The energy consumption of HVAC systems in both residential and industrial buildings is always an important problem. Geothermal sources are becoming one of the most attracting source for building HVAC systems. However, geothermal system operation involves varied conditions, various heat pump parameters, dynamic building loads and different ground loop parameters. Inaccurate design of each component leads to a significant departure from efficient performance which results in high energy use. Objective of this work is to simulate and optimize a geothermal HVAC system with TRNSYS. A control strategy is used to the simulation to find out best parameters combination of heat pump, horizontal ground buried pipes, and the liquid to air heat exchanger. The system has been modeled in Birmingham, Alabama. The results show that there is delicate balance that has to be maintained between the fluid temperature exiting the ground loop and the amount of heat delivered by the heat pump to the house. As a result, the control system parameters are highly important.