Return Air has no physical connection

[Ian Matthews]

This was mentioned in a post under the Families section, but I thought it important enough to bring up here.

Is it possible to design an air conditioning system if the return air is not physically connected to the air conditioning unit? Almost all of our projects have some sort of plenum return - generally via the ceiling void.

[devitg.89838]

Of course you can make the return air to run by plenunm , just locate the proper filter at the air incoming to the evaporator and fan .

[Michael.c]

Of course you can make the return air to run by plenunm , just locate the proper filter at the air incoming to the evaporator and fan .


Um....I think you mis-understood the question there Devit!
What Ian is getting at, (and it's a good question) is that with just a plenum, as opposed to a hard-ducted system, there is nothing for Revit to recognise, to enable the connectivity in the system to be maintained.

Right Ian?

(Oh, and by the way....I don't have the slightest clue to the answer....but I'd like to know, too!)

[mjdanowski]

I had an idea of how you could possibly do this in a wishlist item, but at the moment the only thing you can do is duct it up and then just hide the ducts in view.

http://forums.augi.com/showthread.php?t=84566

[JoelLondenberg]

What specifically are you missing? You can assign the system and equipment without ductwork. You should be manually sizing the plenum drop anyway to account for filters and sound. The system browser will still show total return flow. What's else is needed?

[Steve_Stafford]

I guess being able to acknowledge/eliminate the warning messages that result from the lack of "Physical Connection" would be a good start?

[mjdanowski]

I believe there will still be a disconnect of flow when there is no physical connection as well.

[JoelLondenberg]

I believe there will still be a disconnect of flow when there is no physical connection as well.

My question is, what does that "disconnect" prevent you from either modeling or documenting or calculating?

[mjdanowski]

Lets say you have 10 air diffusers with 100CFM each and connect them all to a system of ducts which go to an supply air terminal unit. If you have the ducts physically connected, it will calculate the appropriate flow, velocity, etc in each duct. If you put a break in the duct though, everything on the SATU side of the break will not calculate the flow within the duct from the air devices.

[JoelLondenberg]

I apologize for being so dense, but what is missing from the system in the attached screen shot, apart from good design that is ;) ?

This is roughly how we would show a plenum return system, return air grilles, lined boots, and open space to the return drop from the unit. Automatic duct sizing doesn't apply because we generally oversize the short bits for plenum return. Scheduling the return air flow would have to be done manually. What's left?

[Steve_Stafford]

I apologize for being so dense, but what is missing from the system in the attached screen shot, apart from good design that is ;) ?

This is roughly how we would show a plenum return system, return air grilles, lined boots, and open space to the return drop from the unit. Automatic duct sizing doesn't apply because we generally oversize the short bits for plenum return. Scheduling the return air flow would have to be done manually. What's left?

What is reported when you use "Check Duct System"...most likely "Return Air System xxx..."? As long as you don't use or care about that...I guess you are good to go! :smile: