The power of modern computing equipment, from small devices such as laptops through a variety of cases up to entire data centres, makes cooling vital. Especially in large-scale systems, delivering the right cooling to the right place at the right time is crucial for both computing performance and energy efficiency. As such, modern cooling systems require a lot of controls. Given the many cases to face, designing and assessing such controls requires tools to rapidly and modularly build and manage computationally efficient simulation models, sometimes concentrating on the thermal policies aboard on a chip, sometimes on the cooling of a rack, sometimes on an entire date centre with its fluid conditioning and transport machinery, and so forth. Though technology exist to address many such cases individually, a holistic approach to embrace them all within a unified modelling methodology and workflow is still the subject of research. In this paper we distil our experience over the last years, and discuss how a solution based on joining purpose-specific chip modelling (using the 3D-ICE simulator) and Equation-Based Object-Oriented Modelling (employing the Modelica language) can help the joint design of a computing system and its cooling.