# [Bldg-rate] Fan Power

Christopher Schaffner chris at greenengineer.com
Fri Jun 17 07:20:35 PDT 2011

```#2 is correct. Allowable fan power is based on the supply volume.  Would
love to see the review comments saying something different.

--

Chris Schaffner, PE
LEED AP BD&C
LEED AP ID&C
USGBC LEED® Faculty
Founder and Principal

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From:  Jason Wendel <jwendel at heatheng.com>
Date:  Fri, 17 Jun 2011 14:00:22 +0000
To:  "'bldg-rate at lists.onebuilding.org'" <bldg-rate at lists.onebuilding.org>
Subject:  [Bldg-rate] Fan Power

So, which is it, I have received LEED Review comments back stating two
different ways to calculate fan power and no matter what I do, the next
reviewer for the next project sees it differently and has me revise it.
1.       Calculate the baseline fan power from the sum of all the supply,
return & exhaust airflows combined to get a total fan power.  Then
proportion out the total fan power between each of the fans (supply, return,
exhaust, etc).

2.       Calculate the baseline fan power for the supply fan only, and then
proportion out the power from that calculation into each of the fans
(supply, return, exhaust, etc.)

Method #1 takes all the airflow in all the fans in the building added
together, calculates the fan power to move that much air, and then
proportions it out to each of the fans.    For example, 100,000 cfm supply
air, 90,000 return air and 10,000 exhaust air.  Totals to 200,000 cfm
airflow in the building.  Calculate the airflow for the 200,000 cfm and lets
say you get 200 kW.  Then you proportion out the fan power between the fans
­ but the sum is 200 kW of power.

So, method #2 doesn¹t make sense to me.  You take calculate the amount of
power required to blow the supply air (lets say 100,000 cfm, for example)
throughout the building (say 100 kW for simplicity), and then say that the
100 kW is then divided out between all the fans in the building, resulting
in the supply fan using 1Ž2 of the power that you calculated it should, and
the rest of the power is divided out between each of the other fans in the
building.  So you calculate that 100 kW of energy is needed to move 100,000
cfm of air, but then you change your mind and say that the baseline system
is actually only going to use 50 kW of energy and the other 50 kW is used in
the return and exhaust fans?  That doesn¹t make sense to me.

So, which is it?  I¹ve received review comments back from LEED stating each
of them, for different projects.  It appears to be interpreted differently
from different LEED Reviewers so apparently they don¹t know either.

-Jason

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