An integrated building load-ground source heat pump model is developed to capture short-term (30 s) and long-term (10–20 yr) performance of ground source heat pumps with vertical boreholes. The model takes advantage of the built-in computation and organization functions of the simulink®/matlab environment to couple the component building load, heat pump, and ground loop models at every time step. The building load model uses the HAMBASE thermal program and is applicable to residential and commercial buildings. The heat pump model uses manufacturer data and sensible heat corrections to accurately model heat pump operation across a wide range of input conditions. The ground loop model is a combination of Hellstrom's borehole tube model, Eskillson's long-term (>10 yr) g-function ground model and the one-dimensional, short-term (<5 min) numerical ground model by Xu. Fifteen year simulation results for a base case residential house are presented to illustrate the integrated model's ability to predict a wide range of time responses and to illustrate a limiting ground loop sizing criterion that reveals the slow degradation in system performance due ground heating effects. Simulations with varying borehole lengths also illustrate the sensitivity of ground loop sizing on the system's thermal and economic performances. The work emphasizes the importance of proper borehole sizing, design, and placement especially in cooling-dominated climates, where the unbalance of heat loads to the ground cause slowly rising ground temperatures.