HVAC plant designers and energy auditors often face the problem to calculate the performance of heat pumps under various boundary conditions. In this paper a model aimed at that purpose is presented. In particular, the model presented is aimed at the simulation of vapor compression based heat pumps or chillers driven by volumetric compressors. It is a numerical model tracing a reference thermodynamic inverse cycle consistent with the specific heat pump or chiller under the requested boundary conditions. The input data necessary are few and easy-to-find, especially if compared with the other simulation models proposed in literature. The proposed simplified model makes it possible to use catalog data in order to get reliable results. Even if the amount of input data required is low, the numerical model showed a good accuracy (within 10%) contrasted against performance declared by manufacturers. Moreover, the model has low sensitivity to user assumptions so it is valid also for the prediction of the performance in old HVAC plants. Moreover, this numerical model is fast and suitable for integration within building energy simulation tools. The present model does not take into account air dehumidification at the evaporator nor defrost cycles, whose implementation is currently in progress. (C) 2011 Elsevier B.V. All rights reserved.