[Bldg-sim] Appendix G Fan Power

Nathan Miller nathanm at rushingco.com
Thu Jul 2 08:38:02 PDT 2009

I think a lot of us were making the mistake of applying the total calculated 
fan power/cfm to both the supply and return/exhaust fans (mostly due to 
ambiguous wording in 90.1). Forcing the modeler to explicitly divide up the 
calculated fan power allows the reviewer to more easily check that the 
energy was properly accounted for.

I played around on one project and found that unsurprisingly the fan energy 
didn’t change whether I put it all in supply air and none in exhaust, or 
divided it up based on the relative CFMs.

Nathan Miller

Senior Energy Engineer/Mechanical Engineer

direct: 206.788.4577

fax: 206.285.7111

From: bldg-sim-bounces at lists.onebuilding.org 
[mailto:bldg-sim-bounces at lists.onebuilding.org] On Behalf Of James Hess
Sent: Thursday, July 02, 2009 10:28 AM
To: Bldg-Sim
Subject: Re: [Bldg-sim] Appendix G Fan Power

We got a review comment on a recent LEED project asking us to break down the 
baseline HVAC power into components even though there is no direction, or 
requirement, from 90.1-2004 to do so.

Can someone please help me understand why this would be necessary?

LEED guidelines say we have to model per Appendix G, but Appendix G doesn't 
say we have to do this.

I do not think it is necessary to do this as the baseline total fan energy 
consumption numbers would still be the same.

The equation in Appendix G is meant to give a total allowance (for each 
system) for supply, return, and exhaust fans. It doesn't matter if we break 
the allowance down into components or not as the total fan energy 
consumption remains the same. Therefore, there is no value in doing this.

If we did break the fan energy numbers into components, how would we do that 
given that there is no direction from ASHRAE or LEED on how to do this?



James A. Hess, PE, CEM

Energy Engineer

TME, Inc.

On Jul 2, 2009, at 8:41 AM, "Jeremy Poling" <jpoling at epsteinglobal.com> 

This issue will start to become a problem for anyone running energy models 
for LEED projects – reviewers on the new review teams are asking to see the 
total fan power for the baseline broken into individual components and 
reported as such in the template for Energy and Atmosphere Credit 1.  The 
90.1-2004 User’s Manual is also silent on the issue; however it raises 
another issue.  In the example (Example G-J on page G-28) the calculations 
use total building supply CFM for an 80,000SF medical office building. 
Since the baseline is System 5 in that example and there would be multiple 
zones for the baseline, it appears that the equation should be used for the 
whole building only and not individual systems.  Is this how you are 
applying the formulas?  The user’s manual notes that fan powered VAV boxes 
are NOT included in the total fan power number as calculated from G3.1.2.9.

Jeremy R. Poling, LEED AP
Senior Sustainability Analyst
Strategic Services

Site Solutions | Operations | Sustainability


Epstein is a firm believer in sustainability. We ask that you please 
consider the environment before printing this e-mail.


From: bldg-sim-bounces at lists.onebuilding.org 
[mailto:bldg-sim-bounces at lists.onebuilding.org] On Behalf Of Ellen Franconi
Sent: Thursday, July 02, 2009 7:37 AM
To: Vikram Sami; Bldg-Sim; Michael I Rosenberg
Subject: Re: [Bldg-sim] Appendix G Fan Power

At my firm, we proportion the baseline total fan power determined as an 
aggregated W/CFM based on  the hp assigned to the fan motors (supply, 
return, exhaust) in the design documents. Of course this assumes that your 
baseline and your proposed design have a one-to-one fan correspondence.


Ellen Franconi, Ph.D., LEED AP
Senior Energy Analyst
Architectural Energy Corporation
2540 Frontier Avenue
Boulder, CO 80301
tel. 303-444-4149
fax 303-444-4303
efranconi at archenergy.com

>>> "Rosenberg, Michael I" <michael.rosenberg at pnl.gov> 7/1/2009 4:48 PM >>>

You have calculated the kW/cfm correctly according to the 2004 version of 
Appendix G. The allowance actually goes up a little in 2007, for your case. 
As for the second part of your question, no you do not apply the same kW/cfm 
to the exhaust fan. The calculated allowed fan power the total allowed power 
for all fans that are part of the system, including the exhaust fan. The cfm 
component is based on the supply fan cfm only. Unfortunately, Appendix G 
gives no direction on how to divide the allowed fan power up among the 
various fans in the system.



Michael Rosenberg
Senior Commercial Buildings Energy Analyst

Pacific Northwest National Laboratory
2032 Todd Street
Eugene, OR 97405
(541) 844-1960
michael.rosenberg at pnl.gov

From: bldg-sim-bounces at lists.onebuilding.org 
[mailto:bldg-sim-bounces at lists.onebuilding.org] On Behalf Of Vikram Sami
Sent: Wednesday, July 01, 2009 3:03 PM
To: Bldg-Sim
Subject: [Bldg-sim] Appendix G Fan Power

G3.1.2.9 Gives you a formula for calculating system fan electrical power for 
supply, return, exhaust and relief. Lets sat I have a zone with a PSZ-HP 
that has a supply volume of 1500 cfm. Using this formula along with table 
G3.1.2.9 I get a fan horse power of 1.29 and kW of 1.17. This equates to 
0.00078 kW/cfm – which seems really high.

The second part to my question is assuming I have 500cfm of exhaust in this 
zone, do I apply the same kW/cfm to the exhaust fans as well as the return 
fans? It seems that the lower the fan volume the more inefficient the 
Appendix G values get from these tables.

Any guidance would be much appreciated.

Vikram Sami, LEED AP
Direct Phone 404-253-1466 | Direct Fax 404-253-1366

1201 Peachtree Street NE, Suite 300, Atlanta, GA 30361
Responsive Design · Technological Expertise · Exceptional Service
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