On the one hand, environmental problems and greenhouse gas emissions are increasing, while societal development has raised energy demand, particularly in the building sector. This study presents a novel direct geothermal heating system (DGHS) utilizing Gavmish Goli hot spring in Sarein, Iran, as a renewable energy source to provide sustainable thermal comfort. A representative building was simulated in EnergyPlus in both non-insulated and insulated configurations, applying DGHS with fan coil units or underfloor heating, and compared to a conventional gas-fired system. The DGHS system maintains the Predicted Mean Vote (PMV) index within the acceptable range, ensuring adequate thermal comfort. Energy simulations indicate that for the non-insulated building, DGHS achieves approximately 90% and 98% energy savings compared to the conventional system with fan coil units and underfloor heating, respectively, while for the insulated building, savings are 87% and 95%. A comprehensive economic assessment was also conducted, calculating the levelized cost of heating (LCOH) to compare overall heating costs. Results show that adopting DGHS reduces heating-related expenses by roughly 92% for the non-insulated building and 88% for the insulated building. These findings demonstrate that DGHS can provide both significant energy and economic benefits by leveraging locally available geothermal resources, offering a cost-effective and sustainable alternative to conventional heating in cold climates.