Hi All,
It seems important to this discussion that the way the Energy Plus defines ?Airflow-First? and ?Temperature-First? seems to be exactly the opposite of how e-Quest does.
The PNNL PRM guide and the E+ Input Output Reference (refer to the SetpointManager:WarmestTemperatureFlow object) state that ?Flow-First? option ?minimizes zone reheat coil energy (or overcoling) and central chiler energy consumption at the cost of possible increased fan energy.?
Whereas, the e-Quest help states that the ?Airflow-First? option ?minimizes air flow and fan energy, at the possible expense of cooling energy?.
Looks to me that in Energy Plus, the starting point is a no load condition (min flow, and maximum SAT), as load increases, flow is FIRST increased, followed by lowering the SAT.
But in e-Quest, the starting point is full load condition (max flow, minimum SAT), as load decreases flow is FIRST reduced, followed by increasing the SAT.
Both are ?Airflow FIRST? but in the opposite sense.
The SAT reset is also implemented in different ways in the two programs.
Best,
Molly
Molly Curtz PE BEMP LEED AP BD+C
Senior Mechanical Engineer
Arup
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I?m going to chime in late with a few affirmations and a couple new thoughts, perhaps:
1. There are situations where reducing airflow, temperatures, or neither first (simultaneous) makes the most sense across all possible applications. There are controls in the real world that get more complicated than this as well, combining/switching to different modes based on different ranges of operation, time of day? I?m one to advocate for simpler sequences personally however ? it?s very easy for over-complicated stuff to get lost in translation and just not work.
2. For those interested in learning more ? consider the fundamentals of this discussion apply very similarly to the hydronic side of things. When you have both variable pump flow and loop temperature reset controls you also have to set a priority. Pump and Fan motors (movers of fluids) all save the most energy at the top end of their part-load operation range (look to your fan law curves), so as a general case you will often find after exploring every option that reducing flow first is ideal sequence in terms of power/energy.
3. Anytime someone mentions ?real-world operations? in a discussion, I have the urge to remind everyone additionally that system controls are not often clearly specified/installed/or maintained to do what a new system designer/specifier might ask for, if put on the spot. Robust simulation platforms like eQuest/doe2 allows us to model both ?intended? vs. ?non-ideal/broken? operations.
4. Anyone applying the 90.1 Standard as a practitioner or reviewer should be mindful that the User?s Manual and similar, more descriptive guidance documents around how one should go about developing a baseline model are not a part of 90.1 when it comes down to brass tacks. Interpretations for best-fit in a given situation are necessary and common.
5. As Coles put forth, the originating comment ("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.") is an interpretation, and is no more or less correct than alternatives. I might agree/disagree with these guys based on other context (for example: Is this a DOAS system with limited airflow turndown range due to ventilation requirements or DCV control parameters?), but would be more interested in ensuring the proposed/baseline case VAV systems are handled in the same manner.
Finally Pasha: My only extra advice is to disregard the whole LEED vs. non-LEED dynamic: The situation sounds much simpler with one practicing engineer holding an opinion you can either agree with or refute. So long as you both hold to arriving at a mutual consensus in pursuing the right interpretation, you needn?t feel beholden to satisfy what an outside party like the LEED reviewing body thinks.
Hope this is helpful!
~Nick
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Nick Caton, P.E., BEMP
Senior Energy Engineer
Regional Energy Engineering Manager
Energy and Sustainability Services
Schneider Electric
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E nicholas.caton at schneider-electric.com
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Lenexa, KS 66219
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