Supply Air Temp Reset Interpretation Please...?

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Hello--- I received this note from a fellow energy model reviewer (non-LEED
review comment) regarding my Baseline energy model that is supposed to be
compliant with Appendix G. I disagree with what they are saying. I
appreciate your feedback please and verification if in fact I am wrong in
how I am interpreting Section G3.1.3.12.

"We believe TEMP-FIRST supply air reset control strategy is not the correct
interpretation of G3.1.3.12 ? rather, AIRFLOW-FIRST should be used."

Thanks!

Pasha

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I had always assumed temperature first but reading the PNNL 90.1-2010 Performance Rating Method Reference Manual, 3.121 (pdf page 151), it states ?For systems 5 through 8, the air temperature for cooling shall be reset higher by 5?F under the minimum cooling load conditions using a reset by warmest zone, airflow first strategy.?

The manual states that the airflow first strategy minimizes zone reheat coil energy ( or overcooling) and central chiller energy consumption at the cost of possible increased fan energy.

Unfortunately, the reviewer may have a case.

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Christopher R. Jones, P.Eng.
Technical Specialist

WSP Canada Inc.
2300 Yonge Street, Suite 2300
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T +1 416-644-0252

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Anonymous

Thanks for the feedback- so if the justification for the approach is solely based on the user manual explanation- I can accept that. I'll cross reference it for the 2007 user manual to put it in my notes.

I've always used air-flow first as a standard reset for all my design energy models, What is counter-intuitive here is that the title of the Appendix G section specifically states 'reset-temp', so why should we not take the section text literally? I've always humbly assumed that the ASHRAE standard writers know more than me. After all these years of creating Baseline conforming models (or so I thought), this makes me feel lost all over again. AND is it not possible to apply code standard requirements without the use of the User Manual?

Thankful- but-frustrated,

Pasha
Ph: 308-763-1593

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Hi,

I think the Standard is quite clear about resetting temperature, and says nothing about modulating airflow. The user manual seems to be wrong.

Shaun Martin LEED-AP, BEMP

Principal

Shaun Martin Consulting

#90 ? 425 Carrall Street

Vancouver, BC V6B 6E3

p. 604-789-1095

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Yes! Thanks Shaun- even one count of validation makes me feel not so crazy ?, especially from an old colleague. Thank you- thank you. Why is there too much room for interpretation on these things? I accept that the User Manual is helpful in most scenarios- but this is one instance that just seems backwards. If it is supposed to airflow reset controls, then why is the section written with "Supply Air Temperature Reset"?

I appreciate your validation of my interpretation.

Pasha
Ph: 308-763-1593

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I wanted to chime in quickly, because this is an interesting question that we?ve gone back and forth on in my office as well.

I don?t agree with the stance that, because the section is labelled supply air temperature reset, therefore temperature first is the correct input. Airflow modulation is a given for VAV systems (baseline systems 5 through 8), so the question is not whether airflow modulates, but what takes precedence between the airflow modulation inherent in a VAV system and the temperature modulation also required by 90.1. The three possible combinations are reflected in the available eQUEST inputs: ?temperature first,? ?airflow first,? or ?simultaneous.?

In my view, the correct order of priority between temperature and airflow modulation is not something the standard directly addresses. Absent clear direction in the standard, I would then look for any formal interpretations of the standard issued by ASHRAE, and if that still doesn?t clear it up, then a user?s manual may be the next best thing. A user?s manual such as the PNNL manual at least provides specific direction by a neutral third party, which is often a great way to make a case to the reviewing authority (GBCI reviewer, code official, etc...).

Coles Jennings PE, BEMP, LEED AP BD+C
Sr. Energy Engineer, Building Sciences Manager | Mason & Hanger
A Day & Zimmermann Company
D 804.521.7045 | O 804.285.4171 | F 804.217.8520
4880 Sadler Road, Suite 300 | Glen Allen, VA 23060
Mason & Hanger
We do what we say.?

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All;

Great discussion.

For context, the text of G3.1.3.12 is: ?Supply Air Temperature Reset (System 5 through8). The air temperature for cooling shall be reset higher by 5F under the minimum cooling load conditions.?

Stepping back, the reset is said to occur at ?the minimum cooling load condition.? Thinking through how a typlica/real VAV system will control, the terminal box dampers will close towards their minimum as the cooling load in a zone decreases. In a ?real? building this should cause the Supply fan to see a pressure increase and slow down to get back to its static pressure setpoint. At some point if all of the boxes are at a minimum position (aka, minimum cooling demand), the Supply Air Temperature should/can be increased.

Looking through the above, this would indicate an Air Flow first reset, before increasing the discharge temperature.

Per the eQuest help file, it would seem to indicate that a ?Temp-First? approach attempts to reset the temperature, without modulating the fan down. This seems fairly odd in a ?real? system since in that if any single zone calls for cooling, no reset occurs, no energy saved. Even if it is a Baseline, it should have some sort of gut check to it. If it were a ?real? system and going Prescriptive path, it would be subject to Section 6.5.2. I didn?t see any requirement (Mandatory or Prescriptive) for DAT reset?.(but I may be missing it?.)

Therefore, I?d think in a VAV system the Air-Flow first modulation, then allow the DAT to reset would be the intended path.

Another minor bit of confusion here: The 90.1 ASHREA User?s Manual does not offer much guidance here. There is new guide from PNNL for the 90.1-2010 standard, which is what is also mentioned below. Just want to clarify for readers that these are two separate (and both are useful) guides.

Thanks,
DARIC R. ADAIR PE, C.E.M.
Henderson Engineers, Inc.?|?Mechanical Engineer, Energy Analyst
dir 913 742 5530? tel 913 742 5000? fax 913 742 5001? tx id #F-001236? email daric.adair at hei-eng.com? www.hei-eng.com
Licensed in KS.?

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Agreed on ventilation. But 90.1 Appendix G addresses the VAV box minimums for Baseline models in G3.1.3.13/G3.1.3.14. The box minimums must take ventilation into account.
Fan energy is a major element. Several of my more recent models have had fan energy on par with heating and cooling energy. Both for baseline and proposed models. Fans unload nicely?which is a blessing and a curse.

Running some quick diagnostic tests on a couple of my most recent models, the difference in the two control strategies didn?t yield a massive difference. But one test does not a trend make?.in any sense, the Temperature Frist reset set was 0.29% lower on a cost basis?

Another factor specific to eQuest, when using Airflow first control, is the ?MIN-RESET-FLOW? variable. This defaults to 0.66. Which from the help files seems to indicate that the it will only allow the fan to reset to this level (0.66 in this case/by default) before starting temperature reset. Modifying this value impacts when the system will start to modify the DAT. This did have an impact on the results, but not very underwhelming at best from my very limited testing pool?.

In short, it gets back to which element should unload first ? fans vs chiller/boiler. Fan curves are dictated for Appendix G (and Fan laws). Chiller curves are not provided/dictated for Appendix G, but performance is. Similar for boilers. In a ?real? system with actual equipment curves, it would be a balance between savings from the fan slowing vs savings from central plant equipment unloadng (which has more complex relationships).

Thanks,
DARIC R. ADAIR PE, C.E.M.
Henderson Engineers, Inc.?|?Mechanical Engineer, Energy Analyst
dir 913 742 5530? tel 913 742 5000? fax 913 742 5001? tx id #F-001236? email daric.adair at hei-eng.com? www.hei-eng.com
Licensed in KS.?

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Hi All-- Thank you for this in depth discussion and taking time to go over
all of this. (I was away from work for a few days, thus my delayed
response.)

I agree with the commentary regarding how REAL LIFE operations happen in
VAV systems, and yes modulating airflow is the first approach. However, i
did always find it odd that temp first was stated in Appendix G, but who am
I to question Appendix G, right?

As for the discussion of direction from the PNNL User's Manual-- that is
interesting, and I hadn't seen that document prior to now because I am
still only working on projects that reference ASHRAE 90.1-2007. Therefore
I have had no need or time to look at the 2010 version or any supporting
docs...which brings us to the next debate:

Why should anyone be referencing a 2010 version document for guidance for a
2007 version compliance model/project? Therefore it seems prudent to stand
firm to follow exactly what the governed/reference standard
states-explicitly which is "temperature reset."

But then- Thank you Daric for confirming that the 2007 version User's
Manual doesn't specifically guide us on how to meet the criteria of that
section. AND thank you Daric for also conducting a minor experiment on
your model. (I will do the same with a couple of mine this week.)

So based on what I have learned from this awesome discussion/debate, I will
assume that my third party reviewer (non GBCI group) was indeed referencing
the PNNL User's Manual for the 2010 version, possibly out of habit, and/or
not even aware that the LEED guided project they were contracted for
reviewing (my model, which I was contracted to produce by the Architect &
owner) is in fact registered as a LEED 2009 project that is referencing and
following ASHRAE 90.1-2007 version.

Hmm, I'm in a quandary of how to take action for my model based on this
questionable review comment- who is not the actual LEED reviewer. This
seems like a weird situation to be in...

Thanks again for all your input on this topic!

Pasha Korber
Ph 308-763-1593

www.korberenergy.com

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I agree with Daric?s analysis that an ?Airflow First? reset priority should be modeled for baseline VAV systems. I don?t see anything in Appendix G. that implies a temperature-first control strategy. Appendix G. states requirements for both SAT reset and VAV minimum flow. The SAT reset is to be applied ?under the minimum cooling load conditions?. That implies that between minimum and design cooling load conditions, airflow rate is adjusted, which means the airflow rate is adjusted first. I tried ?TEMP-FIRST? on one of my baseline models and it saved 3.6% cooling energy, added 1.8% fan energy and saved 0.6% heating energy for a net building energy savings of 0.4% versus baseline with AIRFLOW-FIRST. (So AIRFLOW-FIRST is better in my n=1 case for showing savings in the proposed design vs. baseline.)
~Bill

William Bishop, PE, BEMP, BEAP, CEM, LEED AP | Pathfinder Engineers & Architects LLP
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Precisely.

If you have a project requiring compliance with ASHRAE 90.1 (2007) you are obviously in the clear complying with those values for your baseline and proposed scenarios ? particularly in terms of Appendix G.

Any compliance with newer version of ASHRAE 90.1 would still be acceptable, however, because in essence, they are a little more stringent ? some would differ on the extent of those requirements ? but the fact remains that it illustrates you?ve gone the extra mile by conforming to more stringent guidelines than you needed to.

THAT BEING SAID ? The 2007 version of 90.1 had options for energy modeling that are no longer considered standard practice.

What do I mean by that? Well one example is renovations to existing buildings.

ASHRAE 90.1 (2007) Appendix G Tables had an option to model BASELINE shell and all components to match the building prior to renovations ? given your project was a renovation to an existing structure and the existing windows, doors, and shell were remaining intact (obviously replacing existing doors and windows in place, and maybe adding EIFS over the existing shell as an example).

I don?t recall the exact wording but they removed this option in latter versions of 90.1. But being able to model a building as it sits for your baseline is A LOT easier than modeling the Baseline shell, otherwise. I think everyone would agree..

ALSO, since the more recent versions of 90.1 are slightly more stringent you would likely not see the same kind of energy savings compared to the baseline in the very end by adhering to 2010 or 2013 versions of 90.1 (regardless of your baseline methodology). It might be close. How close obviously would depend on the complexity of your model and what it is, exactly, you are modeling.

Chris Baker
CCI Alliance of Companies
Fort Wainwright, Alaska
907-356-1681 (x105) office
907-750-4922 cell

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One other wrinkle.

RESET-PRIORITY is a VAV box control. The DOE2 dictionary describes the RESET-PRIORITY keyword parameters with this example: ?if a zone cooling setpoint is 76F with a 2F throttling range, the air flow will first modulate in the range of 77F to 76F and the supply air temperature will reset in the range of 76F to 75F.? This doesn?t really match real-world operation, either, and may also explain the results Bill got.

Seems to me, the only way to meet G3.1.3.12 is to use COOL-MAX-RESET-T and COOL-MIN-RESET-T, and let DOE2 do what it wants.

Shaun

Shaun Martin LEED-AP, BEMP

Principal

Shaun Martin Consulting

#90 ? 425 Carrall Street

Vancouver, BC V6B 6E3

p. 604-789-1095

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