FAILED VIGNETTE - DUCT QUESTION?

[harper1980]

This vignette typically involves a section through a two-story volume adjacent to a one story volume (at a different roof height).

My question is, what do you do when a duct crosses these two volumes shown in a 2nd level RCP, and there is NO vertical shaft/mechanical room to change the height of this duct between volumes?

The NCARB 4.0 and 3.1 vignettes both have a vertical shaft/mechanical room at this cross point.

My exam vignette did not.

Dorf indicates you should adjust the plenum for the duct to run horizontally across the two volumes in the passing solution, so I did this, but that meant increasing the plenum of one of the volumes by about 6', which in my case turned out to be wrong.

I was really methodical with this vignette, finished early and double checked thoroughly. I literally spent the last 10 minutes staring at this duct cross point because it didn't seem right, but I was following the logic that Dorf provided. It's frustrating because I passed all the multiple choice sections Level 1, and felt I was adequately prepared for this vignette.

Can anyone explain why this is wrong?

[esagun]

[quote][/quote]

[harper1980]

Hi esagun,

I welcome your comment. I'm not certain this is why I failed, but I am fairly confident.

The only practice example where there is no shaft/mech room at this transition is the Dorf Guidebook (2010 version), and he makes a point to adjust the plenum to allow for this transition in the passing solution. It's the only reason I toiled with the decision, otherwise it's probably a detail I would have overlooked, and assumed that the duct changes height between spaces.

I can send an image of what I'm talking about later, but please refer to the Dorf Guidebook.

Neither the NCARB nor the other youtube videos address this transition because all the examples they use have this shaft.

[harper1980]

p.s. the plenum depth is considered a "major" component, and the fact that I oversized it by 6 feet to accommodate the duct transfer leads me to believe it's enough to fail on it's own.

[esagun]

[quote][/quote]

[harper1980]

I found a previous discussion addressing this issue (partially), but there is still no definitive explanation that I've been able to find.

viewtopic.php?f=10&t=11588

Essentially, I think I failed the vignette because I applied the principles described in the Dorf Vignette #2.

My best guess is that the NCARB assumes the duct can move vertically between volumes, and the two volumes should have separate plenums because it is separated by a load bearing wall.

[CMS-5PS]

I believe the practice exam vignette in the small Ballast/PPI CDS book has a similar scenario.

In the passing solution, they don't show any increased plenum space to account for the ductwork going from the high-ceiling library to the adjacent (higher-ceiling) second-floor office space.

This makes me wonder which direction to take if I get a similar problem on the exam.

[harper1980]

I've been communicating with NCARB officials in the google community. So far all they said is that they do not officially recognize the Dorf material, and I followed the logic of the Dorf sample vignette to a failing solution. I'm pretty sure I had everything else right.

Therefore, based on my experience, I think you should evaluate the plenum of the two different volumes on their own terms, and not adjust the roof of one to accommodate a continuous duct, especially if it mean changing the roof height of one of the volumes by a bizarre amount, like 6'.

This one stings because I waxed that test otherwise. It's a good thing they are doing away with these vignettes in 5.0.

[harper1980]

Furthermore, the definitive answer from NCARB is to focus on the horizontal ductwork for the two volumes separately, based on this discussion:

https://plus.google.com/u/0/11480251042 ... rECXsEfE4P

DO NOT worry about vertical movement, even though the Dorf and the real world says you should.

[Archit3ct-2015]

Harper,

Just to clarify - NCARB's response to this situation is that they are here to address specifically what is in the horizontal section and the plenum and we should not oversize the plenum for the duct transfer from one section to another.

Please clarify.

Thank you

[harper1980]

Yes, although they didn't explicitly say it that way, I interpreted it to mean DO NOT oversize the plenum for a duct to transfer between the two volumes, whether there is a shaft or not. There are other ways for a duct to run outside of the plenum (on the roof with insulation for example). Focus only on the section and joist/duct combo dimension.

I understand this answer is still incomplete, but it's my interpretation of what NCARB wants.