[Equest-users] Oversizing..AHU or Plant?

[Bishop, Bill]

Aru,

 

I don't think the intention of G3.1.2.2 is met by oversizing only the plant and not the systems. The 90.1 User's Manual does not specify what is to be oversized either, although it does describe a chiller when discussing the differences in equipment sizing between the ECB and PRM:

"For instance, if a chiller in the proposed building is 50% oversized, then the chiller in the budget building is also 50% oversized. However with the building performance rating method, equipment in the baseline building is oversized by a fixed amount in all cases."

 

In eQUEST, setting the system Cool Sizing Ratio to 1.15 and Heat Sizing Ratio to 1.25 will also increase the autosized capacities of the plant. I believe this is how most Appendix G modelers (including myself) address the G3.1.2.2 requirement.

 

Note that you could also achieve the same cooling and heating capacity oversizing by setting the overall system Sizing Ratio to 1.15, the Cool Sizing ratio to 1.00 and the Heat Sizing Ratio to 1.087. (1.15x1.087≈1.25). However, this oversizes the system airflow by 15%, and G3.1.2.8, which specifies how design airflow rates are determined, does not mention oversizing. I have had models where unmet heating/cooling hours could only be reduced sufficiently by increasing the system airflow as a last resort.

 

Regards,

Bill

 

 

 

From: equest-users-bounces at lists.onebuilding.org [mailto:equest-users-bounces at lists.onebuilding.org] On Behalf Of Arunabha Sau
Sent: Monday, July 25, 2011 7:47 PM
To: equest-users at lists.onebuilding.org
Subject: [Equest-users] Oversizing..AHU or Plant?

 

Howdy!

 

 

So, I just saw a project which has applied its oversizing factors (1.15 for cooling and 1.25 for heating) to the plant rather than the air handlers.  ASHRAE 90.1 Appendix G page 180, G3.1.2.2 Equipment capacities  does not specify where the oversizing should occur.  Have you seen this method used before?  Is it an acceptable and accurate way of meeting the baseline modeling requirements?

 

I appreciate your help.

 

 

 

 

Much thanks,

Aru

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