The ground-source heat pump systems are highly efficient and energy saving. Its main disadvantage is a significantly higher installation cost compared to conventional systems. The length of the ground heat exchanger (GHX) piping, consequently, the first cost, depends on several factors; one key factor is the undisturbed ground temperature estimations. Xing and Spitler model was developed which provides a new set of ground temperature results for GHX design. There are two common methods in United States to be used for ground temperature estimations - ASHRAE Handbook method and ASHRAE district heating manual method. This paper presents the impact of Xing and Spitler model development on the horizontal ground heat exchanger (HGHX) design. An analytical HGHX simulation tool is developed. 12 geographically diverse sites in United States are chosen for the case study. Three different HGHX configurations are investigated. For each site, HGHX design length using the Xing and Spitler model estimated ground temperatures as inputs are compared to design results based on measured ground temperatures; the calculated HGHX design length percentage error are within ±18.9%. The calculated HGHX design length percentage error using the ASHRAE Handbooks results and ASHRAE district heating manual results are within ±38.3% and ±57.7% respectively.