Embodiments of the invention generally provide a heat engine system, a mass management system (MMS), and a method for regulating pressure in the heat engine system while generating electricity. In one embodiment, the MMS contains a tank fluidly coupled to a pump, a turbine, a heat exchanger, an offload terminal, and a working fluid contained in the tank at a storage pressure. The working fluid may be at a system pressure proximal an outlet of the heat exchanger, at a low-side pressure proximal a pump inlet, and at a high-side pressure proximal a pump outlet. The MMS contains a controller communicably coupled to a valve between the tank and the heat exchanger outlet, a valve between the tank and the pump inlet, a valve between the tank and the pump outlet, and a valve between the tank and the offload terminal.