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

[Joe Llona]

Another point on the cooling energy for the proposed model:

Using a 48 deg SADB eliminates a lot of economizer hours, especially in
Seattle.  Raising the SADB will reduce cooling energy but increase fan
energy.  It's a balancing act to find the sweet-spot between cooling
energy and fan energy.

Joe Llona, P.E., CSBA, LEED(r) AP 
Director of Sustainable Design 
CDi Engineers 
Phone (425) 712-2182
Fax (425) 778-8769 
E-mail jllona at cdiengineers.com 

"You must be the change you want to see in the world." - Ghandi



 

> -----Original Message-----
> From: BLDG-SIM at gard.com [mailto:BLDG-SIM at gard.com] On Behalf 
> Of Tupper, Kendra
> Sent: Thursday, August 24, 2006 1:12 PM
> To: BLDG-SIM at gard.com
> Subject: [BLDG-SIM] High cooling energy results for VAV 
> packagedsystem relative to LEED NC 2.2 Baseline
> 
> That's what I was thinking initially, but it shouldn't be 
> increasing the cooling energy by 30% if the cooling control 
> is set to warmest.  If you set the cooling control to warmest 
> and allow the supply air temp to be reset with OA (up to 62 
> F, for example), then you can avoid most instances where you 
> would be cooling to 48F and then reheating at the zones. 
> 
> -----Original Message-----
> From: BLDG-SIM at GARD.COM [mailto:BLDG-SIM at GARD.COM] On Behalf 
> Of Erik Kolderup
> Sent: Thursday, August 24, 2006 11:59 AM
> To: BLDG-SIM at GARD.COM
> Subject: [BLDG-SIM] High cooling energy results for VAV 
> packagedsystem relative to LEED NC 2.2 Baseline
> 
> Another fundamental point is that a VAV reheat system will 
> almost always
> 
> use more cooling energy than a single-zone system without 
> reheat, assuming they have the same EER and same space loads. 
> In the VAV case, there will be some zones that spend time at 
> minimum primary air flow and
> 
> reheat kicks in. That is wasted cooling energy that doesn't 
> happen in a single-zone system. You can reduce the VAV/reheat 
> cooling penalty by reducing the primary air minimum flow 
> settings to each zone, but the reduction is limited at some 
> point by ventilation requirements. 
> 
> So a VAV saves fan energy in the Appendix G context, but not 
> necessarily
> 
> cooling or heating energy.
> 
> Erik Kolderup
> Architectural Energy Corporation
> 
> -----Original Message-----
> From: Michael Rosenberg [mailto:Michael.Rosenberg at state.or.us]
> Sent: Thursday, August 24, 2006 11:44 AM
> To: BLDG-SIM at gard.com
> Subject: [BLDG-SIM] High cooling energy results for VAV 
> packagedsystem relative to LEED NC 2.2 Baseline
> 
> 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/>  
>  
> 
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