Wouldn't one purpose of the air flow be for cooling whatever's in the room?

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Yes, sorry wasn't clear. I intend to "dump" the cool air into the space from the RTU via a downcomer of ducting (basically a framed opening with some minor ducting from the RTU thru the curb and roof penetration). I was just contemplating whether or not I actually need an air terminal device at the bottom of the downcomer or why not just have the duct wide open.

As long as cold air comes out one end of the room and goes in the other end of the room, it will technically cool the air of that room. I know code calls for balancing dampers for outlets but I can't imagine why you'd need one when there is just one outlet.

nuuvox000: Kind of what I thought, just checking with the HVAC "team" to see if there is some obscure code item prohibiting an open ended duct that I don't know about. I haven't recalled seeing this, although I have seen open ducts for things like combustion air. Regarding balancing, the unit fan VFD will be adjustable for overall system balancing (and we do have a balancing damper on the return duct (as well as dampers and balancing means on the RA/OA in the unit for moving between min position and economizer modes)).

Yeah, you should be fine; we do this all the time but just put a dropbox diffuser on the end of the supply duct but it's almost the same as what you're doing.

You have to make sure you evaluate the static pressure. The motor will look for a certain amount of static pressure, if it can't find it, it will keep ramping up until it burns itself out.

Adrienne Gould-Choquette, P.E.
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Adrienne: Good point, but in this particular situation, the fan/motor won't be running at 0" ESP---the return duct will be routed to the other end of the space and includes straight length, grille, fire damper, couple of turns, etc.; and of course there are internal static pressure (ISP) components as well (filters, coil, mixing dampers, etc...). So the fan/motor will be selected based on TSP and of course there is a little more flexibility with VFDs now as opposed to the old days with constant speed motors and sheave/pulley and belt changes to balance it.

I can't think of any code requirement for providing a diffuser. However, diffusers improve air distribution and help ensure air gets where you need it versus just creating a pocket of cool air.

Depending on who your occupants are, select groups of people tend to be mischievous. Diffusers can help prevent open ended duct from being used to hide things.