Appendix G Baseline Systems 6 and 8 - supply air temperature controls?

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ASHRAE Standard 90.1, G3.1.3.12 requires cooling supply air temperature reset for Systems 5 through 8.
G3.1.3.14 requires "supply air temperature setpoint shall be constant at the design condition" for Systems 6 and 8.
How should we interpret this and model it in eQUEST (for Systems 6 and 8)?

If the "design condition" in G3.1.3.14 means "peak design flow rate", I suggest the following cooling control settings:

- COOL-CONTROL = WARMEST

- COOL-MIN-RESET-T = 56 $ (assumes 76F cooling T-stat; 56F MIN-SUPPLY-T for 20F difference per G3.1.2.8)

- RESET-PRIORITY = AIRFLOW-FIRST

- COOL-MAX-RESET-T = 61 $ (5F reset per G3.1.3.12; assumes 76F cooling T-stat)

Using a RESET-PRIORITY of "TEMP-FIRST" resets the supply air temperature before the airflow, which does not satisfy the G3.1.3.14 requirement.
Using a RESET-PRIORITY of "TEMP-FIRST" or "SIMULTANEOUS" does not satisfy the G3.1.3.12 requirement, since temperature is reset well before minimum cooling load conditions are reached.

Does anyone have an alternative/additional suggestion or interpretation?

Thanks,
Bill

William Bishop, PE, BEMP, BEAP, LEED AP | Pathfinder Engineers & Architects LLP
Senior Energy Engineer

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Bill Bishop's picture
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I also created an input file template for System 6 which could be used for System 8 with some modification. Comments/mods are welcomed.

$ *********************************************************
$ ** **
$ ** Performance Curves **
$ ** **
$ *********************************************************

"G3.1.3.15 VAV Fan Curve fPLR" = CURVE-FIT
TYPE = CUBIC
INPUT-TYPE = COEFFICIENTS
COEFFICIENTS = ( 0.0013, 0.147, 0.9506, -0.0998 )
..

$ *********************************************************
$ ** **
$ ** HVAC Systems / Zones **
$ ** **
$ *********************************************************

SET-DEFAULT FOR ZONE
TYPE = CONDITIONED
MIN-FLOW-RATIO = 0.3 $ Per G3.1.3.14
ZONE-FAN-KW/FLOW = 0.00035 $ Per G3.1.3.14
ZONE-FAN-RATIO = 0.5 $ Per G3.1.3.14
TERMINAL-TYPE = PARALLEL-PIU $ For all zones served by Sys 6 or Sys 8
..

SET-DEFAULT FOR SYSTEM
TYPE = PIU $ For Baseline System 6 and System 8
HEAT-SOURCE = ELECTRIC $ Per Table G3.1.1B and User's Manual p.G-23
ZONE-HEAT-SOURCE = ELECTRIC $ Per Table G3.1.1B and User's Manual p.G-23
HEAT-SIZING-RATI = 1.25 $ Per G3.1.2.2
COOL-SIZING-RATI = 1.15 $ Per G3.1.2.2
COOL-SOURCE = ELEC-DX $ Per Table G3.1.1B
MAX-SUPPLY-T = 92 $ Per G3.1.2.8 (20F above room heat T-Stat
HEAT-SET-T = 62 $ = COOL-MAX-RESET-T plus 1F
MIN-SUPPLY-T = 56 $ Per G3.1.2.8 (20F below room cool T-Stat
COOL-CONTROL = WARMEST $ Per G3.1.3.12
ECONO-LIMIT-T = 70 $ Per G3.1.2.7 and Table G3.1.2.6B by climate zone
ECONO-LOCKOUT = YES $ (Seems like a reasonable baseline default)
COOL-MIN-RESET-T = 56 $ = MIN-SUPPLY-T
RESET-PRIORITY = AIRFLOW-FIRST $ Per G3.1.3.12 and G3.1.3.14
COOL-MAX-RESET-T = 61 $ Per G3.1.3.12 (5F above MIN-SUPPLY-T)
OA-CONTROL = OA-TEMP $ Per G3.1.2.6 and Table G3.1.2.6A
FAN-CONTROL = FAN-EIR-FPLR $ Per G3.1.3.15
NIGHT-CYCLE-CTRL = CYCLE-ON-ANY $ Per G3.1.2.4
FAN-EIR-FPLR = "G3.1.3.15 VAV Fan Curve fPLR" $ Per G3.1.3.15
REHEAT-DELTA-T = 36 $ = MAX-SUPPLY-T minus MIN-SUPPLY-T
..

$ Bill

Bill Bishop's picture
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I'm not sure I follow your interpretation on G3.1.3.14. The supply air
temperature setpoint is constant at the design condition regardless of
whether you reset airflow or temperature first. At the design condition
there will be no reset occurring because you're by definition, at design.
I've always interpreted that whole section of G3.1.3.14 to be referring to
the terminal unit fans only.

TEMPERATURE-FIRST makes the most sense to me for accomplishing G3.1.3.12,
but I may be biased as in a marine climate resetting temperature up
increases economizer hours dramatically.

-Robby

Robby Oylear's picture
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Robby,
My initial interpretation of G3.1.3.14 was similar to yours - of course the SAT is constant at the design condition. But then why bother saying, "The supply air temperature setpoint shall be constant at the design condition"? Since G3.1.3.14 is about fan power and air flow rate, it makes sense that this refers to the design flowrate. My guess is that the intent is for cooling controls to reset the airflow (to save fan energy, since that's what G3.1.3.14 addresses) before resetting the temperature. That strategy can be modeled in eQUEST using RESET-PRIORITY = AIRFLOW-FIRST as I showed below. TEMPERATURE-FIRST can reset the temperature up at the "design condition" flowrate.

~Bill

Bill Bishop's picture
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Bill,

Seems like too big of a guess to me. If the ASHRAE 90.1 committee
specifically wanted your supply air temperature reset priority to be
airflow first, it seems like that would be simply and explicitly stated. I
don't have a copy of the User's Manual for 90.1 in front of me. Perhaps
there's further explanation of the meaning of G3.1.3.14 in that document.

In my experience with these systems, I've never been called on my setting
of TEMPERATURE-FIRST in system 6 & 8 baselines, so it's likely a moot
point.

-Robby

Robby Oylear's picture
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There is no further explanation in the User's Manual ('07 or 2010).
I also checked the LEED template for EAp2 and there is no check box for G3.1.3.14 even when you select Baseline Sys 6.
We are in the elite .00000003% of the world population that cares about that one sentence. So, yes, it's likely a moot point.

Have a great weekend!
Bill

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