LEED Appendix G - System 7 or System 8?

asked 2016-09-30 09:18:22 -0500

Annie Marston
4297 ●4 ●31 http://www.resustain.com

Dear All,

I have a mixed fuel system, gas furnace in the AHU and electric reheat at the zone level. Because of this I have chosen to use System 7 (VAV reheat) in my baseline for my LEED model. However... my AHU also has a heat exchanger and delivers air to zone units between 6.6'C and 10'C because of this, my gas coil never turns on and I have no gas usage reported. My question is, should I therefore change my baseline system to system 8?

All answers, opinions, comments and so on, greatly appreciated as always!

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Which version of 90.1?

JasonGlazer ( 2016-09-30 09:22:03 -0500 )edit

ASHRAE 90.1 - 2007

Annie Marston ( 2016-09-30 10:34:45 -0500 )edit

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answered 2016-09-30 09:50:26 -0500

MatthewSteen

10077 ●18 ●25 https://www.nrel.gov/b...

updated 2016-09-30 09:53:01 -0500

Since there's (non-emergency) on-site fossil fuel usage in the proposed design, the baseline should be System 7.

We recently got guidance from USGBC that the baseline system type should use fossil fuel for a project that had a natural gas back up boiler, which was expected to only run once every ten years, but otherwise had an all electric central plant and buildings.

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Great thanks Matt, that is good to know, my instinct was system 7 but with the proposed case showing no fossil fuel use I am wondering whether GBCI will throw back a review comment on it. Your recent guidance is good to know.

Annie Marston ( 2016-09-30 10:35:53 -0500 )edit

...but only one data point. GBCI is consistently inconsistent in my experience : )

MatthewSteen ( 2016-09-30 11:46:44 -0500 )edit

hahahah yes I agree, who really knows but at least it is one data point.

Annie Marston ( 2016-09-30 12:18:58 -0500 )edit

I had a similar situation to Annie's (though I would not say "recent"): Gas preheat for OA at the central units, but with a substantial majority of airstream heating accomplished at electric reheat coils in the terminal boxes. For the preliminary review I went with the "Electric and other" System type 8. The first round of comments advised my baseline choice should be #7, but citing the "predominant conditions" line following TG3.1.1A & proving the majority of heating was actually electric, I was able to stick with system type #8. Just another anecdote reinforcing Matthew's point.

Nick Caton ( 2016-10-02 11:06:36 -0500 )edit

(continued due to character limit)

... so in summary and with my own experience in hindsight, I'd advise reaching out to GBCI and making the case for System type #8 if it will be beneficial to your project. System type #7 is likely the "safe" route for reasons discussed in that it's less likely to be called into question, and for that reason might suit your priorities better. Ultimately it's your call to decide whether this is a fight worth fighting ;-).

Best of luck!

Nick Caton ( 2016-10-02 11:09:48 -0500 )edit

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answered 2016-10-02 00:12:31 -0500

Anna
373 ●1 ●8

Hi Annie, GBCI reviewers make their comments based on the documentation provided for review. If your mechanical schedules are showing a natural gas furnace, you may get a comment questioning your proposed model, unless you include a narrative explaining the modeling results within your LEED documentation. Regarding the Baseline system choice, I think that making an argument based on LEED Interpretation 10285 should be good enough. LEED Interpretation 10285 states “that the heating source would be considered “Fossil Fuel/Electric Hybrid” if the space is heated by any combination of fossil fuel and electricity. This includes backup heating, heating of ventilation air serving the space, or preheating, but does not include emergency backup heat sources. Note: Emergency back-up heating refers to heating that runs when the primary system fails or needs to be shut off in an emergency, and does not refer to a backup system which may be used to provide additional capacity as needed."