[BLDG-SIM] High cooling energy results for VAV packaged system relative to LEED NC 2.2 Baseline

[Natasha Houldson]

I am modeling a 3-story office building, 40,000 sf (27,000 sf conditioned) in Seattle, WA, consisting of parking garage, retail, and core spaces on the first floor; and core plus open office spaces on the second and third floors.  I am trying to calculate the estimated energy cost savings for LEED NC 2.2, which requires the use of a baseline energy model per ASHRAE 90.1-2004 Appendix G guidelines.
 
The proposed design uses a packaged VAV system to serve the core and office spaces, consisting of: 1) a rooftop packaged AC unit with DX cooling, high efficiency gas furnace, premium efficiency VSD fan motor, and economizer; and 2) series fan powered boxes with electric reheat at each zone.  Cooling supply air is delivered at 48F, and the DX coils have an EER of 11.7 (fan power removed).  The building envelope includes 35% window to wall ratio, average window assembly U-value of 0.385, and external wall assembly R-value of R-11.  The controls strategy for the VAV system cooling is "reset for warmest zone."
 
Per Appendix G, the baseline model uses a constant volume, packaged rooftop unit with DX cooling and electric heat pump.  To reduce the number of hours certain spaces were under-heated, I had to include some heat pump supplemental electric heat and zone reheat.  Cooling supply air is delivered at 55F, and the DX coils have an EER of 11.6 (fan power removed).  The building envelope includes 35% window to wall ratio (same as proposed design), average window assembly U-value of 0.595 (higher than proposed design), external wall assembly R-value of R-8 (lower than proposed design), and lighting power densities higher than in the proposed design.
 
Here is my question:  According to the results, the cooling energy required for the proposed design model is about 30% greater than the baseline model.  This does not make sense to me, since the proposed design model incorporates a VAV system with economizer, slightly improved EER, and improved building envelope.  (Fan power is 72% lower and heating is 9% lower than the baseline model, which seems reasonable.)  Any thoughts on where there might be an error in my modeling?  I would appreciate any insights or suggestions.
 
Thanks,
Natasha Houldson

Natasha Houldson, P.E.
Mechanical Engineer

 <http://www.glumac.com/>  
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