Marty,

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.

Regards,

Kevin Caho, LEED? AP