Fossil/Electric hybrid system under 90.1 App G

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I can't seem to find reference to this in the 90.1 User Manual.

In Appendix G, Table G3.1.1A, there is a reference to a Fossil/Electric
hybrid for determining Baseline system type. Is this a system that, for
example, might have a gas-fired RTU with electric VAV boxes?

In other words, if there is ANY gas heat in your Proposed building, your
Baseline building would have to automatically default to a fuel furnace
or gas-fired hot water heating system, correct?

Thanks,

James Hansen, PE, LEED AP

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There is a CIR floating out there from a LEED-CS project that states the
baseline building can have electric reheat. However, the modeler is to
match the total therms of natural gas consumed for the gas-fired RTU in
both the baseline and as-designed models.

Mitch Dec

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Yes, you are correct. Appendix G requires that if you have any gas heat
in your Proposed building, the baseline system would use gas heat.
However, if this is for a LEED submittal, you may want to do a little
research on relevant CIR's since USGBC has interpreted this a little
differently than was intended by Appendix G.

Michael Rosenberg

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I believe the/a CIR that Mr. Rosenberg is referring to is an NC v 2.2 CIR submitted 9/27/06 and answered on 10/23/06 that hinges on the gas being preheat.

I have not read the CIR Mr. Dec mentioned although that seems fascinating too.

Paul Riemer

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Paul, you are correct, the NC CIR appears to apply. I couldn't find one
under CS. And yes, it looks like as long as the gas heating section is
for preheat (or after-hours heating, or morning warm-up, etc) then you
can have a gas-fired RTU with electric reheat VAV boxes.

Thanks to all,

James Hansen, PE, LEED AP

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I'm assuming this applies to solar hybrid systems as well?

Matthew Higgins, LEED AP

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Can someone please provide input regarding what would be the correct Appendix G baseline system for a 4 storied building > 150,000 sf which has a chilled water system for cooling and uses electric series fan powered boxes for perimeter heating. There is a gas fired dedicated outside air unit that supplies air to all the Air handling Units. The gas fired OA units use about 35% of the total heating energy for the building. The rest 65% is from the perimeter boxes. The total heating utility cost for gas is very small ($6500) compared to the total building utility cost ($270,500).

We intend to submit this project for LEED and are struggling since the choice of the base system significantly changes the performance of the building. We have implemented a number of energy saving measures in the design like better insulation in walls and roof, high performance glass, daylight sensors, occupancy controls, reduced lighting power density, heat recovery, water side economizer. Our building saves about 15%energy compared to the baseline system when using System 7 as the baseline. However in terms of Utility this imposes a major penalty and barely meets the baseline Utility Cost when compared to a hydronic system i.e., System 7.
When the baseline is modeled as System 8 we can the 14% Utility Cost savings.

Has someone modeled a system similar to this before and submitted it for LEED. What would be the correct choice for the baseline system.
There is note on page 176 which states" Where attributes make a building eligible for more than one building system type, use the predominant condition to determine the system type for the entire building. "

We think this exception would apply to our design since most of our heating is electric and System 8 would be the right system to use.

One option around this is to change our Outside air units as electric and then there would be no question regarding the base system as it would be electric System 8. However we do not think this is the right design approach since the proposed design would have more utility cost than the current scenario and meet LEED 14% requirements.

There was a discussion regarding this a few days back on the forum and there was a mention of a CIR dated 9/27/2006. Would this apply to our case?

I know there are a number of experienced professionals on this forum who have been involved with the LEED energy and atmosphere credits documentation evaluation for different LEED projects for this credit. I would appreciate your insight.

Please advise.

Thanks
ARVINDER DANG, LEED(r) AP

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