Return Air Systems

[dmb.100468]

I have a building that uses the ceiling plenum for return air. There are 3 Rooftop units that each have a supply air and a return air connection. I have gone through the process of creating all of my supply air systems. I am now trying to create return air systems. It allows me to select one of the return air grilles in the ceiling and create a return air system, but it will not allow me to select equipment (one of the rooftop units) for the system.

So in the systems browser all of my return grilles are part of a system, but my rooftop units are still "unassigned" under Return Air.

I guess my question is do I need to bother creating return air systems for each unit if I'm using the plenum? This is all a little over my head, but I want to make sure I have everything assigned to the appropriate system.

[neb1998]

I havent tried, but have you tried to create a custom return air duct thats very thin, like 1/256" of an inch that is the size of the ceiling...and connected to it?

I did this with an early version of systems in beta and just changed the duct work to the size of the ceiling and it worked ok, but i wasnt sure if my SP calculations were correct.

[ham128]

Did you get the plenum return to work correctly using an extremely large and extremely thin duct to tie the systems together? I am in the same process of trying to tie my return grilles to the plenum return intake in the ceiling.

[alan.jackson]

okay. a couple things here. i believe i know what your problem is with the rooftop unit, as i have experienced similar problems. you need to make sure that the rooftop unit is not already part of a return air system, even a default return air system. you should be able to find the return air system in the system browser and remove the rooftop from it by deleting the system. you should then have the rooftop available in the drop down menu for equipment when editing the return air system created from the terminal.

as far as the other topic on plenum returns. has anyone tried to use the return air connector to define the plenum instead of a large thin duct as described. i have not, but i believe it may be an option.

-
aj

[kyle.bernhardt]

Don,
Check out the Connector Flow Direction parameter on your Rooftop and be sure that it is set to "In". Does that make a difference?

Kyle B