HVAC Design

Once you have a load-optimized building design, it is time to decide on the type of HVAC system you want to use to meet these loads. In this design exercise, you will first select an HVAC system type based on resulting carbon emission, costs and space requirement (Task A). You will then modify the duct geometry and riser position to spatially and aesthetically complement your design (Task B).

Task A: System Selection

In this portion of the exercise, you will evaluate the performance of four different HVAC systems choices,

  • Variable Air Volume (VAV),

  • Variable Air Volume with Economizer,

  • Fan Coil Units with Dedicated Outdoor Air System (DOAS) and

  • Variable Refrigerant Flow (VRF) Heat Pump with DOAS.

To do so, you first need to set up your thermal model in Grasshopper and then run an energy simulation to compare the systems based on first cost, operating cost, operating carbon, payback period, and space requirements.

To get started, open a Rhino 3D file which includes your thermal zone geometry and template description. Open the HVACer.gh grasshopper definition from the buildVAV GitHub repository (in case you have trouble getting access, reach out to Ali Irani) and follow the instructions in the GH definition, starting at the top left.

Step 1 Define Location

Step 1: Define your weather file and ASHRAE climate zone for your project site

 

Step 2 Define Geometry And Templates

To et started, select your main zone description and save it as a new template in your Rhino file’s ClimateStudio library. Make sure that the fuel costs and carbon emissions in the template have been set correctly.

Figure 1: Save the thermal zone description for your building in the Rhino file’s template library

Step 2a Select the thermal zone BREPs from your Rhino thermal model and select the previously saved zone template for your building.

Step 2b Select the window geometry and apply the same glazing type as for your Rhino thermal model.

Step 2c Select the ground plane (where your building touches the ground) and define the boundary condition.

Step 2d (optional) Define exterior shading objects such as neighboring buildings or external light shelves.

Step 3 Initiate Energy and Sizing Simulation

Step 3a Specify a directory on your PC where you would like to save the simulation results. The path name should not include any blanks.

Step 3b Run simulation

After your energy model has finished running, you should see a detailed energy use breakdown for the above listed HVAC systems along with a series of additional performance metrics for evaluation. An example output is shown in Figure 2. The figure shows that the all-electric VRF with DOAS system has the lowest operational energy use and required floor area along with the highest first cost. Given the performance metrics from the table, the calculated payback period, as well as your general knowledge of and preference for HVAC systems, indicate which system you select going forward. Submit a screenshots of your version of Figure 2 and a brief motivation for your system choice.

Figure 2: Example energy use predictions for four different HAVC systems for an office building in Boston

Task B: Spatial Design

In this portion of the exercise, you will design the spatial layout of your selected HVAC system. Follow the steps below to modify the geometry of the HVAC system based on spatial and aesthetic considerations.

Step 1 HVAC Spatial Layout

Step 1a Draw and select riser points on the roof of your building. You need a minimum of one riser.

Step 1b Select your desired HVAC system from the dropdown menu and assign the points for your risers. You an modify the riser position by moving the base point along the roof surface.

Step 1c Select the desired aspect ratio for your ductwork

Step 1d Modify the placement and geometry of the main ductwork vertically and horizontally

Step 1e Modify other geometric parameters to refine the spatial layout

A summary of the various geometric parameters is provided in the GH script. Consider placing risers near core locations or toward he back of of the building and route the primary supply air ducts through circulation areas.

Once you have completed your initial design in Grasshopper, bake the various elements in Rhino. You can access the different geometries using the various Preview components in the script (Figure 3). Right-click on each component and then bake the corresponding geometry to a Rhino layer.  

Figure 3: Baking HVAC layout geometry to Rhino

Once the duct geometry has been converted into Rhino geometry, you can modify it in order to, for example, edit zone branch placements, etc. When you have completed your design, please provide a perspective and representative section of your final design (Figures 4 and 5). Please also submit the Rhino geometry and Grasshopper script to CANVAS. 

Figure 4: Axonometric view of the central plant, fan rooms, terminal equipment, and distribution for every floor

Figure 5: Representative section showing riser, main branch duct and terminal equipment

Related Video Tutorial/Handbook Chapter