[Trace-users] Unmet Load Hours and Baseline Outside Air

Michael Collarin MCollarin at elmengr.com
Tue Dec 7 08:07:59 PST 2010



I think it's first important that you understand the unmet load hour and how it is defined. From the TRACE 700 User's Manual:


Unmet load hours are triggered in TRACE when space conditions are outside the thermostat setpoints. Unmet hours occur for a wide variety of reasons. They are restricted to hours when the excess load on a coil is at least 1 percent greater than the design coil capacity, and at least one room attached to that coil is in excess of 1.5°F of the thermostat setpoint for that hour.


Also, included in the User's Manual are strategies to help reduce or limit the number of unmet hours. The User's Manual should be located on the computer that TRACE is installed in the following folder: C:\Program Files\Trane\TRACE 700\Documentation


Is the classroom modeled to be occupied the entire time or is it fluctuating between occupied and unoccupied?

The server room seemingly would be unoccupied.


It is important to know that while the room is occupied, the cooling/heating system will try to maintain thermostat setpoint. However, when the room becomes unoccupied, the system will allow the temperature to drift to whatever you have the range or drift points set to. So while your thermostat may be set at 75F for cooling, if your cooling drift point is 85, when that space is unoccupied the system will allow the space temperature to reach 85. Now, when the room becomes occupied, the system will activate and try to meet setpoint (75F) which it will not be able to do from 85F within one hour, hence the unmet hour.


Per Table G3.1.4 Schedules and Section G3.1.2.5 Ventilation of Appendix G in ASHRAE 90.1, the Ventilation schedules should be modeled the same in the Proposed and Baseline cases except when demand controlled ventilation (DCV) is used but not required to be used per Section When DCV is required, you cannot take credit for using it. Without knowing more about the project, I cannot determine if DCV is in fact required by code. The Ventilation rates shall be the same for the Proposed and Baseline as well.


I hope this helps.




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/> 


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