## Question about plenum depths and sketch rectangles

Building Section Vignette and Multiple Choice

### Question about plenum depths and sketch rectangles

Let's say for a floor my largest duct is 20" and my largest joists in the zone/space is 24". That would make my plenum depth 48" for the entire floor.

Going off the example of a 48" plenum depth. So your biggest joist depth is 24" and duct is 20" BUT lets say your section cut goes through 12" joists and not 24", (so smaller joists then what your plenum requires are what you're required to show because of where the section is cut).

Ideally when using sketch rectangles to lay this out, people typically follow the order or light fixture clearance, plenum depth and then place their joists in the drawings on top of the plenum depth.

Question: If my plenum depth is 48" and my joists are 12" (smaller than what is in the largest duct joist combo) would you draw your joists INSIDE the plenum depth sketch rectangle or resting on top of the plenum depth sketch rectangle like how pretty much every youtube video and tutorial shows?

My thought would be that you would draw a smaller joist that's being cut INSIDE the plenum depth sketch rectangle, because ideally a smaller joist would fit in there as you made the plenum depth account for a much bigger joist. And the whole point of this thing seems to be that you're drawing for the worst case scenario WHILE being efficient. If you put the small joist on TOP of the plenum depth (that's already accounting for the worst case scenario) aren't you technically overcompensating and adding 12" to what you already adequately have?

hangryarch

Posts: 15
Joined: Thu Dec 07, 2017 7:00 pm

### Re: Question about plenum depths and sketch rectangles

You confused the hell out of me. Can't imagine how confused you are.

Auto thinking so hard.

When you set it up for the largest combo, that's it. Top of joist is the same for all joists within that combo area.

Coach

Posts: 13242
Joined: Wed May 23, 2012 2:08 am

### Re: Question about plenum depths and sketch rectangles

Okay, here's what you need to think about. Floors are level and ceilings are level. You need the deepest joist and the deepest duct at that joist. You need the deepest duct and the deepest joist at that duct (these might be different). Choose whichever one is bigger. That plus 8" is your plenum. In the example you give, you would have 52".
thd7t

Posts: 400
Joined: Mon Aug 22, 2016 1:23 pm

### Re: Question about plenum depths and sketch rectangles

thd7t wrote:Okay, here's what you need to think about. Floors are level and ceilings are level. You need the deepest joist and the deepest duct at that joist. You need the deepest duct and the deepest joist at that duct (these might be different). Choose whichever one is bigger. That plus 8" is your plenum. In the example you give, you would have 52".

NO!

The deepest combo doesn't necessarily include the deepest joist or the deepest duct.
Let's say you observe these combos in an area with the same ceiling height:
J/D
12/20
24/16
30/8

Which do you use?

Coach

Posts: 13242
Joined: Wed May 23, 2012 2:08 am

### Re: Question about plenum depths and sketch rectangles

I see. 24/16 since it's the total deepest at 40.
Thanks for bringing joist/duct combo discussion to the forum.
nicolenalvarez

Posts: 42
Joined: Sat Feb 01, 2014 4:16 pm

### Re: Question about plenum depths and sketch rectangles

Oseler

Posts: 7
Joined: Tue Jun 16, 2015 7:01 pm

### Re: Question about plenum depths and sketch rectangles

No.

The deepest combo doesn't necessarily include the deepest joist or the deepest duct.

Use the largest co-located combination.

Coach

Posts: 13242
Joined: Wed May 23, 2012 2:08 am

### Re: Question about plenum depths and sketch rectangles

Dumb question, but just for clarification... If the largest J/D combo is somewhere other than the section cut, you simply draw the plenum space to equal that combo and then simply draw the joist as it occurs at the cut line, correct?
ChappyEight

Posts: 4
Joined: Mon Mar 26, 2018 10:40 am

### Re: Question about plenum depths and sketch rectangles

ChappyEight wrote:Dumb question, but just for clarification... If the largest J/D combo is somewhere other than the section cut, you simply draw the plenum space to equal that combo and then simply draw the joist as it occurs at the cut line, correct?

Correct. Ceiling height and floor level are consistent.
thd7t

Posts: 400
Joined: Mon Aug 22, 2016 1:23 pm