VAV Box CFM (Air Flow) Parameter

[lillianB5TFU]

Why does a VAV box family (OOTB Revit or made in house) have to have a value for CFM in the Air Flow parameter in order for it to work?  I could not get my VAV box to show the sum airflow from diffusers unless in the VAV box family the Air Flow parameter had a value of 350 CFM.

I only reached this conclusion because that is how the Revit OOTB family is setup.  Why is that?  I was assuming everything should be zero so you don't interfere with the sum.

Thanks,

[HVAC-Novice]

VAV just pass the flow through. So technically you only need to connect the 2 connectors. The one discharging air needs o "Calculate" airflow to add up the downstream diffuser flow.

I use the airflow instant parameter to verify and schedule the flow. but it isn't needed for the system to function (i.e. pass the flow to AHU)

[lillianB5TFU]

Connecting connectors is only useful if it's set to Global, which I don't want.

When you select the VAV box it should give you the CFM of all the diffusers connected to it.  The family doesn't do that unless there is a value to the CFM for airflow parameter.  And it it's not registering the diffuser cfm it can't pass it along up stream.

What is an airflow instant parameter?

[HVAC-Novice]

Why not global? That way it works in supply and exhaust systems. I realize a regulars VAV box with RH coil isn't used in exhaust. But airvalves, for example work in both and global would make it work in both.

You also can just use the instance parameter if you don't want to connect connectors. Then you see that value in properties when you select the VAV device.

Instance vs type parameter. Airflow should be instance. 

[lillianB5TFU]

I am not against Global, I just couldn't get the CFM to add with connectors linked so I moved on.

Set to Global, no parameter, connectors link - doesn't add up the CFM.

Set to Supply, instance parameter, connectors not linked - doesn't add up CFM

Set to Supply, instance parameter with a value of 350 CFM, connectors not linked - Works! Adds up CFM from diffusers.

Why? I've tried other variations to that too and the sweet spot seems to be the last one I mentioned. That seems broken to me.  I am working in 2020... but that is also how the Revit standard families for VAV boxes are set up as well.

I have a VAV box working, so I'm happy with that.  I just couldn't find anything online that was helpful for explaining what was happening.  I appreciate the responses! 🙂