VAV Box Not passing CFMs through

[jessem5CN9L]

Hi! I have a home-grown VAV bow that, until recently, was working great. Unfortunately, it looks like it's not passing the CFMs from downstream to upstream in the duct systems. See images.

What's wrong with the family that it won't pass the CFMs through?

[bjlottis]

Are you sure it's the box? Are there any open ducts upstream of the VAV box that can be messing with the system? You can check this by temporarily deleting the reducer just upstream of the box and then checking the CFM of duct in your screenshot.

[hmunsell]

Duct connectors have 3 settings depending on how they are being used. 

• System
• Preset
• Calculated

I can't say I totally understand the difference, but I believe if you set the connector to Calculated, it will accept the flow from up/downstream and add/subtract depending on the elements connected to it.  It depends on where the information is coming from/to.

https://help.autodesk.com/view/RVT/2023/ENU/?guid=GUID-99B57EF9-BBDD-4C34-8AE2-50EA6AE446E7

[jessem5CN9L]

Thanks for the reply.

I changed the connectors to have calculated values and that didn't seem to be the issue. I also just for fun changed both connectors to bi-directional but I didn't see any change either.

[jessem5CN9L]

Not sure. I disconnected and reconnected the ducts multiple times but it didn't resolve the issue. I did an analysis of disconnects and everything is connected well. When I click on the box it even says that it has CFM flowing thru. but on the upstream that CFM doesn't show up. 

[hmunsell]

as @bjlottis mentioned, it may not be the VAV, what is on either side of the equipment?

• would you be able to upload the model for us to look at?
• maybe just an empty file with a mockup of the equipment and how that system is connected.

[bjlottis]

So when you delete this reducer and check the CFM at the duct it's still 0?

[hmunsell]

does this piece of duct have a flow?

[iainsavage]

..

[iainsavage · Accepted solution]

You need to associate both of your connectors with one of your flow parameters such as CFM. 

This will pass the flow from the Calculated, Out connector to the Preset, In connector.

Alternatively you can make both connectors Global and then link them (linking ONLY works with Global connectors, it has no effect with Supply Air connectors):

[jessem5CN9L]

Yes

[jessem5CN9L]

No

[jessem5CN9L]

Associating the connectors with a CFM parameter worked! I clicked on the Flow parameter and associated it with the CFM param, reloaded it into the project and the CFMs carried through.

I'm going to keep that Global Link Connectors in mind tho, that will be a handy solution in the future! Thank you!