simulating different spaces with the same thermal zone

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First, to answer the question about internal loads. If only one of those spaces has 30 people, and the other spaces have 0 people. then then zone would have 30 people. The people from the spaces are added together for a single zone. If instead you had a conference room space type with 30 people, and then assigned that space type to all 4 spaces, you would have 120 people in the zone. OpenStudio is just aggregating the space loads to the zone.

As far as the wall that is between two spaces that are part of the zone, that is not modeled as a heat transfer surfaces, as it is not part of the bounding box of the zone. Generally the zone should not have any excess heat transfer surfaces that do either connect to another zone or the ground or outside. There is an option to have these surfaces that are not converted to heat transfer surfaces result in internal mass objects.