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why is 90.1 Appendix G Baseline one system per floor

asked 2016-12-20 10:48:21 -0500

xchen's avatar

updated 2016-12-20 11:39:30 -0500

This is a general question, I would like to understand the reasoning behind it. I remember that 90.1-2004 did not have the "one system per floor" requirement and the zoning was the same as the design case. But starting 90.1-2007, the requirement became "each floor shall be modeled with a separate HVAC system". Thus the zoning is very different from the design case. And I personally feel it is very difficult to compare the two models (design and baseline) and do trouble shooting etc this way. Also the one system per floor does not seem to be a better representation of design in the actual world. Can anyone explain a little bit why they have this requirement? Thx.

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answered 2016-12-20 12:01:47 -0500

updated 2016-12-20 12:07:10 -0500

In ASHRAE 90.1-2007 (and 2010 and 2013 and probably 2016, too) Appendix G, the baseline thermal blocks should be the same as the Proposed Design model. See Table G3.1 No. 7 and 8. Nothing in the Performance Rating Method requires (or even allows) the model zoning to be different.

The 'system per floor' requirement for baseline systems 5, 6, 7 and 8 simply means that all the thermal blocks (except the ones that meet the G3.1.1 exceptions) on the same floor are conditioned by a single baseline system. I do not know the exact reason why this requirement exists, but would guess that since those systems are multi-zone, ASHRAE has to set some standards for which zones are served by which system, and 'per floor' is a pretty straightforward standard to follow.

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thanks Eric. I guess what I would like to know is why they decided to stop using the same zone/system assignment as the design case and switch to one system per floor. Isn't it easier to just use the same "which zones served by which system" as is in the design case?

xchen's avatar xchen  ( 2016-12-20 12:17:24 -0500 )edit

I don't see where 90.1-2004 says to use the same zone/system assignment as the design case. Neither the 2004 Standard or the User's Manual (that I can find) explicitly states how to assign zones in the baseline. Which is probably why they amended the standard with Addendum u to (as the addendum forward states) "provide guidance for complying with the intent of the baseline building design for HVAC systems 5,6,7, and 8, which shall be modeled as floor-by-floor HVAC systems."

ericringold's avatar ericringold  ( 2016-12-20 12:47:05 -0500 )edit

In fact, the 2004 User's Manual says that part of the intent of the PRM includes that defining the baseline HVAC independently from the proposed provides opportunities for more credit through energy efficiency measures such as system choice and design (pg G-22).

ericringold's avatar ericringold  ( 2016-12-20 12:49:06 -0500 )edit

I read through 90.1-2004 Appendix G and I think you are right, the standard does not say the zone assignment should be same as the design case. Only ECB (section 11) says that "each hvac system in a proposed design is mapped on a one-to-one correspondence with one of eleven hvac systems in the budget building design". I guess that's where I got the idea from.

Like you said, ASHRAE has to set some standard. Whether "system per floor" will give adequate credit for good design (system design and arrangement) is a different topic. But at least my misunderstanding is now clarified. thanks.

xchen's avatar xchen  ( 2016-12-21 13:42:22 -0500 )edit

Oh yeah, I forgot about ECB, thanks for the reminder!

ericringold's avatar ericringold  ( 2016-12-21 13:52:46 -0500 )edit

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Asked: 2016-12-20 10:48:21 -0500

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Last updated: Dec 20 '16