A plenum can be used to separate the envelope loads between the occupied space, and the unoccupied void above the ceiling. Doing so can provide a fine-tuned understanding of the heating and cooling loads, and their impact on the return air temperature.
A typical default is the model each room as slab to slab, ignoring the plenums. This is helpful to simplify the geometry and in many cases is sufficient. Of course where there is no ceiling or exposed slab this is also the correct approach. When we do want the added detail of the plenum, these can be created as their own Floor in the drawing.tool.
For some project types, such as labs or hospitals, the volume of each room plays significant role. Other projects may need to see the heating and cooling loads separate from the occupied rooms.
For these, you can add an elevation representing the ceiling plenum with its own walls, floors, etc. defined, along with its own room. In the image below, the plenum spaces are highlighted in blue and are modeled at an elevation considered to be the ceiling plane. All plenums are assumed to be unconditioned and unoccupied, so they should be assigned a Void Room Template. Additionally, the floor of these spaces is acting as the ceiling of the space below, and the ceiling of the occupied spaces is acting as the floor of the plenum. Thus, it's important to match the plenum's Floor U-Value and the occupied space's Ceiling U-Value to that of the actual ceiling material to be installed.
Depending on the needs of your project, there are two methods to model the plenum. The first is as shown above, where each occupied room gets its own plenum, often these will be assigned to the same Zone in the simulation. To create this configuration easily follow these steps:
Create the main floor layout as desired with all rooms defined.
Create two new floors above, one will be for the ceiling, and the second will be the slab, or floor above
Copy the geometry from the main floor to the ceiling floor.
Adjust the heights of the ceiling floor and slab above floor to the require dimensions and rename the rooms as needed.
The second is as shown below, which is where the plenum of every room is joined together as one to form one large plenum zone that sits atop the divided rooms below. To create this configuration follow the above steps and finally remove the interior walls so that there is a single open plenum.