ASHRAE 90.1 baseline HVAC sytem description

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All,

Can anyone share their experience with the modified baseline model HVAC description in ASHRAE 90.1 - 2007 and 2010? (copied below) ASHRAE 90.1-2004 did not require the baseline model for systems 5,6,7 and 8 to have separate HVAC systems per floor and I'd like to hear how people have interpreted this and, if anyone can explain the reasoning for this.

For example, the proposed model of a 15 story office building might have 2 AHUs, one serving the east wing and one serving the west wing. For the baseline per ASHRAE 90.1-2007 or 2010, will these 2 AHUs be replaced with 15 separate VAV units? Or can the 2 AHUs be kept as long as it satisfies the statement "Floors with identical thermal blocks can be grouped for modeling purposes"?

ASHRAE 90.1 - 2004
G3.1.1 Baseline HVAC System Type and Description.
HVAC systems in the baseline building design shall be based
on usage, number of floors, conditioned floor area, and heating
source as specified in Table G3.1.1A and shall conform
with the system descriptions in Table G3.1.1B.

ASHRAE 90.1 - 2007 and 2010
G3.1.1 Baseline HVAC System Type and Description.
HVAC systems in the baseline building design shall be based
on usage, number of floors, conditioned floor area, and heating
source as specified in Table G3.1.1A and shall conform
with the system descriptions in Table G3.1.1B. For systems 1,
2, 3, and 4, each thermal block shall be modeled with its own
HVAC system. For systems 5, 6, 7, and 8, each floor shall be
modeled with a separate HVAC system. Floors with identical
thermal blocks can be grouped for modeling purposes.

Thanks!

Linda Lam

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Yes, for ASHRAE 90.1-2007 and 2010 you will have 15 separate VAV AHUs (one per floor) in the baseline. I'm not sure of the reasoning for the change unless it is simply for standardization of the number of baseline systems.

Cory Duggin, EI

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I ALWAYS model using the "Floors with identical thermal blocks can be grouped for modeling purposes" allowance, as long as it's thermodynamically appropriate.
Same operating schedule on all 15 floors? Same setpoints? Same plug loads and lighting? What difference would it make to model 15 separate air handlers? (none)
One of my assessment criteria is "Can I explain what I did in front of a group of peers and expect that they'll agree with what I did?" In this case, I think it would be very hard for them to disagree.

James V Dirkes II, PE, BEMP, LEED AP

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I Agree with the interpretation, baseline (5 to 8) should be modelled with
one AHU system per unique floor. If say, the office building had 15
identical floors, then you would have three AHUs. first and last floor
would have different boundary conditions to the rest, so the simplified way
to model the baseline would be One AHU system for the first floor one for
the middle identical floors (with a load multiplier of 13) and one for the
last floor.

Santiago.

2013/12/11 Duggin, Cory

Santiago Velez

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I agree with the interpretation, paraphrased for clarity's sake: 15 separate baseline systems (one per floor), each with a number of VAV terminal units equal to the number of thermal blocks per floor (one per zone).

I speculate prior to 2007 folks (myself included) would generally match the quantity and arrangement of proposed systems where possible for baseline modeling. Without the direction of "one per floor" it would mean, all else being equal, two different baseline models could have varying performance based solely on the design of the proposed systems... I speculate the 90.1 committee didn't want the baseline systems to influence/inform actual designs in such a fashion, so they chose to lock down system quantities using the "one per floor" language. Arbitrary, yeah, but probably has a net effect of making baselines more uniform in performance across projects. That's my best guess, fwiw.

I would add to Jim/Santiago's multiplier suggestion that while floor/system multipliers are something I'd also expect to use for a 15-story building, assuming you can arrange for identical zoning and so forth, you may find it less work on the documentation/review side of things to model each floor & system separately. I caution this as I've run into reviewers needing hand-holding to understand what DOE2 system reports should look like when floor multipliers are in play, and I've also found on the front-end that baseline system calculations can become more complex considering the goal to clearly document how you arrived at your answers. To be fair, more complex calculations for half as many (or fewer) systems in quantity can balance out and may be a wash too.

Food for thought - hope that helps =),

NICK CATON, P.E.

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Thanks all for your input!

I agree that there isn't much room for interpretation regarding the floor-by-floor, and that the overall energy use between a baseline AHU configuration that isn't floor-by-floor versus one that is, probably won't differ so much-especially for buildings with homogeneous space types (ie all offices). The scenarios in which there might be a slightly bigger difference in annual energy use is for buildings with varied spacetypes and schedules (ie varied hourly power densities) and/or different comfort setpoints across a single floor, which may mean running this 'fictitious' floor-by-floor unit at the lower part of the system efficiency curve.

Does anyone know who added this line? Or how we can find out who wrote it?

Linda Lam

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Bldg-sim,
Standards have to provide some direction of how the baseline AHUs are configured. While surely not perfect, the AHU per floor is the simplest and most sensible solution I have read. Yes, I know different numbers of systems between two models is a pain in some software. A one to one correlation with the proposed design often fails, for example when the proposed is zone heat pumps. You can still overlay your other exceptions based on load intensity, hours of operation etc. Before you gloss over the differences as negligible based on one homogenous project, remember all of the aspects that are tied to AHU size including fan power, economizer thresholds, and DX cooling efficiency. On some projects in some locations those are very significant differences.
Happy modeling,

Paul Riemer, PE, LEED AP BD+C

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