Calculating Baseline Fan Power

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Sorry in advance for a confusing question, but determining baseline system fan power is terribly confusing to me. I am currently using ASHRAE 90.1 - 2007.

According to section G3.1.2.9 I need to calculate fan power=(bhp*746)/(fan motor efficiency) for a system 7.

According to Table G3.1.2.9 I need to calculate bhp=CFM(s)*.0013+A for a system 7.

According to Table I need to calculate A=Summation of [PD*CFM(d)/4131] where:

PD= each applicable pressure drop adjustment from table in wc and
CFM(d)= the design airflow of each applicable device from table

So basically I take the airflow rates from my design building systems and the pressure drops from my components in my design building systems to figure out what my baseline building break horsepower will be. What if I have systems serving multiple floors in my design building? Since 90.1 requires me to have a system per floor in my baseline building how would I handle this? What if I have systems with different components serving different portions of my design building?

I guess a simpler question would be how are people determining baseline fan power?

Thank you,

Scott Tomlinson, PE, LEED-AP

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Joined: 2011-09-30
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The note for Table G3.1.2.9 states to use the design flow rate of the
baseline building system to calculate A. There is still the issue of
whether or not to apply the adjustment from Table when the
baseline system serves a different area than the proposed system. I'm
still trying to decide that as well. I can think of at least two
1.) Apply the pressure drop adjustment to any baseline system that
conditions at least half of the area of a proposed system that qualifies
for the adjustment,
2.) Apply the pressure drop adjustment in a weighted manner based on
the proportional area conditioned by the baseline system relative to the
proposed system.

I think option 1.) would be quicker but option 2.) more closely reflects
the intent of the pressure drop adjustment.

On a related note, if anybody has figured out a way to use a VLOOKUP
function or equivalent in Excel for looking up motor efficiency in a
baseline Pfan calculation, let me know. (The function returns the
efficiency of the next smallest motor in the table when there is no
exact match, but G3.1.2.9 requires the efficiency of the next largest
motor to be used.)


William Bishop, EIT, BEMP, LEED(r) AP

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Joined: 2011-09-30
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I think I can help with the Excel formula...

The VLOOKUP function uses 2 columns of data: the first is where the
function is looking to match numbers (or find "closest to") and the
second is the value to return that corresponds with the first.

Make your first column have the min and max values in a range for a
single efficiency. Say you're increasing from A to B to C for every
interval of 5:

0 A

4 A

5 B

9 B

10 C

14 C

And so forth.. and make sure your VLOOKUP is set to "closest to"
instead of "exact match." Does that get the job done?

Greg Collins

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Joined: 2011-09-30
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If you just start the table with:

0 0.825

and set =vlookup(*,*,2,TRUE) you should get the results you want.

John Grando LEED AP

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Joined: 2011-09-30
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Thanks John, Greg for the Excel advice.

I got LOOKUP to work by shifting the range of cells for "result_vector"
by +1. The only time there is a problem is if the calculated motor bhp
is exactly equal to one of the selections in the table, which is very


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Joined: 2011-09-30
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