How can I check ground heat flux?

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EnergyPlus requires some type of floor surface to interact with the ground. The material can be soil if you want. There are also multiple ground models to choose from. The most current ones are Foundation:Kiva and Site:GroundDomain:*. Once the surface and boundary conditions have been set up, the rdd output file will list available output variables for the ground model and for the surfaces.

Depending on how the data for ground heat transfer was actually measured, a possible variable to report is Opaque Surface Inside Face Conduction. Be sure to read the definition in the Input Output Reference to understand exactly what this variable represents. Also, this is an advanced output variable, requiring Output:Diagnostics, DisplayAdvancedReportVariables; to see it.