Heat pumps, as a process recirculating environmental heat back into useful heat production, fit the environmental requirements of present global policy. They are very useful tools to reduce the need of fossil fuels in space heating and cooling. Considering the geological specifications of most regions of Iran, the closed loop systems are usually the best option and due to limitations in available surface area in our case study, a vertical GCHP system was selected. For given heating/cooling loads, and various climatic conditions, the optimum values of heat pump design parameters (saturated temperature/pressure in condenser and evaporator) as well as VGHX optimum design parameters (inlet and outlet temperatures of the buried pipe and its diameter, the number and depth of the boreholes) were estimated. Different parameters such as climatic conditions, system capacity, soil types, refrigerant types and number of parallel boreholes are among the studied. Results from optimum values show that for regions with temperate climate in Iran, the GCHP system has the best performance. Also compare to the regions with dry, low density soil, installing the system in conditions with dense, moist soil may lead to a significant increase in the system performance and consequently lower costs. Furthermore, it is shown that there is a reverse effect on objective function with increasing the number of boreholes which results in an optimum amount.