Energy geostructures rely on efficient heat exchange between buried structural elements and the surrounding ground, the performance of which is governed by the soil’s moisture-dependent thermal behavior. This study experimentally evaluates how the moisture state of silty-sand soil controls heat dissipation around a buried source, comparing dry and fully saturated conditions. Two model-scale tests were performed under identical geometry, instrumentation, and a constant 25 W heating power, using a centrally embedded cylindrical source and a horizontal array of five MPS-6 sensors logging temperature and matric potential at 0.1-0.5 m over 21 hours. Under the same power, the saturated soil held the source markedly cooler than the dry soil (about 33°C versus 56°C), distributed heat more uniformly, decayed less steeply with distance, and reached a quasi-steady state faster because pore water forms continuous conductive bridges that the air-filled pores of dry soil cannot. Unlike studies reporting single-point conductivity values, this work provides a controlled side-by-side comparison of the full spatial and temporal temperature field for both moisture states under identical conditions, quantifying a moisture effect directly relevant to the thermal design of energy piles and ground heat exchangers.