[Trace-users] Trace-users Digest, Vol 15, Issue 1

Julia Beabout juliabeabout at yahoo.com
Wed Oct 28 00:05:18 PDT 2009

Hi Marty,
It's not unusual that your proposed fan KW and annual energy consumption is more than the baseline.  ASHRAE 90.1 is kind of stingy with respect to the allowable fan KW for the baseline.  If you are using the 2004 edition of 90.1, make sure you are using Addendum ac to calc your baseline fan energy.  This addendum is allowed by LEED and should help your situation some.  If you are using the 2007 edition of 90.1, the fan energy equation in Appendix G  should already include the revisions from addendum ac along with any subsequent others.  In all cases, though, you are only allowed to calc your  baseline fan energy allowance on the SUPPLY cfm.  You can't add up your supply, return, exhaust cfms etc and use that calc your fan energy.  Nor can you apply the equation to each of your component fans separately.  This question comes up a lot since Appendix G it is a little confusing in this regard.  There is a good bid of traffic on the subject in the Bldg-sim
 archives ( including some responses  directly from LEED reviewers - although in an unofficial capacity) if you'd like a little more clarification or confirmation on the issue.  

In terms of Trace inputs, I usually calc the total fan break KW for my baseline systems and proportion that out to the individual component fans as applicable based on their cfms.  I input these numbers directly into Trace in lieu of allowing Trace to automatically calculate it - but it sounds like that method would work too based on Kevin's email below.   Then I apply the ASHRAE 90.1 Appendix G fan curve to the baseline case fans (as applicable).  For the proposed case, I input the break KW for each fan based upon the actual fan selections and then apply the appropriate fan curves types based on the actual fan types being specified.

Hope that helps some.

From: Kevin Caho <kevin.caho at gmail.com>
To: trace-users at lists.onebuilding.org
Sent: Tue, October 27, 2009 8:39:28 PM
Subject: Re: [Trace-users] Trace-users Digest, Vol 15, Issue 1

I had the same question previously and I would encourage you to check to ECM/PRB sizing option yourself and verify that it is correct if this worries you.  All you need to do is apply it for one simulation, open the "equipment energy consumption" report, and do the hand calc with the cfm shown for each fan and compare these values to the kW that Trace has calculated.  I think you'll find that they are very close (within 1%).  
You say your results are higher and I guess I'm not sure if you are referring to the fan size or to the fan energy consumption.  In the particular model that I have in front of me (using the ECM/PRB option) the proposed is also higher. CFM by 16%, kWh by 19%, and kW by 23%.  This is not surprising as the proposed case has 70% glass vs. 40% for the baseline so their is more air needed to satisfy the load.  The kW/cfm however are nearly the same.  I suggest checking items like that to make sure things are proportional because your total % difference doesn't necessarily tell the whole story.
My other thought is that you used the wrong units.  In your post you said you used G3.1.2.9 to calculate the horsepower but your result from that equation would be in watts.  If you used kW from that equation and entered as hp in your 90.1 model, then you'd have 25% less power than you should leaving your proposed more than 30% ahead.  
Hopefully I answered your question somewhere in there.
Kevin Caho, LEED®AP
T: 312.360.4432  F: 312.360.4545 
Kevin.Caho at Som.Com 

On Tue, Oct 27, 2009 at 2:10 PM, <trace-users-request at lists.onebuilding.org> wrote:

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>Today's Topics:
>  1. Appendix G Fan Energy (Snarski, Marty)
>Message: 1
>Date: Tue, 27 Oct 2009 13:33:28 -0500
>From: "Snarski, Marty" <msnarski at shwgroup.com>
>To: <trace-users at lists.onebuilding.org>
>Subject: [Trace-users] Appendix G Fan Energy
>       <078AC5207CEDE145A8C7C74FFB3975A05470DE at DALEXC1.shwgroup.com>
>Content-Type: text/plain; charset="us-ascii"
>Hello, all.
>I'm running an energy model (for LEED), and am having a particular
>problem with the Fan Energy results.  I am using the actual fan
>horsepower (for the supply and for the return fans) for the proposed
>building, and the Appendix G equation in G3.1.2.9.  The results I'm
>getting for the proposed design are considerably higher (about 31%) than
>the baseline building.  This leads me to believe that I may be
>misinterpreting the equation.
>As stated in the 90.1 Users manual, the equation accounts for supply,
>return, relief, and exhaust fans.  However, I've been simply inputting
>the Supply CFM into the equation.  Given that it accounts for all fans,
>should I use the accumulated CFMs for each of those fans (i.e. if I'm
>supplying 32,000, and returning 24,000, should I use 56,000 cfm)?
>While I understand TRACE has the automatic baseline fan sizing
>capability, I'm not sure I trust it completely, and would like to know
>where the calculation comes from.
>Thanks for any help you can offer!
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Kevin Caho
T: 309.716.0084
kevin.caho at gmail.com
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