Duct Sizing tool not working on high side of VAVs in 2025.

[juliefL3JKF]

I have 5 Titus VAV boxes run out to diffusers. the VAV boxes pick up the CFMS from the diffuser as they should. I combined the high side going to the boxes to run a single duct into the shaft. In 2023, the duct sizing tool would size the high side for me. In 2025, I get error messages saying the high side ducts do not have any air flow. What can be the cause of this? The Titus VAVs have the duct connectors flow config set to Calculated & the FLow direction set to in. The ducts are fully connected except for the open end in the shaft. Does it have to be connected to an air handler in order for the ducts to pick up the air flow or is there something else I am missing?

[robert2JCCH · Accepted solution]

Flow Configuration for an In-Line device (like VAV) is:

• Calculated + Out on the diffuser side
• Preset + In on the Air Handler side.
• Flow Parameter assigned to both Connectors.

You should not need a final upstream mechanical equipment (Air Handler/RTU/etc.) to manage the flow calculations. You also don't need to Link Connectors, the common parameter performs that function just fine.

I have noticed some weird system behavior starting in 2024 with excessive generation of Duct Systems that occasionally interfere with calculations + flow direction. If it's not related to the family, it might be related to that.

Fastest way to troubleshoot this would be to either provide a stripped down version of the project file (keep only the duct section of interest and remove everything else), or provide some screenshots for clarity. What generally helps is:

• Floor Plan with Check Duct Systems and Show Disconnects enabled. If there's a Duct System warning then copy the warning dialogue.
• Screenshots of the VAV box family connector settings
• Screenshot of the System Browser. When I say excessive generation of Duct Systems:
  • 
  • You'll have extra systems with no Drop-Down button that have 0 flow, but you can still 'find' them and they'll point to ductwork.

Also worth checking - are your duct systems split properly? If you slice a duct system to then add in a VAV box, sometimes it does not properly split the system in two: all downstream components should be a different system than the upstream components.

[juliefL3JKF]

Thank you Robert! The issue was that the Flow parameter was linked to different air systems, design conditioned air on the out & design supply air of the in conector. That is not something I would have figured out on my own, so thanks for the great explaination.