Trace-users Digest, Vol 15, Issue 1

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I had the same question previously and I would encourage you to check to
ECM/PRB sizing option yourself and verify that it is correct if this worries
you. All you need to do is apply it for one simulation, open the "equipment
energy consumption" report, and do the hand calc with the cfm shown for each
fan and compare these values to the kW that Trace has calculated. I think
you'll find that they are very close (within 1%).

You say your results are higher and I guess I'm not sure if you are
referring to the fan size or to the fan energy consumption. In the
particular model that I have in front of me (using the ECM/PRB option) the
proposed is also higher. CFM by 16%, kWh by 19%, and kW by 23%. This is not
surprising as the proposed case has 70% glass vs. 40% for the baseline so
their is more air needed to satisfy the load. The kW/cfm however are nearly
the same. I suggest checking items like that to make sure things are
proportional because your total % difference doesn't necessarily tell the
whole story.

My other thought is that you used the wrong units. In your post you said
you used G3.1.2.9 to calculate the horsepower but your result from that
equation would be in watts. If you used kW from that equation and entered
as hp in your 90.1 model, then you'd have 25% less power than you should
leaving your proposed more than 30% ahead.

Hopefully I answered your question somewhere in there.


Kevin Caho, LEED? AP

Kevin Caho's picture
Joined: 2011-10-01
Reputation: 0

Hi Marty,
It's not unusual that your proposed fan KW and annual energy consumption is more than the baseline. ASHRAE 90.1 is kind of stingy with respect to the allowable fan KW for the baseline. If you are using the 2004 edition of 90.1, make sure you are using Addendum ac to calc your baseline fan energy. This addendum is allowed by LEED and should help your situation some. If you are using the 2007 edition of 90.1, the fan energy equation in Appendix G should already include the revisions from addendum ac along with any subsequent others. In all cases, though, you are only allowed to calc your baseline fan energy allowance on the SUPPLY cfm. You can't add up your supply, return, exhaust cfms etc and use that calc your fan energy. Nor can you apply the equation to each of your component fans separately. This question comes up a lot since Appendix G it is a little confusing in this regard. There is a good bid of traffic on the subject in the Bldg-sim
archives ( including some responses directly from LEED reviewers - although in an unofficial capacity) if you'd like a little more clarification or confirmation on the issue.

In terms of Trace inputs, I usually calc the total fan break KW for my baseline systems and proportion that out to the individual component fans as applicable based on their cfms. I input these numbers directly into Trace in lieu of allowing Trace to automatically calculate it - but it sounds like that method would work too based on Kevin's email below. Then I apply the ASHRAE 90.1 Appendix G fan curve to the baseline case fans (as applicable). For the proposed case, I input the break KW for each fan based upon the actual fan selections and then apply the appropriate fan curves types based on the actual fan types being specified.

Hope that helps some.

Julia Beabout's picture
Joined: 2011-10-01
Reputation: 3


I am under the assumption that you shall use the 32,000 cfm in the calculation and the equation accounts for all other fan energy. In other words, inputting the 32,000 cfm accounts for the power of the supply fan, return fan, and exhaust fan.

Keep in mind that the 32,000 cfm should have been calculated based on a 20 degree temp difference. Verify that you have modeled that correctly.

Please re-post and let me know if you agree and what you have concluded.


Tony Kriel's picture
Joined: 2011-09-30
Reputation: 0