Unmet Load Hours and Baseline Outside Air

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I am having problems with unmet load hours while modeling a school in Trace. I have checked all my error messages and gone through the reducing unmet load hours paper from Trace. I am modeling a VRV system, so each classroom has its own fan coil unit in the room and there are dedicated outside air units. There are also small server rooms with their own fan coil units. Those 2 types of rooms, classrooms and server rooms, seem to have the highest unmet load hours. Does anyone have any suggestions on reducing these unmet load hours?

Also, our dedicated outside air units for the school will be controlled more with a timer than CO2 sensors. So the ventilation is scheduled from 8-3 for classroom occupancy and offices from 7-5. I have all fans at available 100% though. For the baseline, I have just put all outside air on from 7-5. I am uncertain if this is the correct way to do the outside air for the baseline. Does anyone have suggestions or can anyone point me to the section in 90.1? Thanks for any help you can give me!


Jason Boehning

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Joined: 2011-10-01
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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 

Michael Collarin's picture
Joined: 2011-10-01
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