Question-and-Answer Resource for the Building Energy Modeling Community
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Looks like you've found the issue - it's the ground boundary conditions. For a model like this, the result will be highly sensitive to the ground heat transfer. A ground construction is required, because the Kiva ground model must have some type of surface as a boundary condition to the zone. For this application, I would try a rather thick slab of soil, with the foundation settings such that the soil surface is completely in-grade.
