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

Carol Gardner cmg750 at gmail.com
Tue Jul 26 12:49:40 PDT 2011


Leave it to Bill and Bruce to give you great answers. Bruce, you are so spot
on about the equipment sizing. If LEED buildings don't "live up to their
promise" I would start looking for the cause right there.

I keep thinking that a specification section for LEED compliance might help.
Similar to the Building Commissioning one, in that there's a single place to
put all the requirements of the task so people don't have to search 00800
and the 16440 and then xxxxxx in order to be sure you have seen all that's
spec'd.

Aru, I remember when using a system that had baseboard heating, or some
system with no heating fan, that I was unable to apply the heating oversize
in the air handler. I could oversize the cooling, just not the heating. In
that case I applied the oversize to the boiler and let er rip. The model
output seemed reasonable.

Carol

2011/7/26 Arunabha Sau <runabha at gmail.com>

> Thanks Bill. Awesome clarification. I appreciate your time.
> So I was wondering, if you oversize the Plant does the AHU get oversized
> too by default, in eQuest?
> Any thoughts?
>
>
> Much thanks,
> Aru
>
>
> 2011/7/26 Bishop, Bill <wbishop at pathfinder-ea.com>
>
> 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****
>>
>> ** **
>>
>> [image: Senior Energy Engineer]****
>>
>> ** **
>>
>> *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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-- 
Carol Gardner PE
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