Appendix G Fan Power

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I am a little confused on how to apply fan power to the baseline
building using Appendix G. Per G3.1.2.8 ".... 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 and sized for the
baseline system supply fan air quantity less the minimum outdoor air, or
90% of the supply fan air quantity, whichever is larger."

Following are comments from two USGBC reviewers on different projects:

* Please note that the Baseline fan power calculated in Section
G3.1.2.9 refers to the sum of the supply, return, exhaust, and relief
fans for each HVAC system.
* Finally, the fan power for the Baseline Case must be calculated
according to Table G3.1.2.9 where CFM refers to the supply CFM (not
supply + exhaust + dedicated outside air), and where the fan power
refers to the sum of all supply, return, exhaust, and relief fans for
each zone.

Should the airflows be summed, calculate the fan power and then prorate
the power to each fan based on cfm?


Robert Vogt

Bvogt's picture
Joined: 2010-10-05
Reputation: 1


The CFM used in the 90.1 calculation is the CFM through the main coil for
the system. Size the return/relief fans for 90% of this number or this
number less the minimum outdoor air which ever is larger.

Make the 90.1 calculation then check your fan selections against the allowed
HP. This allowed horse power is the sum of all the fans that operate at
design. That normally does not include relief fans or parallel fan box
fans. According to ASHRAE fi an exhaust fan does not exhaust from the
building it is not counted either.


Mack Wallace's picture
Joined: 2011-10-01
Reputation: 0