Appendix G Fan Simulation

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Sorry to rehash this but I've been going through the baseline fan power, and still don't have the answers I need.

I've got a 113,000 cfm (from the SV-A System Design Parameters report) VAVR system on my baseline that when I go through all the calculations for ASHRAE 90.1-2007 (from Table G3.1.2.9, Table and b) I get 176.13 bhp. I put that in the wizard entry for BHP for the VAVR system and don't get any errors, BUT when I put in the bhp that I solved for the packaged multi-zone AHU's that will be used in the building also (92,000 cfm, 153.88 bhp), using the same calculations, it gives me an error that the static is at 56 in w.c., and it needs to be below 15.

What's going on? Also, if you use the equations from Table and b, then the break horsepower is higher than the allowable nameplate hp. Isn't that backwards?


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

Ok, so here's the spreadsheet I set up to calculate the bhp with the pressure drop allowances

System #

Calculated CFM

Ducted Return (0.5)

Particulate Filtration Credit: MERV 13 thru 15 (0.9)

Sound Attenuation Section (0.15)

"A" (Table

Allowable Fan System BHP (Table G3.1.2.9 also)

Allowable Nameplate Motor hp (Table


Packaged VAV System with HW Reheat








Packaged Multizone with HW reheat









Unit Heaters






Split System Single Zone DX with Electric Heat







. From what you're saying, do I not even bother with the bhp input in the wizard mode, just set up the system and run the simulation, then calculate the kW/cfm and put that in from the detailed mode? Do I use the total fan KW from the SV-A output and the cfm from that output (shown below)?
[cid:image002.png at 01CA74D8.49B16180]

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

Yes, that is the correct place to find it. However, The fan power that you get from your 90.1 calculation is the total fan power for the system which includes associated supply, return, and exhaust fans. I usually portion out that kW/CFM to each fan being used in the design (if you have 0.00120 kW/CFM from calcs and you have a supply and return that are equal, I put .00060 kW/CFM for each fan). This is important because of fan heat.

John Grando LEED AP| Mechanical Engineer

Grando, John's picture
Joined: 2011-09-30
Reputation: 0


You do not need to use the Allowable Nameplate Motor HP (Table
equation to determine the system fan power for an Appendix G baseline.

For System 1, I can confirm that you've calculated the allowable fan system
BHP correctly as 173.02. Using the fan CFM reported in the SV-A report is
also correct.

the following:

Fan Motor Efficiency = the efficiency from Table 10.8 for the next motor
size greater than the bhp using the enclosed motor at 1800 rpm.

Then, use the equation provided in G3.1.2.9 for system types 3-8, as

Pfan = bhp x 746 / fan motor efficiency = 173.02 x 746 / 0.95 = 135866 W =
135.87 kW

I find it easiest to leave the wizard as a default value, simulate the model
after all other parameters are defined, then calculate the fan power based
on the CFM reported in the SV-A report. Then, input a kW/cfm in the
detailed mode. Re-simulate and cross check the fan power reported in the
SV-A report. The fan power reported here should be equal to 135.87 kW.
Additionally, the peak demand for ventilation fans should also correspond to
this value (or a sum of the fan power for multiple air systems). FYI, you
may need to make some small adjustment to the kW/CFM to get to the correct
total fan power. I typically do this using a ratio of input vs. output to
come up with an adjusted kW/CFM.

On another note, in the table below System 2 and 4 are described as a
packaged multizone with HW reheat and split system single zone DX with
electric heat, respectively. These system types do not correspond with any
of the Appendix G Baseline HVAC Systems, as described in Table G3.1.1A. You
may want to have another look at this. Also, be careful with modeling a
unit heater in the baseline case. Be sure that the space can be considered
semi-heated, otherwise this system will need to be modeled with both heating
and cooling. For more information about this see the definitions provided
in Section 3, Table 3.1, and Table G3.1 Item 10.

Good Luck.

Amanda Bogner, PE LEED AP

Amanda Bogner's picture
Joined: 2011-09-30
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