Spacecraft thermal management is a complex endeavour that requires expertise from many different domains and employs several different strategies. Heat pipes are a common device used to move thermal energy from one part of the system to another. Modeling heat pipes can be very complex due to the inherent two-phase nature of the heat transfer. While systems-level models and correlations can be used for modeling heat pipes, the true three-dimensional nature of the heat transfer is not captured. Conversely, modeling the process fully in 3D can be both cumbersome and time-consuming. In this presentation, we demonstrate a way to connect system models and 3D models. The combined models are then solved in co-simulation mode, where each respective solver performs the calculations under its purview and exchanges data with the other at specific interfaces and times. The solvers communicate directly with each other. Data mapping and interpolation is employed at the interfaces to enable data exchange between surfaces in the 3D model and point data used in the system model components. This co-simulation approach allows us to leverage the strengths of each solver to gain more insight into the system dynamics. We present an example case of a mechanically driven heat loop and show how the inclusion of the 3D model helps to improve the results. We discuss the challenges inherent in modeling physics with very different time constants and the strategies we employed to mitigate them, including parameter sensitivity studies. We conclude with a discussion on how the model data may be improved, such as accounting for orbital heating and methods to accelerate thermal radiation computations for orbits.