[Bldg-sim] ASHRAE App G VAV Fan Curve

Porter, Fred AEC FPorter at archenergy.com
Wed Feb 26 20:17:23 PST 2014


All,
This is a great example in contrasting styles of applying Appendix G for energy rating and I'm torn between the two as my message indicated. Both provide reasonable justification for their conclusions. Thanks Paul and Nathan. This is the kind of thing USGBC/GBCI should be addressing in the LEED Energy Modeling Guide and eventually ASHRAE. It's also an example of why California is going to the CBECC-Com process for determining compliance with energy models.

Fred Porter, BEMP, LEED(c) AP
Senior Energy Analyst
Architectural Energy Corporation
A Member of NORESCO Energy and Sustainability Services
2540 Frontier Ave, Suite 100, Boulder, CO  80301
Phone 303.459.7425 |  fporter at archenergy.com<mailto:fporter at archenergy.com>
www.archenergy.com<http://www.archenergy.com>

From: Paul Riemer [mailto:Paul.Riemer at dunhameng.com]
Sent: Wednesday, February 26, 2014 12:04 PM
To: 'Nathan Miller'; Porter, Fred AEC; bldg-sim at lists. onebuilding. org (bldg-sim at lists.onebuilding.org)
Subject: [External] RE: ASHRAE App G VAV Fan Curve

Section 6.5.3.2.3 requires static pressure reset for VAV fan system with DDC of individual zones and I believe that requirement predates the fan curve in Appendix G.  Given the prevalence of DDC zones, it seems disingenuous to claim savings for static pressure reset under Appendix G.  Maybe there is room for claiming improved reset based on oversized ducts and boxes?  It wouldn't take much to run the different published VFD curves out there and use whichever one gives your project the most savings but that is clearly gaming the system.  Since tools are not explicitly modeling the physics of static pressure reset, I suggest it should at least be isolated as an exceptional calculation for review.

Paul Riemer, PE, LEED AP BD+C
DUNHAM

From: bldg-sim-bounces at lists.onebuilding.org<mailto:bldg-sim-bounces at lists.onebuilding.org> [mailto:bldg-sim-bounces at lists.onebuilding.org] On Behalf Of Nathan Miller
Sent: Wednesday, February 26, 2014 12:42 PM
To: Porter, Fred AEC; bldg-sim at lists. onebuilding. org (bldg-sim at lists.onebuilding.org<mailto:bldg-sim at lists.onebuilding.org>)
Subject: Re: [Bldg-sim] ASHRAE App G VAV Fan Curve

I think it is pretty clear the way 90.1 App. G is written that you SHALL use that specific curve for the baseline VAV system fan power. Now it is a different discussion about whether that curve properly accounts for any ASHRAE required controls strategies, fan performance requirements, etc, but that doesn't really matter, here is the curve to use.

Proposed, as always, should accurately reflect how the system will operate. If you think a more advantageous fan curve is appropriate, such as to account for static pressure reset, go ahead and use it, just as you could create a custom curve for boiler or chiller part-load efficiencies. Don't hide the fact, make it clear to the reviewer that you are using different curves, state the source for your proposed curve and see if they want to comment or challenge the results.

For what it is worth (keeping in mind that past rulings do not set precedent), on multiple projects I have worked on I have used "static pressure reset" fan curves in the proposed, the mandatory baseline fan curve in the baseline, and it has never even been questioned by the reviewer.

Nathan Miller - PE, LEED(r)AP BD+C, CEM
Mechanical Engineer/Senior Energy Analyst
RUSHING | D 206-788-4577 | O 206-285-7100
www.rushingco.com<http://www.rushingco.com/>

From: bldg-sim-bounces at lists.onebuilding.org<mailto:bldg-sim-bounces at lists.onebuilding.org> [mailto:bldg-sim-bounces at lists.onebuilding.org] On Behalf Of Porter, Fred AEC
Sent: Wednesday, February 26, 2014 9:40 AM
To: bldg-sim at lists. onebuilding. org (bldg-sim at lists.onebuilding.org<mailto:bldg-sim at lists.onebuilding.org>)
Subject: [Bldg-sim] ASHRAE App G VAV Fan Curve

All,
Does anyone know, particularly through having claims accepted by GBCI, whether the ASHRAE App G VAV fan curve is supposed to represent a fan system with or without static pressure reset. I always thought it was supposed to represent a system with sp reset, conforming to 6.5.3.2.3. However, most of the curve power values are even higher than curves representing VAV/VFD systems without sp reset. A confusing aspect is that compared to other VFD fan curves, the power values are high at high flows, but low at low flows, even going to zero at zero flow, though that is really meaningless.

As an aside, the 2013 Title 24 ACM now defines fan curves for central VAV systems with and without sp reset, taken from Taylor's VAV design guide. SP reset is the "baseline" or "standard."A

Thanks for any experience or thoughts on this.

Fred
Fred Porter, BEMP, LEED(c) AP
Senior Energy Analyst
Architectural Energy Corporation
A Member of NORESCO Energy and Sustainability Services
2540 Frontier Ave, Suite 100, Boulder, CO  80301
Phone 303.459.7425 |  fporter at archenergy.com<mailto:fporter at archenergy.com>
www.archenergy.com<http://www.archenergy.com>



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