Abstract: Offshore substations present unique HVAC challenges due to constrained layouts, exposure to harsh marine environments, and critical limitations on auxiliary power consumption. This study investigates the integration of free heating and free cooling strategies as a means to optimize energy efficiency in extreme cold climates. Conventional winter operation relies exclusively on electrical heating to condition incoming fresh air, resulting in significant power consumption (≈136 kW for −20 °C ambient). In the proposed approach, chilled water is circulated through AHU cooling coils even during winter operation, effectively preheating fresh air and reducing electrical heater load. Analytical evaluation demonstrates that using chilled water supply at 7 °C reduces electrical heating demand to 38 kW, while further utilization of chilled water return from FCUs at 12 °C lowers it to 19 kW, corresponding to energy savings of 72% and 86%, respectively.