Abstract A modular framework to model the steady off-design behavior of micro-CHP natural gas boilers based on Rankine technology is presented. The system charge integration into the model eliminates the use of any assumptions (i.e. subcooling, superheating, condensing pressure, etc.) which makes the presented model completely predictive. It is illustrated in the modelling a micro-CHP that satisfies the hot waters and central heating domestic needs (35 kWt) and produces electricity (≤1.5 kWe). A library of sub models of components with empirical (rotary vane pump and vapor scroll expander), semi-empirical (compact plates condenser) and spatially detailed physical (gas burner and evaporator) models is used to construct a model, using R245fa as thermal fluid. The model is calibrated and validated in tests in which 0.1 kg/s of water was heated from 20 °C to 30–36 °C, and 80–500 W mechanical power was delivered at the expander shaft, sweeping restrict ranges of three control variables: burner thermal power of combustion (10–14.5 kWt), pump (500–740 rpm) and expander (2500–2750 rpm) rotation speeds. The model predicts most output variables with acceptable errors, e.g., less than ±10% for the expander outlet pressure (190–220 kPa, abs) or the temperatures at the outlet of the evaporator (80–150 °C) or the expander (60–120 °C).