Plenum Return - Return Airflow Path (Walled Room vs Open Office)

[MECHJAMIECTA]

I had an issue come up this week that I am curious about. I have always been told something on other jobs by my superiors at work but it is getting questioned on a job I am working on now by the architects and I can't find it explicitly in the code.

The issue is this: In an office environment, using a full plenum return, there always tend to be big open office areas mixed with enclosed offices and conference rooms that have walls extending to the drop-ceiling. I have always been told that there needs to be a dedicated return grilles in these walled offices and conference rooms since you cannot, by "code", base your design around the air being supplied to these enclosed offices but then transferring to the adjacent open office area to be returned.

I cannot find anything in the code against this other than stating that '*Corridors* shall not serve as supply, return, exhaust, relief or ventilation air ducts.' Well, these open office areas don't seem to fall under the definition of a corridor in the code.

Now, I am still currently designing my building this way with dedicated return grilles to the plenum but the architects are wanting me to do something different and code-wise I can't find a reason to back up the way I am doing it.

Has anyone else run into this? Other than the fact that the doors to the offices might not always be open and you'd have to provide door grilles to physically allow the air to transfer out to the open office area, what in the code would prevent me from transferring air from a walled office, to the adjacent open office area, and then to the the plenum?

 

[walkes]

No return; then limited airflow when the door is closed and potential whistling through cracks (depending on the available static pressure from the fan). We put acoustically lined transfer elbows at high level with a grille on the outside wall (to the open office)

 

[MECHJAMIECTA]

Thanks for the suggestion and I guess that also answers my question:

So, other than the fact you would have to account for area since the door would be closed at times, you see no code reason the enclosed office HAS to have a dedicated return grille?

The scenario I ran into was that, on the southern wall of our building (imagine a square office building) they have a row of walled offices with a drop ceiling (walls go to the drop ceiling). North of that, for the open office area on the side side, along the entire span east-west, they wanted the no true ceiling, just open structure, with a floating "cloud" ceiling/surface above the cubicles. This "cloud" isn't an enclosed plenum, just hung, horizontal acoustical boards with no vertical piece that encloses it like a normal dropped ceiling and plenum.

This essentially meant that the ceiling plenum of my southern offices were now completely isolated from the plenum in the northern half of the building by this completely open, no-ceiling area. My building was designed as a full plenum return with a completely enclosed plenum until this point.

My only option was to design it so that the return air from my southern offices "transferred" to the open office area and then transferred to the ceiling plenum on the north half of the building via some transfer grilles. This felt "off" to me since I was always told to only transfer air if I am using it as make up, and otherwise to return directly out of a room I supply to. However I am also not as experienced in doing commercial work.

 

[marcoh]

Typically your options are to (1) provide a door grille, or (2) a ceiling return air grille in the enclosed office (plus a transfer duct if the partitions are slab-slab). Door grilles are cheap, but offer limited acoustic attenuation.

A suitable return air path must always be provided, because if air cannot escape from the room, the supply air will not go in and it will get hot. This is not really a 'code' compliance issue, but is considered normal engineering and design.

 

[MECHJAMIECTA]

sorry, I think I may have misled you. As you probably know, engineering problems can be a bit tricky to describe simply by a forum post...

I definitely do understand that I need to supply a suitable return air path out of the room - this is definitely normal engineering like you said. My plan was for a perforated return air grille in the ceiling of these walled offices.

My concern was that the open office they are adjacent too has an exposed ceiling. So, the walled offices have a 9ft drop ceiling, then the open office area (cubicles) adjacent to them is open to structure with exposed ductwork etc. Once the return air from my walled offices would go through their respective return air grilles, it would essentially just be migrating into the open office area and would then drift toward the middle of the building that had an enclosed drop ceiling ceiling plenum again and would be picked up into the plenum via return grilles there.

My concern was that, even though I am providing a suitable path out of the open wall office, I would then essentially be using the open office area as the return path to the plenum in the other part of the building. I was concerned that this might not be acceptable. I know that if these offices were off of an egress corridor, I would not be allowed to transfer the air to the corridor and then to the return (even if I provided a suitably sized path to the corridor) due to code. I have heard of code officials arguing that, unless you are using transfer air to makeup for exhaust, they will not accept you transferring return air from one space to another before returning it to the system.

 

[Drazen]

i think that is not good for general functional reason, as you can have different occupancy and pollutants in different offices, and your open office would work as used-air sink. You cannot control air quality in open office that way, and main reason to install ventilation system at all is to achieve specified air quality.

 

[willard3]

Ceiling plenums in general do not work well.

Hard to balance, hard to control.

 

[MECHJAMIECTA]

Drazen: I agree. That was my concern. Even though I can't find anything in code against it, it seems against good engineering practice as an HVAC engineer.

willard3: I can understand that, but it is still very common practice in many office buildings. The job I am working on has a real budget constraint and an extremely fast schedule for HVAC. A ceiling return for this VAV system was by far the cheapest option and allowed my designer to bypass having to route most return ductwork.

 

[walkes]

You should likely review ASHRAE 62 and determine if transfering the air reduces the classification of the space. If it is a concern, another option may be to provide a return fan ducted from the south offices to the north plenum, avoiding tranfer through the open office.

If you had a displacement system, the return air being warmer should remain near the ceiling and not mix with the ventilation in the open office. With standard ceiling diffusers, some of the return air will be induced in to the airflow in any case.

 

[Shophound]

>>>>>You should likely review ASHRAE 62 and determine if transfering the air reduces the classification of the space. If it is a concern, another option may be to provide a return fan ducted from the south offices to the north plenum, avoiding tranfer through the open office.

If you had a displacement system, the return air being warmer should remain near the ceiling and not mix with the ventilation in the open office. With standard ceiling diffusers, some of the return air will be induced in to the airflow in any case. <<<<<

+ 1 what was said above.

I think I can picture pretty well what the OP is describing, and what I see happening with the transfer grills/ducts for the southern row of enclosed spaces is the supply pressurizing each room as their respective VAVs stroke open, thereby forcing air into the common space through the transfer means, which in turn will mix with the air in the common space.

There is no way to assure a direct line of sight return to the AHU with this configuration without a dedicated r/a duct spanning from the north side of the structure to the southern section. This was probably not desired by the architects or interior designers of this space for aesthetic reasons (or it was simply "value engineered" out of the plan), but it is the only way to assure dedicated return without mixing the southern office space air with the common space air.

My general experience with drop ceiling plenum return schemes is that they don't always work as intended, and even where they do it can be spotty. I've seen air actually entering an office through the perforated return grate in the drop ceiling from the plenum space above, and in another case with the door open the return in the office drew in air from the common space outside the office, because that particular return in the ceiling was near the intake for the dedicated return duct back to the AHU.

I also don't like ceiling plenum return schemes because you essentially suck on the building, which unless great care is taken is far from airtight.

 

[willard3]

You may have been backed into a problem that can only be solved by a proper air distribution system, including return air ductwork and proper balancing.

I don't know who set your schedule or your budget, but now is the time to tell your client that what he wants won't work and the design needs to change.

 

[MECHJAMIECTA]

Willard3:

That is basically exactly what I had to do. Once they saw the increase in cost (both materials and labor) for all the return ductwork above what they were already expecting, they forced the architects hand to instead to a raised/pop-up ceiling in the area where he wanted to completely remove the ceiling. I now have a completely enclosed plenum.

I do of course now have the pleasure of trying to squeeze a main supply duct through the 10.5' ceiling portion, but a bad aspect ratio for 15-20' is better than the situation I was in before I feel like