A two-phase effort was conducted to define the architecture of a 21 Century Heating Ventilation and Air Conditioning (HVAC) System for Future Surface Combatants and then, in phase 2, to quantify the benefits of that concept. The 21st Century HVAC System is a complete, revolutionary new approach to shipboard HVAC. It features automation at the ship level, variable air volume flow, integration with the damage control and firefighting systems and other ship networks. A complete new catalog of advanced HVAC system components (fans, cooling coils, ductwork) make up this new architecture. New design philosophies/paradigms such as lower chilled water temperatures and flow rates, increased cooling coil face velocities and increased duct velocities are adopted within designs. Simplified macro level HVAC system models were developed and applied in phase 2 to estimate the benefits of the 21st Century HVAC System architecture using six legacy design sub-systems of DDG 91 as a baseline. The model results were a 30-50% reduction in weight and volume, regardless of system complexity or function. The benefits of the automation and variable air volume technology were calculated using a simplified Total Ship HVAC Model. The ability to provide cooling and replenishment air appropriate to each space in a dynamic, rather than static, mode was shown to lower the total ship HVAC cooling load. An intermediate heat sink water system for cooling equipment, directly or indirectly in a closed system, is proposed as an innovative method to dramatically lower the power required for Future Combatants with large, new thermal loads.