indoor air quality.