[Equest-users] Baseline vs. Proposed Fan Curves

[Bishop, Bill]

Bernie,

Nick is spot on. I would also add that you should have a justification for using a different proposed VAV fan curve than the baseline VAV fan curve. There have been some good discussions on the forums. See these for reference:
http://lists.onebuilding.org/htdig.cgi/equest-users-onebuilding.org/2012-February/034577.html
http://lists.onebuilding.org/htdig.cgi/bldg-sim-onebuilding.org/2014-February/045383.html

Regards,
Bill

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

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From: Equest-users [mailto:equest-users-bounces at lists.onebuilding.org] On Behalf Of Nick Caton
Sent: Tuesday, October 28, 2014 11:41 PM
To: 'Bernie Hont'; equest-users at lists.onebuilding.org
Subject: Re: [Equest-users] Baseline vs. Proposed Fan Curves

Hi Bernie,

>From your phrasing, I'd caution to avoid a mistake I once made regularly until corrected: A single value should be determined for the baseline system fan power (Pfan).  Put another way, Pfan should not be calculated separately for each fan.  The language in Appendix G preceding the Pfan calculation method specifies this value accounts for supply, return, relief, and exhaust fan energies for the system (as they may occur)... If you wish to explicitly model both supply and return fan energies for your baseline system, you can apportion the Pfan quantity/result between those kW/CFM inputs, but the resulting sum between the two should not exceed the singular Pfan calculated for the system.

As you touched on, kW/CFM is not the only input affecting fan energy end-use sums.  How those proposed/baseline fans operate relative to each other for 8760 hours can be a much bigger deal.  It's almost always a worthwhile QC check to state how both baseline and proposed system fans SHOULD operate (VAV/CV?  Cycling?  On 24/7? Setback behavior?), then run a couple system and/or zone hourly reports to track the flow rates simulated and confirm whether they match your expectations.  There are enough "gotchas" between the various system types and enough potential for simple human error between all the involved inputs to potentially trip up even very experienced eQuest-ers on this front.

This advice may already be on your radar, but if I've presented a new perspective it may help you shape/understand where the reviewer is coming from in anticipating an overall performance reduction.


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360 Analytics
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From: Equest-users [mailto:equest-users-bounces at lists.onebuilding.org] On Behalf Of Bernie Hont
Sent: Tuesday, October 28, 2014 5:38 PM
To: equest-users at lists.onebuilding.org<mailto:equest-users at lists.onebuilding.org>
Subject: [Equest-users] Baseline vs. Proposed Fan Curves

I have just revised a baseline and proposed model for LEED submission and the fan energy savings has changed dramatically as a result. The first go-round the fan energy savings was 27% between baseline to proposed. The formulas in G3.1.2.8 and G3.1.2.9 were used incorrectly though in the first round, so I had to revise the supply and return fan power calculations. The result is that they both were higher (0.001217556 KW/CFM supply and 0.001049567 KW/CFM return now). The baseline fan curve is the equation from G3.1.3.15 Method 2.

The proposed model fan power matches the actual units purchased, (0.001164 KW/CFM supply and 0.000435 KW/CFM return). This represents a 4.4% reduction in supply fan power and a 58.6% reduction in return fan power. The fan curve used for the proposed model is Variable Speed Drive FPLR.

There is a huge savings on the return side and a slight savings on the supply side, so I can visualize how a 60% overall savings between the two models is feasible, but is the sizing for the return fan under the baseline scenario accurate? I followed the G3.1.2.8 method of sizing them based on the supply flow minus the ventilation (or 90%) but the calculations still come out with a substantially higher return fan power than the proposed model. This may be perfectly acceptable, but USGBC always put the disclaimer on the review comment that "the comments are perceived to reduce projected savings". That would be the case otherwise, but since the fan energy has changed so dramatically after revising the baseline calculations that the savings actually increased from 17.99% to 21.77% solely on the fan energy. Does anyone have experience where, with an explanation of why this occurred, that USGBC will accept the revised results even though the savings is actually higher now?

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Bernie Hont, PE, LEED AP
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