Quirk I've wondered about - Fan EIR-fPLR

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eQuest wizards, if specifying a VAV, will generate systems using a library curve and the following 2 lines:

FAN-CONTROL = FAN-EIR-FPLR
FAN-EIR_FPLR = "Variable Speed Drive FPLR"

Looks like this in the interface:
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Is there a practical reason this approach is taken in lieu of FAN-CONTROL = SPEED? Does one input option perhaps predate the other? Do these approaches result in different effective curves to describe net fan/motor/VSD effects as a function of flow? Do these approaches perhaps interact differently in the presence of a MIN-FAN-RATIO?

I am being lazy here - but if someone has a slow afternoon, some of these open questions would be easy enough to quantify with a dummy model and some custom hourlies =).

~Nick

[cid:image003.jpg at 01D5EE14.E92A2E70]
Nick Caton, P.E., BEMP
Senior Energy Engineer
Energy and Sustainability Services
Energy Performance Contracting
D
M
E

913 . 564 . 6361
785 . 410 . 3317
nicholas.caton at se.com
15200 Santa Fe Trail Drive
Suite 204
Lenexa, KS 66219

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Nicholas Caton2's picture
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Hey Nick,

In the updated version of eQUEST, most of the descriptions have been
replaced by curves - a more transparent approach, imo.

So, the first field defines the curve. The second field defines that you
are indeed using the curve. This is likely because one method predating the
other as you suggest.

If you select anything else in the lower field, even "variable speed", it
will use one of the historical curves from eQUEST, instead of the default
VAV curve. BTW - the default variable speed curve is different than if you
select "Variable Speed"!

Thanks to your help Nick, and going through almost every new keyword, I
will be going over some of these things in an upcoming 3 hour workshop.
http://energy-models.com/content/equest-doe23-webinar

Best,

Bob
Simulate.Energy
Energy-models.com

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Thanks Bob!

As a precaution for your forthcoming training and framing of the matter, I don?t think this particular variety of input methods is ?new,? so much as it has been around in the interface as long as I can remember. Certainly longer than doe2.3 has been around.

It was highlighted for me off-lists that the FAN-EIR-FPLR approach is certainly preferable (over FAN-CONTROL = SPEED) for 90.1 Appendix G-based modeling, wherein the modeler is supposed to use the specified cubic curve coefficients. For clarity however, I?ll point out that the library default curve (illustrated below) is not in total compliance with the coefficients required by Appendix G, so you still need to punch those coefficients in for the likes of a LEED model.

I?d also suggest for those modeling ?to reality? for calibrated modeling that while the fundamentals of the affinity laws and motor / drive losses can only vary so much, neither the library nor Appendix G curves should be considered ?one size fits all? for variable flow systems. The curves in question may not account for a variety of real-world factors that can limit the reasonable application of these curves in a ?blind? fashion, but in practical terms I wouldn?t typically raise these concerns where the range of operation is limited to within +/-0.40 PLR. Put another way: If you?re going to reduce design flows from 100% down to 50% or less, and those savings are a critical component of your projections, then you might want do ?do the math? to ensure you?re projecting something reasonable at the bottom of the expected range of operation.

While we?re on topic, I happy to share that I spent years of my career over-estimating how low a VAV system ?on paper? could turn down, before I had the pleasure of learning how the limits of occupant comfort have a heavy vote in the matter. I would advise a younger version of myself to be sure to have conversations with your mechanical engineers of record (even if that means talking to yourself) to explore ?at what point would a central fan turndown negatively impact my air register throws and cause cold air to dump??

~Nick

[cid:image002.jpg at 01D5EE4B.3B9B2980]
Nick Caton, P.E., BEMP
Senior Energy Engineer
Energy and Sustainability Services
Energy Performance Contracting
D
M
E

913 . 564 . 6361
785 . 410 . 3317
nicholas.caton at se.com
15200 Santa Fe Trail Drive
Suite 204
Lenexa, KS 66219

[cid:image003.png at 01D5EE2D.E959C090]

Nicholas Caton2's picture
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When the answer to a Nick question seems simple I do doubt myself.....

I assume yall know this but the reason I use EIR FPLR is to change the curve used based on the application. Ie ASHRAE 90.1 fan curve.

There is an ASHRAE paper out there that defines curves for static reset. . Jeff Stein and Mark Hydeman's "Development and Testing of the Characteristic Curve Fan Model, ASHRAE Transactions, January 2004
I use these in utility studies to justify energy savings

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Melissa Crowe, LEED AP, BEMP
508-647-9200 ext. 1225

Melissa

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