Why both the heating and cooling load are reduced when decrease the thickness of ground slab in cold region?

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When I used the Ideal load air system and reduce the thickness of ground slab, both the heating load and cooling load will reduce, but in reality the heating load should increase. The ground surface temperature was set as 2C lower than indoor temperature. and the building located in cold region. Heating and Cooling setpoint are 18C and 24C respectively.

I have tried to solve the problem, but it was still existed. only when I reduce the groud surface temperature to 10C in winter, this problem was rectified but the heating load was too large.

So if you guys have the project with ideal load air system, can you try to reduce the thickness of ground slad to see the heating load performance. Because this is the main suspect target in current.