[Bldg-sim] Adjusting Capacity for Unmet Cooling

[Michael Collarin]

Neeraj, it is in fact the contrary. 

 

ASHRAE 90.1-2007 Appendix G requires you to model Baseline Airflow Rate using a 20-degree F temperature difference between the supply air and room air temperatures. Therefore, if your room set point is 75F for cooling, set your cooling leaving air temperature to 55F and vice versa for heating; 72F heating set point requires a 92F leaving air temperature. This establishes your CFM airflow within the model. Do not vary these to adjust unmet hours.

 

If you have unmet heating or cooling hours (over 300 or greater than 50 between baseline and proposed), you may incrementally adjust the baseline cooling and heating capacities (originally oversized 15% and 25% respectively) accordingly to reduce the overall unmet hours (below 300) or the difference between baseline and proposed (within 50).

 

Before taking this step, be sure to look at your thermostat drift points. If the cooling set point is 75, but the cooling drift point is 84, when the space is unoccupied this would allow the space to move toward 84 degrees. When the space becomes occupied and the simulation tries to cool to 75, the system will not be able to achieve this within the allotted time (1 hour) and you will have an unmet cooling hour. 

 

There are some changes to ASHRAE 90.1-2010 that will effect unmet hours, but for the time being, if you are modeling a project using 2007 or 2004, this method should help you eliminate the unmet hours.

 

Regards,

 

Michael M. Collarin, EIT, LEED AP BD+C | Elm Engineering, Inc. | 212 S Tryon St | Suite 1375 | Charlotte, NC 28281

PHONE 704-335-0396 Ext. 108| FAX 704-335-0399 | www.elmengr.com <http://www.elmengr.com/> 

 

From: Nearedge [mailto:near_ej at yahoo.com] 
Sent: Friday, November 12, 2010 12:51 AM
To: cjaigath at yahoo.com
Cc: bldg-sim at lists.onebuilding.org
Subject: Re: [Bldg-sim] Adjusting Capacity for Unmet Cooling

 

Hello Jaigath,
Using the performance rating method (PRM), in my experience it is usually an issue of low supply CFM.
Correct me if I am wrong but on the contrary I think that ASHARE baseline PRM fixes the cooling and heating capacities to 1.15 and 1.25 respectively and the user is allowed to bump up the supply CFM in increments if necessary.
I have a feeling that the issue of unmet load hours has been discussed many times before -- so, the archives will be a good resource as well.
Best,
Neeraj

Neeraj Kapoor
t: +91.99581.70018

e: neeraj[at]kalpakrit[dot]com
Kalpakrit Sustainable Environments Pvt. Ltd.
www.kalpakrit.com

Office Address:
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On 11-11-2010 10:30, Jaigath Chandraprakash wrote: 

Hello All,

I would like to know how other increase their baseline capacity when they have unmet load. I have a PSZHP system and in one system, I got more than 400 hrs unmet load. I usually just increase the TR cooling capacity until I get a lower unmet load but in this case, I think the unmet load is due to low supply cfm. Is it ok to adjust also the supply fan cfm? I am thinking that G3.1.2.2 only requires me to adjust only the cooling capacity. Do others adjust both cfm and cooling load?

Thanks,

Jaigath

 

 




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