Fan Power

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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 ? 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.


jwendel's picture
Joined: 2010-10-05
Reputation: 0


Appendix G is clear in stating that the system total fan power is based on the baseline system maximum design supply fan airflow rate, which is itself determined on the basis of "a supply-air-to-room-air temperature difference of 20?F or the required ventilation air or makeup air, whichever is greater", as per G3.1.2.8.

What is not clear from 90.1-2007 is how you split this total fan power between supply/return/relief/exhaust. 90.1-2010 however add the provision that the "calculated system fan power shall be distributed to supply, return, exhaust, and relief fans in the same proportion as the proposed design."


Demba Ndiaye's picture
Joined: 2011-09-30
Reputation: 200

You seem to be directly correlating fan energy consumption and airflow rate. Remember, fan energy is proportional to the product of airflow rate and differential pressure.

Here are my thoughts:

If we have:
SA fan: 100,000 cfm @ 4"wc
RA fan: 90,000 cfm @ 1" wc
EA fan: 10,000 cfm @ 1" wc

And let's say they result in:

SA fan: 95 KW
RA fan: 20 KW
EA fan: 2 KW

The total airflow of the system is 100,000 cfm, since the RA and EA fan are in series with the SA fan. Remember, in a series arrangement the differential pressure of the fans are summed, not the airflow quantity; therefore, we meet the LEED criteria that says that the system total fan power is based on the baseline system design supply airflow rate. The system total fan power, however, is 117 KW, which can be calculated by adding the individual fan's KW together, or by using total system airflow (100,000 cfm) and total system static pressure (5" wc) to calculate total system KW directly. The system total differential pressure is 5", not 6", because the RA and EA fans are in parallel with each other, and both are in series with the SA fan.

Then I guess you'd simply apply that proportion of fan power (81% for SA fan, 17% for RA fan, and 2% for EA fan) to the proposed system configuration.

Can anyone else confirm? I haven't done this calculation for LEED before so I am not saying this IS the correct approach. This is simply my initial reaction to your question.

Michael Eustice, PE, LEED AP

Michael A. Eustice's picture
Joined: 2011-10-02
Reputation: 0


ASHRAE 90.1 2007 (and 2004) uses CFMs = "the baseline system maximum design supply fan airflow rate in cfm"

So it should be method 2. As Michael points out, you're not actually calculating the amount of power it takes to supply 100,000 cfm. You're calculating your "system allowance" based on that supply / total system airflow. The calculations already are based on the idea that a supply of that quantity will have to return and/or exhaust that quantity as well.

The Advanced Energy Modeling for LEED manual also points out that a common error is "Additional fan power allowance is claimed in baseline for return, exhaust, or relief fans".


corinnebenedek's picture
TRACE 700 User
Joined: 2010-10-18
Reputation: 0

Thank you to all who commented. It makes more sense now that the calculated fan power is more of a building or system "allowance", not an actual calculation of power to move the air. I'll stick to that method.


jwendel's picture
Joined: 2010-10-05
Reputation: 0