Juliet Simpson of the National Renewable Energy Laboratory presents the HEATNETS model, a techno-economic tool used in HEET's LeGUp project to evaluate geothermal energy networks (GENs). Part of MIT's IAP 2025 course on Geothermal Energy Networks, the session walks through how the model estimates costs, capacity, and performance tradeoffs for neighborhood-scale ground source heat pump systems designed to replace fossil gas heating. Simpson explains the inputs the model requires, such as building loads and borehole configurations, and how planners use its outputs to compare network designs before committing to construction. The talk sits within a broader push to convert gas utility infrastructure into thermal energy networks, and it gives a concrete look at the analytical tools engineers and utilities use to make that case financially credible rather than purely aspirational.