Can A Room Be a Return Plenum?

[CuriouslyGeorge]

I am working with sketchy details of an area I'll never get to visit. Even worse, I can't contact the one who surveyed the area because of budget cuts. Anyway, I have a room with a Liebert which has supply ducts shown coming off it. The supply feeds the immediate room and remote areas.

As for return air, it appears there is a vent shown in the far wall with an indication that return air is being dumped back into the room itself (from the supply air feeding the remote spots). I assume the intake to the Liebert is simply an open hole sucking all this air back into the Liebert.

Does this sound right, or should the return be ducted? Note: The Liebert is in a computer room, and wouldn't the return air be dumping a lot of residual heat back into the computer room? But, if we accept this as a poor design, is it possible to return air in this fashion?

Thanks for helping me with both hands tied behind my back!
Curiously George

 

[HVAC68]

I trust that the Liebert AHU is in a separate room and the return air is just dumped back into this room. This would mean that the entire room acts as a return air plenum. This would also mean that there will be heat addition on account of the walls, roof, floor, etc., of the AHU room, which will of course get added as a return air heat gain.

Ducting the return air back to the AHU is not a must. There are many installations where the return air is just dumped into the AHU room. I suppose that multiple AHUs serving areas with different room conditions are not existing in this location.

HVAC68

 

[imok2]

I suppose the question to ask is...is this system working now? What you say is not all that unusual.

 

[CuriouslyGeorge]

Yeah, it's working, but: The Liebert is NOT isolated from the conditioned space (such as being placed in an adjacent mechanical room). Actually, two Lieberts are in operation. With my limited experience, I've only seen Lieberts recycle the air within a computer room ONLY...so it surprises me.

My surveyor's details are sketchy. I guess I can only go on "the facts of the case"*. Based on the sketch I have, it seems the return air is just dumped back into this computer room. The adjacent areas are offices which, I have to speculate, have no unusual heat loads.

*unless there is interest in re-surveying the area. This is a low-budget, band-aid type project so I doubt that'll happen. Still, I hate doing "fly-by" engineering work!

 

[McCormick93]

CuriouslyGeorge,

Congratulations, you have challenged the lousy status quo known as computer room air conditioning. Unfortunately, mechanical engineers have been outnumbered by IT folks for some years now, and we have been stuck with horribly inefficient air distribution schemes.

Lieberts typically do not have ducted returns; they pull air directly from the room. You are correct that residual heat from the computers gets dragged through the space. This is common practice, and has created problems everywhere. Ducted returns work much better from a heat management perspective, but aren't widely used, maybe because they don't look nice.

If there is a return air plenum above the ceiling, I suggest ducting the Liebert return to it, and locate return air intakes above heat sources.

 

[friartuck]

I'm interested to know how the controls are configured. I assume the Liebert has a return air sensor and so only controls the space its in.
This arrangement isn't untypical of many I've seen mackled together.

The client gets what he can afford. Usually they want to condition three or four areas but can't afford to. The result is a poor compromise.



Friar Tuck of Sherwood

 

[NNACons]

CuriouslyGeorge,

I have a few concerns with the system you have described.

Firstly, computer room units tend to be cooling only systems which also provide humidification and dehumdification to the space in an attempt to provide close tolerance db and humidity control. Firstly i would check to see if the adjacent space from which retur air is being dumped back requires heating? If so, there is a good chance occupants in this space are experiencing less than ideal comfort during winter or early morning occupancy periods where heating requirements may be greatest. If there is local space heating to this area, then this system is being comprimised by the PCU cooled air.

Secondly, during normal operation, there is a good chance that the adjacent space is experiencing a loss of conditions as the PCU in the computer room is responding to a local thermostat and not to remote area load requirements. Zoning issues may thus be inherent in this setup.

Thirdly, Check to see that you are complying with ventilation requirements to the remote area as well as the computer room. Are these spaces being served by another system which has fresh air provisions. If not and their are no measures for natural ventilation, then you may need to review the design.

Fourthly, you are faced with a very uneconomical system design which provides continous air conditioning to an unoccupied remote space on account of continous operation to the computer room. Not ideal.

I would suggest you try to isolate the computer room conditioning system from the remote office. Blank of ductwork, rebalance (reinstate to original design intent) and then install a dedicated office unit to serve the remote office space.