[Bldg-rate] Fan Power

Jason Wendel jwendel at heatheng.com
Fri Jun 17 07:00:22 PDT 2011

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 doesn't 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 ½ 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 doesn't make sense to me.

So, which is it?  I've 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 don't know either.


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