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Guidance on 90.1-2013 Appendix G system selection, Exception 2 (for spaces with differing peak thermal loads)

asked 2020-12-29 16:20:44 -0500

JustinLueker gravatar image

updated 2021-04-07 13:03:23 -0500

Hi all, a question on 90.1-2013 Appendix G baseline system selection, specifically the exception related to areas with differing internal gains from the predominant space type.

I note the Exception 2 in the 90.1-2013 Appendix G section covering baseline system selection:

"If the baseline HVAC system type is 5, 6, 7, 8, 9,10, 11, 12, or 13 use separate single-zone systems conforming with the requirements of System 3 or System 4 (depending on building heating source) for any spaces that have occupancy or process loads or schedules that differ significantly from the rest of the building. Peak thermal loads that differ by 10 Btu/h·ft2 or more from the average of other spaces served by the system or schedules that differ by more than 40 equivalent full-load hours per week from other spaces served by the system are considered to differ significantly."

It would seem the spirit of this exception is to apply PSZ baseline systems to areas with HIGHER internal gains than the predominant space type, though this is not explicitly stated. Could this clause also apply to spaces with LOWER internal gains than the predominant type?

For example: if the predominant building type is 100,000 ft2 of laboratory space modeled with significant plug loads (on a System 7) but there are 30,000 ft2 of other circulation/office space with plug loads that are >10 Btu/h/ft2 LESS THAN the labs, would this circulation space qualify for this exception, and thus qualify for a System 3 or 4 baseline?

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answered 2021-04-02 13:31:32 -0500

Yes for sure the rule applies to LOWER internal gains too.

You have to do the area weighted average of internal peak load (process, lights, people) of the spaces originally served by your main system (at the beginning they probably will be all conditioned zone on a floor if there are not 40-h schedule exceptions, or mixing use). Then starts to take off all spaces whose internal peak load differ more than 10 Bth/h-ft2 till to have no one exceeding this threshold value .

You may want to see PNNL-26917 PRM Reference Manual, pag 3.4 (pag 37 PDF)

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Asked: 2020-12-29 16:20:44 -0500

Seen: 91 times

Last updated: Apr 02