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

[Michael Rosenberg]

Natasha,

A couple of things to point out. 

1. Your proposed system is a gas/electric hybrid system, so your
baseline system should be System 3, packaged single zone with dx cooling
and gas furnace heating, not system 4 with heat pump heating. (see TABLE
G3.1.1A)

2. When the baseline building requires a single zone system, each
thermal zone should have its own system assigned to it. This is implied,
though not clearly stated in the published version of Appendix G. It is
discussed in the 90.1 Users Manual. This has also been clarified in
Addendum U to 90.1 2004, which addresses system assignments in Appendix
G in more detail. 

3. I would guess that at least part of your building requires
economizers in the baseline system according to TABLE G3.1.2.6A. 

I am not sure this solves your problem, but I think you may want to
address these issues and then see if the mystery remains. 

Mike

Michael Rosenberg
Oregon Department of Energy
625 Marion St. N.E.
Salem, OR 97301-3742
Phone : (503) 373-7809
Fax: (503) 373-7806

>>> "Natasha Houldson" <nhouldson at glumac.com> 08/24/06 09:55AM >>>
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/>  
  _____  

320 SW Washington, Suite 200
Portland, OR 97204-2640
T.  503.227.5280  F. 503.274.7674
D. 503.345.6285

Thinking. Inside the building.
www.glumac.com <http://www.glumac.com/>  
 


=====================================================You received this
e-mail because you are subscribed
to the BLDG-SIM at GARD.COM mailing list.  To unsubscribe
from this mailing list send a blank message to
BLDG-SIM-UNSUBSCRIBE at GARD.COM

======================================================
You received this e-mail because you are subscribed 
to the BLDG-SIM at GARD.COM mailing list.  To unsubscribe 
from this mailing list send a blank message to 
BLDG-SIM-UNSUBSCRIBE at GARD.COM