Europe’s energy transition requires an increase in both renewable electricity generation and renewable heating and cooling supply. Achieving these goals calls for flexible energy system capable of converting, storing, and dispatching energy across multiple vectors. Reversible Organic Rankine Cycle/Heat Pump (rev. ORC/HP) systems offer such versatility, as they can switch between power production and heating modes while sharing key components. Although recent research has examined the steady-state behaviour of rev. ORC/HP systems, their transient operation – particularly during start-up – remains insufficiently understood. This work experimentally investigates the start-up dynamics of a fully rev. ORC/HP system equipped with fixed-speed reversible volumetric machines. The paper examines different electrical grid-connection strategies for reversible volumetric machines, highlighting their respective operational constraints, benefits, and limitations. The start-up procedures for both HP and ORC modes are then analysed in detail, with a focus on how fixed-speed operation shapes transient behaviour. Finally, the study proposed practical measures to mitigate the challenges associated with fixed-speed reversible machines.