Prior to using the BCVTB, I suggest that you set up your IDF without any external interface objects and that you use an EnergyPlus schedule object (for instance) to add heat thermal load to your system. Once you see the effect on your output variable (e.g. freezer temperature) and you are satisfied with it, you could then replace this schedule with an external interface schedule where the heat load calculation will be done externally prior to being passed to E+. You may also report the external interface schedule value as an E+ output variable to see if you are passing the right thing.