Method and apparatus for operating a combined heat and power system with greater flexibility, reliability, control and stability, for providing operational flexibility and energy efficiency in operating a combined heat and power plant which includes a backpressure steam engine that expands a high temperature heat source of a thermodynamic fluid to generate mechanical power and discharge its spent heat for a beneficial use comprises a vessel subsystem for the spent heat, said vessel subsystem including: at least one main indirect heat exchange device or vessel (7) in heat exchange communication between its primary space (10) and its secondary space (11). The present invention also discloses the use of a method and apparatus to operate a combined heat and power system.