Although siting of thermal energy storage systems in the vadose zone may be beneficial due to the low thermal conductivity of unsaturated soils, water phase change and vapor diffusion in soils surrounding geothermal heat exchangers may play important roles in both the heat injection and retention processes that are not considered in established design models for these systems. To better understand these roles, this study incorporates recently-developed coupled thermohydraulic constitutive relationships for unsaturated soils into a coupled heat transfer and water flow model that considers time-dependent, nonequilibrium water phase change and enhanced vapor diffusion. After calibration of key parameters using a tank-scale heating test on compacted silt, the subsurface response during 90 days of heat injection from a geothermal heat exchanger followed by 90 days of ambient cooling was investigated. Significant decreases in degree of saturation and thermal conductivity of the ground surrounding the heat exchanger were observed during the heat injection period that were not recovered during the cooling period. This effect can lead to a greater amount of heat retained in the ground beyond that estimated in conduction-based design models.