The Surface element defines the boundaries of a space or zone. In most cases, a surface will be an opaque construction such as a wall, roof, ceiling, floor, and more.
Sometimes a large space needs to be zoned into separate “HVAC zones” for purposes of modeling a VAV system. This large space may not have physical walls separating it into these “zones”. By inserting “air surfaces” or “air walls” into the virtual model, it communicates to the energy modeling software that the space is zoned. In other words, the user is inserting an invisible wall into the space that delineates the space into one or more zones.
The way this is done is the user creates a wall inside the space and assigns the Surface’s SurfaceType attribute as type “Air”.
Another way to do it is to create a regular Surface and assign an Opening to that Surface of OpeningType “Air”. Make sure the area of the Opening is equal to the area of the Surface.
Now, EnergyPlus will interpret the space as having one or more zones while still conveying the fact that it is one large space.