How to calculate the heat flux from boundaries in E+

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Thanks for your advice. Yes, I've tried that, but there are too many surfaces in my case. In order to calculate the heat flux from the rock to the cavern, I should pick up the surfaces between them and it seems like a mission impossible.

Why not? This is exactly the variable you need, all the rest is a question of post processing - you can sum them in Excel or whatever. There is another variable Zone Air Heat Balance Surface Convection Rate - it gives you the sum of convection rates from surfaces in each zone.