[BLDG-SIM] High cooling energy results for VAV packagedsystem relative to LEED NC 2.2 Baseline
Joe Llona
jllona at cdiengineers.com
Thu Aug 24 14:01:46 PDT 2006
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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