HVAC plant configurations vary dramatically in their conceptual design, and in the complex building-and-plant topologies. Analysis of plant performance often relies on a powerful and flexible simulation tool to produce realistic results based on appropriate system and component models. The stress is thus on the credibility and applicability of contemporary systems simulation packages and the component libraries that underpin these systems. This paper describes an approach to plant component modelling, which is based on the notion of ＇primitive parts＇ (PP) to represent the fundamental heat and mass transfer processes. The principal advantage of the approach lies in its flexibility in relation to the multiplicity of plant systems, possible modelling abstractions, and converging models at the theoretical level. The PP approach, as implemented within the ESP-r system (Version 9 Series), is described in terms of its features, applications, limitations and planned developments.