Oversizing..AHU or Plant?

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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

Arunabha Sau's picture
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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

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Bruce Easterbrook's picture
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Thanks Bruce. Great reply.
I completely agree that modelers need to have a good technical knowledge.
Oversizing AHU is justified now. I appreciate your help.

Much thanks,
Aru

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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

Arunabha Sau's picture
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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

--
Carol Gardner PE

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Bruce Easterbrook's picture
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But what about the case where you can't input the over-sizing in the systems
side because all the necessary inputs are grayed out?

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