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NC-2009 EAp2:Minimum Energy Performance

System fan power - Baseline

Hi, I have a doubt related to the system fan power calculation described in section G.1.2.9. This section states that "system fan electrical power for supply, return, exhaust, and relief (excluding power to fan-powered VAV boxes) shall be calculated using the following formulas..." So the formulas present the term "CFMs", where the subscript is for supply. So, the value that I calculate for the bhp accounts for the total fan power for each system (supply, exhaust, return and relief fans) using the baseline system maximum design supply fan airflow rate? Or, should I use the formulas for each of the fans, separately, considering the flow through each of the fans? Thanks, Luana

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David Hubka
Director of Operations
Transwestern Sustainability Services
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355 thumbs up

Tue, 10/16/2012 - 20:15

Use the formula for each fan seperately, with the fan's airflow.
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Marcus Sheffer
LEED Fellow
7group / Energy Opportunities
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Tue, 10/16/2012 - 20:53

Just want to be clear - the baseline fan power for each system as calculated in G3.1.2.9 does include all of the fans associated with that system (supply, return, exhaust, relief). The formulas are based on the supply air cfm. So your first question represents the correct path. You do not do separate calculations for the supply fan and the return fan and the exhaust fan. All these are included in the G3.1.2.9 baseline system fan power as part of the same formulas.

Thu, 10/18/2012 - 16:10

So just to be sure if I understood the explanation correctly, I'll give some numerical examples to illustrate. For example, if the supply air volume determined by the simulation program was 120,000 cfm and if my baseline system is #5, then I would have: bhp=0.0013 x 120000 = 156 If I have an exhaust fan whose air flow is 12000 cfm (10% of the supply air flow), I shouldn't calculate the bhp for it, according to the following formula, is this right? bhp=0.0013 x 12000 = 15.6 Thanks.
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Marcus Sheffer
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7group / Energy Opportunities
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Thu, 10/18/2012 - 16:56

Nope. The exhaust fan would already be included in the first calculation. If you have a separately controlled exhaust fan entirely independent of the HVAC system and not necessarily running at peak then it is modeled identically to the Proposed in the Baseline. There is a discussion above this one regarding the issue of how to account for the fans in the baseline.

Thu, 10/18/2012 - 17:31

Hi Marcus, I also read the discussion above. Now things are clear! Thanks a lot!

Fri, 12/21/2012 - 17:56

Hi, I just didn't understand one matter. The supply air volume calculated according to section G3.1.2.8 is used to calculate the total fan power for baseline system design, which reflects the sum of power modeled for supply, exhaust, return and relief fans. Though, in section G3.1.2.8, it is mentioned that "if return or relief fans are specified in the proposed design, the baseline building design shall also be modeled with fans serving the same functions...". But, weren't they already accounted for in the formula of section G3.1.2.9? Is it necessary any additional modeling? Thanks! Luana
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Marcus Sheffer
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LEEDuser Expert
4454 thumbs up

Fri, 12/21/2012 - 18:35

The second part refers to the sizing of the baseline supply air quantity. Use G3.1.2.8 to determine the quantity of air. The fan power for all fans associated with the HVAC systems is accounted for in G3.1.2.9 so no need to add any fan power.

Fri, 02/08/2013 - 12:25

Hi! In section G3.1.2.9 it is specified that "system fan eletrical power for supply,..., shall be calculated using the following formulas:" Then, it is presented some formulas indicating the fan system input and fan system motor nameplate. Just to confirm, in the simulation model, the energy consumption related to these fans shall be calculated based on the fan system motor nameplate, right? Or should it be calculated based on fan system input? Thanks!
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Marcus Sheffer
LEED Fellow
7group / Energy Opportunities
LEEDuser Expert
4454 thumbs up

Fri, 02/08/2013 - 19:09

The baseline is calculated based on the auto-sized supply air cfm. There is a spreadsheet tool that does these calculations under the resources section at the top of this page. The baseline would not have a motor nameplate because the fans do not exist. The proposed is calculated based on the actual design.

Fri, 02/08/2013 - 19:26

Hi Marcus. I understand that the baseline fan power is calculated based on the supply airflow rate, whose formula is presented in Table G3.1.2.9. Though, in section G3.1.2.9, based on the value previously calculated, we calculate the Pfan, which would correspond to the eletric power to fan motor. My question is, in the simulation model, should I use the value calculated through the Table or the Pfan value?
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Marcus Sheffer
LEED Fellow
7group / Energy Opportunities
LEEDuser Expert
4454 thumbs up

Mon, 02/11/2013 - 15:34

Hard for me to say how it gets modeled as it could be software specific. The value calculated in the table gives you the bhp which is then used to calculated Pfan. What are your options for inputting this data in the software you are using? For LEED the reviewer will ideally want to see the kW of the baseline fans.

Mon, 09/22/2025 - 14:16

Hello Marcus,I’m currently working on a LEED v4 project where the proposed HVAC system includes both supply and return fans. The baseline model has been developed in accordance with ASHRAE 90.1-2010 Appendix G, and the baseline system type also includes a return fan.While filling out the LEED MEPC form, specifically in the Air-Side HVAC tab, I noticed that the form requests only the supply airflow (CFM), but requires separate entries for supply fan power and return fan power.Since the baseline fan power is calculated using the supply airflow only (per G3.1.2.10), the total fan power (kW) for the baseline is significantly lower than that of the proposed system, which includes both supply and return fan power based on actual equipment data.My question is:
Are we expected to assign both supply and return airflow (CFM) in the baseline model to justify the return fan power entry, or should the return fan power be derived as a portion of the total baseline fan power calculated from supply airflow only?
Thanks in advance!

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