thoughts on App G system type for mixed use building

7 posts / 0 new
Last post

I've got a 92,000 square foot Industrial building that I am modeling.
If you went by Table G3.1.1A, the baseline system type would be System 5
- Packaged VAV with reheat.

However, of the 92,000 square feet, approximately 16,000 sq ft is
conditioned to standard 75/72 degrees, while the remainder is a mixture
of storage, manufacturing, etc, and is heated to 55 and cooled to 120
(no actual cooling in proposed model, but you've got to insert some sort
of cooling system per App G).

So I really have 16,000 sq ft that is fully conditioned, and 76,000 sq
ft that is only partially conditioned (ie the peak thermal load differs
by 10 Btu/h-ft2).

Any guesses how I configure this?

Do I have a packaged VAV system for the 76,000 sq ft of shop/storage and
isolate the 16,000 ft2 of real conditioned space as system 3 per the
Exception to G3.1.1?

Or do I have a packaged VAV system for the 16,000 ft conditioned space
and isolate the 76,000 sq ft as system 3?

To further complicate things, since the 76,000 sq ft is split between
storage and manufacturing, where there is theoretically an operating
schedule that differs by 40 hours or more a week between these two
different use types, do I actually just end up with ALL system 3 even
though the total net building area is above the 75,000 sq ft cutoff
where we're supposed to go to System 5?

I'm confused! Any help would be appreciated.

To recap:

16,000 sq ft office, heated/cooled to 72/75

30,000 sq ft of storage with basically no operating hours per week,
heated/cooled to 50/120

36,000 sq ft of manufacturing / workshop area that is used approximately
50 hrs a week, heated/cooled to 50/120

Mixture of System 3 and System 5, or ALL system 3?

James Hansen, PE, LEED AP

James Hansen's picture
Joined: 2011-09-30
Reputation: 200

Hi James,

It sounds like the storage and workshop area may be semi-heated as defined by Section 3. I'd recommend looking into that. If the spaces are semi-heated then they should be modeled only with heating in the baseline case and the requirements of Table G3.1.10 don't apply. It's also worth noting that if the spaces are semi-heated they should be modeled with different envelope values in the baseline case (based on the semi-heated in Table 5.5).

Best Regards,

Gregg Liddick, EIT, LEED(r) AP

Gregg Liddick's picture
Joined: 2011-09-30
Reputation: 0

Thanks Greg, but based on the heating density, I can't classify these as

James Hansen, PE, LEED AP

James Hansen's picture
Joined: 2011-09-30
Reputation: 200

I can't find an addenda on this, but which publication is correct for
defining the baseline system of a non-residential building less than 3

The base 90.1-2004 document says that if the building is 3-stories or
less AND < 75,000 sq ft, System 3 and 4 are to be used.

The User's Manual, on the other hand, says "For smaller non-residential
buildings that are less than 75,000 ft2 (any height) OR less than three
stories (any area)"...the baseline system is PSZ (System 3 or 4)

Which is correct?

James Hansen, PE, LEED AP

James Hansen's picture
Joined: 2011-09-30
Reputation: 200

90.1-2007 says "or", so I suspect that's the updated / corrected
understanding. (Makes more sense, too.)

James V. Dirkes II, P.E., LEED AP

James V. Dirkes II  P.E.'s picture
Joined: 2011-10-02
Reputation: 0


It has been awhile since I have really looked at 90.1-2007, but your space type has no bearing on what the system type is going to be. You just need to know whether your building type is res or not, this should be whole building. Seeing that you reference 2004 and assuming your project is 2.2 (also you should be referencing the actual standard not the manual for finality), you should be using system 5 or 6 due to the square footage. I got jammed up on the square footage vs. number of floors once myself. I got reviewed on a job where the building was one floor and 80k sqft but I had used system 3. When I changed to system 5 the savings doubled, I resubmitted and since the project was on the cusp of silver to gold, they got gold instead of the expected silver. I am assuming everyone was just glad things worked out there.

Secondly, I don't think whether the space is conditioned or not matters either. G expects all spaces to be conditioned. I say again, it has been awhile since I have thoroughly looked at this standard and I know a few people have quarks about the whole space conditioning expectations (just trying to get some good discussion going) seeing as how us HVAC/energy guys are so stuck on carbon-based realties. But, in reality (and to answer your question), the entire charade is all up to interpretation anyways :)

Kevin Kyte2's picture
Joined: 2011-09-30
Reputation: 2

To add my thoughts onto Kevin's

The whole building may need to be system 5, but they can be multiple system
So one system type #5 for the office area, and a completely separate
system type #5 for the storage area and another for the manufacturing. This
will make the return air for each system closer to the supply. Using three
separate systems would also make your baseline more efficient making your
design improvement lower.

I agree with your idea of modeling a cooling system but making the
set points extremely high and extremely low so they never turn on. I have
received this same advice on other projects.

Jeurek's picture
Joined: 2010-10-07
Reputation: 0