Hello, all.
I'm running an energy model (for LEED), and am having a particular
problem with the Fan Energy results. I am using the actual fan
horsepower (for the supply and for the return fans) for the proposed
building, and the Appendix G equation in G3.1.2.9. The results I'm
getting for the proposed design are considerably higher (about 31%) than
the baseline building. This leads me to believe that I may be
misinterpreting the equation.
As stated in the 90.1 Users manual, the equation accounts for supply,
return, relief, and exhaust fans. However, I've been simply inputting
the Supply CFM into the equation. Given that it accounts for all fans,
should I use the accumulated CFMs for each of those fans (i.e. if I'm
supplying 32,000, and returning 24,000, should I use 56,000 cfm)?
While I understand TRACE has the automatic baseline fan sizing
capability, I'm not sure I trust it completely, and would like to know
where the calculation comes from.
Thanks for any help you can offer!
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Hello Marty,
I would suggest you to calculate the baseline fan energy on a spreadsheet using 90.1 users manual and use this value for the energy model. I know trace internally takes the summation of cfm's and applied appendix G. Also look for the optimum start option under Systems ? Options ? Advanced Options ?Schedules.
Other strategy what you can adopt is make similar models and run the model for only one month (Actions ?Change Load Parameters?First month and last month).
Hope this helps you little bit.
Thanks,
Fareed Syed EIT, LEED? AP
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Addendum p of ASHRAE 90.1-2007 has modifications that incorporate
healthcare and lab usage.
-Christine
Marty,
As you know there are many ways to model fan energy. I normally use the
Apply ECB/PRM rules under the Change Energy Parameters tab for the
baseline building. Trace somehow accounts for the Appendix G
requirements. I do not modify airflows, I let Trace keep track of those.
For the proposed building sometimes I make a fan selection at the
desired static, divide the selected kw by the cfm and static pressure
and input the results in the systems fan tab by inputting the static
pressure and Full Load Energy Rate units of kw/cfm-in wg.
Can your system use a system exhaust in lieu of a return fan?
Hope this helps.
Bob Vogt
Marty,
The equations in Section G3.1.2.9 and Table G3.1.2.9 are only for the
baseline building. For the baseline building, the supply CFM is used to
model the efficiency of all of the fans in the building based on
assumptions about minimum design characteristics. If you are using the
equations, you should use the Supply CFM for the system, run through the
equations in Table G3.1.2.9 to determine the BHP for the baseline, then
run that through the equation in G3.1.2.9 to determine total fan power
in kW. That value can then be input into Trace for fan power.
For the proposed building, you should use the actual rated fan kW based
on the fan power curve for your CFM and the fans specified.
As Bob stated, Trace does have an option where it will do the
conversions for the baseline building, but I typically have not used it.
I shy away from the "black boxes" in energy modeling when putting things
together for a LEED review team since ANYTHING is up for grabs when they
review the credit and if I can't explain a black box process they may
deny the entire credit.
JEREMY R. POLING, PE, LEED AP