Revit issued a few warnings about creating duct connections in the wrong direction. These required deleting the elements the automation tried to place.
I'll check what HVAC content the project has: round duct types, supply system types, air terminal and air handler families.
Sliding along the trunk line moves the riser toward the atrium edge. Instead I'll pull the south riser back toward the corridor wall, between the two ring beams.
A south riser offset of 20'-7" from the exterior wall line clears everything. Next I'll pick each rooftop unit's orientation so its footprint sits on the roof deck clear of the atrium opening.
The south unit clears the atrium opening by just over 2'. Exporting the duct layout so I can send it to Revit:
Placement pass: fix the three corridor beams, place the 42 diffusers (face at 8'), set both rooftop units on the roof, and cut riser shafts through the floors and roof.
That call timed out on the computer side. It isn't safe to repeat, since it would duplicate the diffusers, so I'll check what landed first.
The flip worked (all 42 necks now face up), but the move didn't take, because the diffusers are hosted on their work planes. I'll set the offset from host instead.
The offset isn't taking effect either. Moving the three diffuser reference planes up ½" will carry the hosted diffusers with them.
Diffuser faces now sit exactly at 8'-0" on every floor. Next I'll build the L1 ductwork as a test: the 18" trunks, 7 taps per side and the 12" branches. If that's clean, I'll repeat it on L2 and L3.
L1 built cleanly: both trunks, all elbows, and 14 branches with taps. I'll quickly confirm every L1 diffuser is actually connected, then build L2 and L3 the same way.
Both systems exist with 21 diffusers each, but Revit still sees a break in the physical network. I'll find any unconnected duct or fitting ends.
L1 works: each drop is a 12" elbow plus a short straight piece into the diffuser neck, and all 14 diffusers are connected. Same for L2 and L3:
All 42 diffusers are connected. Last pieces: cap the six trunk dead ends, rebuild the two supply systems, and confirm there are no open connections.
Each diffuser is correctly at 225 cfm, so the system-level figure may be measuring something else. I'll check the flow Revit computes in the ducts at each rooftop unit.
All 42 diffuser connectors read 225 cfm "In", so the terminals are right and the issue is in the roll-up. I'll check the system type's calculation setting.
The cap utility only works on pipes. I'll place Round Endcap fittings directly instead: orient each to face its trunk, size it to 18", then connect it.
Capping fixed the flows. Each system now carries 4,725 cfm, building up 225 cfm per office toward each riser. One detail: the caps came in at 12" instead of 18". I'll size them to match the trunks.
Caps are now 18". A quick HVAC-only 3D view to check it visually:
The supply-air HVAC system is in. Both systems are continuous from rooftop unit to diffuser, with no open ends and no warnings. I added a "Supply Air HVAC - 3D" view that shows only the ductwork over transparent floors.
What's in the model:
- Diffusers: 42 supply diffusers, one per office on each floor. Each is 24×24 with a 12" round neck, face at 8'-0" above the floor, set to 225 cfm.
- Branches: each office has a 12" branch using the "Taps / Short Radius" duct type. It takes off the trunk with a round tap, runs at 9'-4", and drops into the diffuser through an elbow.
- Trunks: 18" round trunks using the "Taps" type run along the corridor on each side of the atrium on all three floors. They're capped at the west end.
- Risers: one 18" round riser per side using the "Tees" type. Each starts with an elbow at the L1 trunk, takes tees at L2 and L3, and connects to its rooftop unit through a round-to-rectangular transition. Shaft openings cut through L2, L3 and the roof.
- Rooftop units: RTU-N and RTU-S, 12.5-ton packaged units with bottom supply, set on the roof deck clear of the atrium opening.
- Systems: "SA-N (RTU-N)" and "SA-S (RTU-S)" each serve 21 diffusers at 4,725 cfm. Revit's flow roll-up checks out: 225 cfm accumulates per office along each trunk, and 1,575 cfm per floor feeds each riser.
Frame changes to make room for the ducts:
- Corridor-line ring beams: all 42 (one per bay, per floor) are now 10¾"×18" instead of 21". This let the 12" branches cross underneath with enough height for the elbow drop. At 22' spans that's on the shallow side for glulam, so it's worth flagging to your structural engineer.
- One corridor beam line: the 24" beam on that line was also reduced, on all three floors.
Decisions for you:
- Riser and trunk velocity is high. The specified 18" size carries the full 4,725 cfm only in the riser above L3, which works out to about 2,700 fpm. The trunk segments nearest each riser run around 900 fpm. Upsizing the risers to 22–24" would bring them into a typical low-pressure range.
- This is supply only. There's no return or outside-air path yet; the rooftop units have return connections ready.
- Ducts pass through the corridor walls without cut openings.
Saving is still on you: use Save As on "Project3", and after that I can save from here whenever you like.
