Designing a Supply Air System: Step-by-Step Guide in Revit and BP

Welcome back to the CAD Teacher VDCI video course content for the BIM 321 course, Introduction to Revit MEP. Having successfully configured our second-floor return air system in the previous video, we'll now tackle the supply system configuration—a critical component that requires precise coordination with our existing infrastructure.

Let's begin by navigating to the Duct tool. Ensure you're working with an 18 × 12 duct configuration—if you need to adjust this, simply pull down the menu and select 18 × 12. Most importantly, verify that your system type is set to "supply air." If you currently see "return air," make this adjustment now to avoid system conflicts downstream.

Now we'll begin routing our supply ductwork strategically. Starting from this position, I'll extend the duct slightly outward, then run it along this wall. Upon reaching the gypsum board roof assembly, we'll transition to the opposite side of the hallway corridor. This routing approach maximizes space utilization while maintaining code compliance for accessibility.

Continue the main run until it aligns with that register location, then bring the connection over to complete the circuit. Use the Escape key to end this drawing sequence. Notice the duct positioning relative to the wall—we'll need to select it and use the directional arrows to adjust the spacing slightly for proper clearance.

Here's where coordination becomes critical: both this supply duct and our existing return duct are positioned at the 10-foot elevation, creating a collision conflict. To resolve this systematically, we'll elevate our return air system, maintaining consistency with our first-floor approach. This creates a standardized vertical coordination strategy across all building levels.

Let's refine these connections by bringing this section closer to the wall, providing adequate space for fittings and future maintenance access. I'll adjust this element downward slightly to optimize the overall layout geometry.

To modify this main run, I'll use the Split command (SL shortcut). Select points approximately here and here to create the necessary break. Rather than dragging the unwanted segment, we can simply select and delete it—a more efficient workflow approach.

Creating a section view will provide essential vertical coordination information. Access the Section tool, establish your start point here, and drag downward to generate the cutting plane. This three-dimensional visualization is crucial for MEP coordination in complex building systems.

Double-click the section marker to open that view, then use WT (Window Tile) to display both plan and section simultaneously. This dual-view workflow is standard practice for professional MEP design. Adjust the detail level to Medium to display complete duct geometry, and switch the line style to Wireframe for clearer visualization of overlapping systems.

The section view reveals our coordination challenges more clearly. Select the duct system in plan view, then observe the corresponding selection in the section view. Now we can precisely adjust the offset elevation to 11 feet 2 inches and press Enter to confirm. This is where Revit's parametric modeling demonstrates its power—because all branch connections are linked to the main trunk, elevating the primary system automatically adjusts all connected components. The flexible connections will adapt accordingly, streamlining what could otherwise be a tedious manual adjustment process.

Zooming into our connection detail, I'll draw the required 45-degree transition fitting. Input "135" for the complementary angle, then use the Trim command to clean up the intersection geometry. These angular transitions are essential for maintaining proper airflow characteristics and minimizing pressure losses.

The next phase involves populating our supply air distribution points. Remember, supply air systems in modern commercial construction typically utilize Variable Air Volume (VAV) boxes for zone control and energy efficiency. We need to identify optimal locations where VAVs can serve multiple supply outlets effectively. For larger spaces, we'll specify 14 × 14 ductwork, while smaller rooms will use our standard 12 × 12 configuration.

Navigate to the Duct settings and change the size to 14 × 14. Locate "14" in the dropdown menu and set your elevation offset to 10 feet. I'll create a tap connection here and route it to approximately this location. This will accommodate our VAV installation.

Here's an important workflow reminder: I neglected to change the system type before drawing, which is why you see the color coding mismatch (blue and pink). Rather than redrawing, simply press Escape to end the Duct command, select the mistyped segment, and change the system type to "supply air" in the properties panel. Maintaining consistent system types prevents scheduling errors and coordination issues in construction documentation.

---

Let me demonstrate this correction process again, as it's a common oversight. Use DT to restart the Duct command and begin the next segment here, routing to this approximate location. Again, I'll need to manually correct the system type—select the segment and change it to supply air to maintain consistency.

Returning to the Duct command (DT), I'll switch to 12 × 12 sizing for the smaller zone distribution. Now I'll systematically create tap connections: tap off here and route inward, repeat this process for each required connection point. This methodical approach ensures we don't miss any supply points while maintaining proper system organization.

Perfect—all VAV locations are now established. Let's modify that end connection to 12 × 12 for consistency. Adjust the width parameter to 12 × 12, and Revit will automatically generate the required transition fitting. However, avoid positioning transitions within wall assemblies, as this creates construction complications. Select and slide this element back so the transition occurs in accessible space.

Now we'll populate the VAV equipment. I'll specify larger 12-inch VAVs for substantial spaces and use the 8-inch units we implemented on the lower level for smaller zones. Navigate to Mechanical Equipment and select the 12-inch VAV. Set the offset to 10 feet and rotate the unit so the inlet orientation aligns correctly with our ductwork approach.

Position the first unit here, then the second unit in this location. To ensure precise vertical alignment, I'll bring the section view down to this area. Select the VAV, then use RO (Rotate) to turn it 90 degrees for proper orientation.

Now I'll draw a short duct connection from the VAV outlet. Use the Match Attributes command (MA) to ensure this new duct segment matches the existing system parameters—select the source element first, then the target element. Apply the Align command (AL) to position these elements precisely, then use Trim (TR) to create clean connections.

Continue routing from this point, extending outward before transitioning downward. Let's adjust this to 14 × 14—the oversized 17 × 14 isn't necessary for this application. Route the duct outward and create the vertical drop to serve the lower distribution level.

From this main distribution point, we'll tap off to serve additional registers within the same zone. Return to the Duct command, specify 12 × 12 sizing, and use the 10 feet 3 inches elevation. Create the branch connection here, then continue to the next connection point. Adjust the equipment position slightly closer to the ductwork for optimal connection geometry.

Modify this connection to 12 × 12 as well, changing from the 14 × 14 to maintain consistent branch sizing throughout the zone. The system integration is developing well with these coordinated adjustments.

Notice the elevation conflict here: one system at 10 feet intersects with another at 10 feet 3 inches. Select the conflicting element and drag it clear, then rotate 90 degrees to resolve the interference. In the plan view, use the Split command (SL) to break this main run at the edges of the conflict zone. Rather than removing the center segment, drag it back to maintain continuity, but delete the unnecessary union fittings. Elevate this segment to 11 feet 2 inches to create proper vertical separation.

Draw the connecting 45-degree fittings, extending slightly beyond the immediate connection to provide adequate fitting space. Position these elements using the directional arrows to achieve efficient clearance over the intersecting ductwork. Use the Trim command systematically—remember to click on elements you want to preserve during the trimming operation.

Now connect the terminal units with flexible ductwork. Select each connection point, right-click, and choose "Draw Flex Duct" to create the final connections to the supply outlets. This flexible connection accommodates minor misalignments and provides vibration isolation.

Let's configure the upper VAV installation. Bring the section view up to this area for precise positioning. Rotate the unit 90 degrees and adjust the position as needed. Draw the outlet duct connection, use Match Attributes (MA) to ensure consistency, then align and trim the connections for clean integration.

---

Create the distribution tap from this VAV, using 14 × 14 sizing initially, then transitioning to 12 × 12 for the branch connections. Notice I'm using a 45-degree takeoff here rather than a 90-degree fitting—this approach is gentler on the airflow, reducing pressure losses and system noise. Thoughtful fitting selection like this distinguishes professional MEP design.

Continue with DT for the next duct segment, maintaining 14 × 14 sizing before transitioning to 12 × 12 for the final branch. Complete the terminal connections with flexible ductwork using the right-click "Draw Flex Duct" option for each supply outlet.

Now we'll install the remaining VAVs for this zone's completion. Navigate to the Systems tab, select Mechanical Equipment, and choose the 8-inch VAV. Set the offset to 10 feet—the unit orientation is already correct, so position it approximately here. We'll establish precise positioning for the first unit, then copy it to maintain consistent spacing throughout the installation.

Position the section view at this location—this "working section" approach is fundamental to professional MEP coordination. When dealing with three-dimensional systems, simultaneous plan and section views prevent costly coordination errors. Draw the connection duct from the VAV outlet, use Match Attributes to maintain consistency, then align the elements precisely.

1. Critical consideration when installing VAVs: always provide adequate duct length (minimum 1 foot 6 inches to 2 feet) between the VAV outlet and any fittings. This spacing is essential for proper air measurement and system performance. Size this outlet connection as 10 × 10 to maintain appropriate velocity.

With the first VAV properly positioned and connected, copy the entire assembly for the remaining installations. Select all related elements, access Copy from the Modify tab, use the endpoint as your reference point, and place the copies aligned with the remaining connection points. Enable the Multiple option to streamline repetitive placement.

Fine-tune the positioning as needed—precise alignment isn't always achieved on the first placement. Adjust this element back slightly to avoid interference with adjacent components.

To facilitate alignment in plan view, we need centerline visibility. Change the visual style to Wireframe, but note that centerlines don't automatically display. Access the Visibility/Graphics dialog (VV), scroll to the Ducts category, and enable centerlines display. Click Apply to implement this change. This centerline visibility is crucial for accurate alignment of MEP systems.

Now systematically complete all connections using the Align (AL) and Trim (TR) commands. Work methodically through each intersection: align first, then trim to ensure clean, coordinated connections throughout the system. This attention to detail is what separates professional-quality models from amateur work.

Complete the system by connecting all supply outlets with flexible ductwork. Select each connection point, right-click, and draw flex duct to the corresponding supply register. This completes our supply air distribution for this zone.

Zoom to extents and save your work with Control+S—regular saving prevents data loss during complex modeling sessions. We'll conclude this video segment here, having successfully established our second-floor supply air distribution system.

In upcoming videos, we'll route all ductwork systems up through the mechanical shaft and connect them to a rooftop air handling unit that we'll create and configure. This vertical integration completes our building's mechanical system design and demonstrates advanced coordination techniques essential for modern BIM workflows.