Numerous workers have made effort to harness the flexibility of spreadsheets to analyse thermal problems. One approach has been to execute a thermal analysis within the spreadsheet itself, however the usefulness is limited by the performance of the built-in solvers. In addition, spreadsheets must either be custom built for each application or founded on parameterised models from similar analyses. An alternate approach has been to add spreadsheet like capability to existing tools. Again, this loses the full flexibility of a spreadsheet.To overcome these limitations, ThermXL has been developed. ThermXL is a thermal lumped parameter network solver and generic model builder that is fully integrated within Microsoft® Excel. It is very flexible easy-to-use tool that is particularly suited to preliminary design and “What if…?” type parametric analyses.This paper describes the utility, functionality and architecture of ThermXL.